Abstract:
A wire testing method and apparatus tests wire ( 30 ) in-situ in a rigidly installed state, such as an aircraft ( 100 ). Stray currents are compensated by an apparatus for rapidly switching the excitation voltage and ground. The apparatus outputs a stray current signal ( 721 ), a detected current signal ( 731 ), and a difference signal ( 830 ) indicative of a measurement with the stray current signal removed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS.  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/201,262 filed May 2, 2000, which is incorporated by reference in its entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to a method and apparatus for testing wire in-situ, in a rigidly installed state, in which the effects of stray currents are compensated.  
           [0004]    2. Related Work  
           [0005]    Weight is an important factor in aircraft design. Using lighter components is one way of reducing weight in an aircraft.  
           [0006]    Wiring is an important component in every aircraft, and contributes significantly to the weight of an aircraft. In this context, a wire may be understood to include a conductor provided with a protective outer layer referred to as insulation. Furthermore, the term “wiring” as used herein may be understood to include conductive wires that carry signals, current, and the like. The term “testing” of wiring as used herein may be understood to include testing various qualities of the conductor, the insulation, or both.  
           [0007]    Wires may be bundled together and enclosed with a common shield made of various materials such as nylon, rubber, and the like. On the other hand, it is frequently the case that a bundle of wires is not enclosed with a common shield. In either case, however, such a bundle of wires may be referred to as a cable. Other common terms for a cable are a harness or a loom.  
           [0008]    There are various approaches to making the wiring in an aircraft design weigh less. One approach is to reduce the length of wire as much as possible. Another approach is to use the smallest conductor that is considered to be safe for the load. Yet another approach is to equip the wiring with a lightweight insulation.  
           [0009]    Although the use of lightweight insulation is a prominent approach to reducing the overall weight of the wiring component in an aircraft design, this approach includes risks not heretofore appreciated. One such risk is the risk of degradation of the insulation. Degradation may result from vibration, chafing, or impact Degradation may result from the simple aging of the insulation and from the exposure of the insulation to harmful environments. As used here, degradation means damage to insulation from any source whatever.  
           [0010]    The insulation is an important part of the wiring because it keeps conductors of different wires from undesirably coming into contact with each other and from undesirably coming into contact with an electrical ground. When different wires undesirably come into contact with each other, short circuits can occur. Insulation also keeps the conductor of a wire from undesirably coming into contact with the environment surrounding the wire. When the conductor of a wire undesirably comes into contact with its surrounding environment, sparks, fires, and explosions can result Another possible result of such an undesirable contact is the malfunction of the connected or nearby equipment, or the propagation of invalid data or unwanted noise.  
           [0011]    This invention relates to the testing of wires for degradation. The degradation tested is primarily degradation of the insulation. Even more particularly, this invention relates to the testing of wiring in situ, or in its normal place of operation. This invention was developed in the particular context of aircraft wiring, and the examples described herein are given in that context. Nevertheless, the invention is extremely useful for testing wires for degradation in situ in any setting. In particular, it is envisioned that the invention may be used with wiring installed in such settings as the home, in ships, underground, in buildings, or the like.  
           [0012]    There are certain problems with the testing of in-place wiring insulation.  
           [0013]    First of all, a cable may contain numerous wires, each with its own insulation that needs to be tested.  
           [0014]    A second problem is that the wires to be tested may be inaccessible except at the ends. In an aircraft or in an underground wiring system, cables containing wires may extend for hundreds of yards, or even miles, without the opportunity for access. Ideally, a test that identifies wiring degradation should take inaccessibility into account. Moreover, it is very often the case that only one of the two ends is available or accessible. Where only one end is accessible, a loop-back type of test is substantially impossible. A test that accounts for single-end accessibility is required.  
           [0015]    A third problem with testing wires for degradation is that some testing methods may actually cause damage to the insulation, or to the conductor, or to the connected equipment For example, testing a wire using high voltage may damage equipment connected to the other end of the wire, or may even further injure degraded insulation.  
           [0016]    The following patents are representative of the art, and each is incorporated by reference in its entirety for its useful background information: U.S. Pat. Nos. 464,125; 2,526,891; 2,942,181; 3,349,324; 3,820,011; and 4,891,597.  
           [0017]    In U.S. Pat. No. 464,125, there is disclosed a technique for testing insulated wires. In particular, a bundle of insulated wire is immersed in a tank of water. An electromotive force is provided at one end of the wire at the conductor of the wire. A conductor is placed in the water. The insulation on the wire is determined to be defective if a current can be induced to flow from the one end of the wire connected to the electromotive force, through the insulation, and to the conductor that is in the tank of water.  
           [0018]    This method and apparatus for testing wire insulation is unsuitable for use in aircraft or the like because it requires the complete removal of the wire or cable, and its immersion in a big vat of water. Big vats of water cannot reasonably be used in an aircraft or the like in view of the risk of harmful effects to the surrounding environment. Another reason that big vats of water cannot reasonably be used for in situ testing is that wires are often affixed to elevated parts of the interior structure, and it is very hard to remove the wire far enough from the interior so as to submerge it in such a vat. That is to say, the wire to be tested normally has to remain substantially rigidly fixed to the aircraft.  
           [0019]    In U.S. Pat. No. 2,526,891, there is disclosed a method of testing electrical conductors. According to this method, a twisted pair of wires is tested by applying a DC polarizing potential across the two wires for a period of time sufficient to create electrolytic polarization in the moisture of the sheaths, disconnecting the source of polarizing potential from the wires, and measuring the electrolytic polarization potential residual in the moisture after disconnection of the polarizing potential.  
           [0020]    According to this approach, the wire must be a twisted pair of copper wires each sheathed with a seamless sheath of paper pulp. It is assumed that the paper pulp sheaths already contain as much as 15 percent water weight, either residual from the making of the sheaths or outdoor from the atmosphere during storage. The teachings of this patent relate to the manufacturing stage of wire and, more particularly, to the detection of breaks in a wire when the amount of moisture in the paper pulp sheaths is enough to cause a false reading that the conductor is not broken. No conductive medium is applied to the wires, and there must be at least two breaks in the insulation for a detection to be made.  
           [0021]    U.S. Pat. No. 2,942,181 describes an apparatus for testing cables. This cable-testing apparatus tests a cable as it is wound from one reel to another reel. As the cable moves between the two reels, it is immersed in a tank of water. One end of the cable is connected to an electromotive force. Submerged in the tank of water is an electrode. If the electromotive force can cause a current to flow from one end of the cable through a crack in the insulation and to the electrode submerged in the water, then a defect in the cable is detected.  
           [0022]    This approach is unsuitable for use where the wires to be tested cannot be removed and dipped in the tank of water. Furthermore, this United States patent does not contain any teaching or suggestion relating to how much electromotive force should be used in the attempt to cause a current to flow to the submerged electrode. In addition, in the foregoing approach assumes that there is no sensitive or delicate equipment connected to the cable.  
           [0023]    U.S. Pat. No. 3,349,324 describes an apparatus for locating faults in insulated electric cables. The apparatus has an elongated tube that contains a conducting fluid electrode. According to this approach, a cable is transferred from one reel to another. During the transfer, the cable passes through a tube filled with water. The tube is pressurized. The tube itself is made of an electrical conducting material. Under this approach, an electromotive force is applied to the tube itself, while one end of the cable is grounded.  
           [0024]    With the foregoing approach, it is necessary that the entire tube be insulated from the ground. Also, it is assumed that the cable is completely disconnected from any sensitive or delicate equipment, and also that the cable is not installed, but is passed from one reel to another. This method is therefore entirely unsuitable for use in an in situ test.  
           [0025]    U.S. Pat. No. 3,820,011 provides for a trough into which a wire is submerged so that faults in the insulation around the wire can be detected. The testing described in this document describes the use of low voltage in the detection of faults. Under this approach, however, it is assumed that the wire is fed through the trough. Thus, the wire must be in a state in which it is not installed in a restrictive area, such as an aircraft or the like.  
           [0026]    U.S. Pat. No. 5,206,597 describes a capacitive moisture detection apparatus. In particular, the apparatus is meant to measure the volume of water intrusion in underwater cables without having to take the cables apart. According to this method, a cable is fed from one reel to another through an elongated tube. In the elongated tube, which is a cylindrical brass shell, there are two curved electrical conducting plates which serve as capacitance plates. The volume of water that has infiltrated the cable is determined by measuring the capacitance.  
           [0027]    None of these patents provides for an acceptable in situ testing of a wire for degradation, and none provides for any protection of the equipment that remains connected to the cable during the test.  
           [0028]    Furthermore, in situ testing of a wire in an aircraft or the like often involves wires that are suspended overhead or are affixed or attached to walls of the aircraft.  
           [0029]    In-situ testing of wire in an aircraft may be accomplished by a method described as follows.  
           [0030]    In-situ Testing of Wire  
           [0031]    A practical and effective wire testing method is provided in the DelTest™ system, available from Lectromechanical Design Company™, 45000 Underwood Lane, Unit L, Sterling, Va. 20166-9557. DelTest™, Lectromechanical Design Company™, and LectroMec™ are trademarks of Lectromechanical Design Company.  
           [0032]    The DelTest system is an example of a wire testing method described in the above-identified related application. The methods described in the related application will now be described here to provide useful background in understanding the present invention.  
           [0033]    [0033]FIG. 1 shows a cable, indicated in general by reference numeral  10 . The cable shown includes a cable shield or covering  20 . The covering  20  may include multiple layers and a protective mesh (not shown). Although FIG. 1 shows a cable with a cable shield, it is typically the case that the cable has no shield at all, and sometimes has only a lacing to hold it in place. Inside the cable  10  there is at least one wire, indicated generally by reference numeral  30 . In FIG. 1, seven wires  30  are shown, although a cable may contain an arbitrary number of wires. Each wire  30  includes a conductive part  40 . The conductive part  40  will be referred to hereafter as a conductor, even though the conductive part  40  may include many smaller individual conductors. Each wire  30  also includes insulation  50 . The insulation  50  insulates the conductor  40 . Inside the cable  10  there may be interstitial spaces  60  between the wires. The spaces  60  typically contain air.  
           [0034]    [0034]FIG. 2 shows another cable  10  and, in particular, the details of the end of a cable which may be used in an aircraft. The cable covering  20  is not shown and, indeed, it may not be present in some cables. The cable  10  includes a plurality of wires  30 . The conductor  40  of each wire  30  is electrically connected to a respective pin  45 . It should be appreciated that, instead of respective pins  45 , a cable might just as well have sockets instead. The pins  45  are normally in a one-to-one correspondence with the conductors  40 . The pins  45  may be protected and rigidly held by an outer metal housing  12  which may be provided with a rotatable locking ring  14 . The rear portion of the housing  12  may be an externally threaded rear portion  16  adapted to receive an internally threaded backshell member  18 . The backshell  18  may have integrally formed with it a part of a strain relief  19 . A strain relief clamp portion  19 A may be provided to cooperate with strain relief  19  so as to secure wires  30 . Threaded fasteners  19 B may be provided to fasten the strain relief clamp  19 A to the strain relief  19  of the backshell  18 . The outer circumference of the housing  12  may be provided with raised areas  47  that act as keys for precisely aligning the end of the cable  10  with corresponding recesses in the inner circumference of a mating connector.  
           [0035]    [0035]FIG. 3 shows, in a simplified schematic form, a wire  30  connecting Unit A and Unit B. The wire  30  has a conductor  40  surrounded by insulation  50 . Of course, it will be appreciated that the wire may be a bundle of wires, may be several lengths of wire connected together at intervening connectors (not shown), and that certain ones of the bundle of wires may extend from some equipment other than Unit A or may connect to some equipment other than Unit B.  
           [0036]    [0036]FIG. 4 shows an aircraft  100 , and the approximate locations of Unit A (A) and Unit B (B). Aircraft  100  is an example of a situation in which removing wire  30  for testing is impractical.  
           [0037]    [0037]FIG. 5 shows an exploded view of the aircraft  100 , revealing Units A and B, and the wire  30  connecting the units. Wire  30  passes through bulkhead  110  of the aircraft  100 .  
           [0038]    [0038]FIG. 6 depicts Unit A and bulkhead  110 . Wire  30  is shown connected to Unit A, and extending toward Unit B after passing through bulkhead  110 . Bulkhead  110  represents a point at which the wire is substantially inaccessible. This point of inaccessibility may exist because of a physical obstacle, such as bulkhead  110 , or may exist because access to the wire beyond the point of inaccessibility is merely inconvenient. For the remainder of this discussion, reference numeral will be referred to, in the main, as the point of inaccessibility or, for linguistic convenience, as point  110 . It will be appreciated that point  110  may, in other settings, not relate at all to a bulkhead. For example, where an underground wire is concerned, point  110  may be where the wire enters an enclosed conduit. Where a dwelling is concerned, point  110  may be where the wire enters behind a dwelling wall or into an attic.  
           [0039]    Degradation of the wire  30  may occur at the part of the wire between the point of inaccessibility and the equipment to which the wire is connected. In FIG. 6, this area is shown by reference numeral  200 . This area may be referred to as a high degradation area. This high-degradation area  200  may be subject to much more wear and tear than many other parts of the wire  30 . Since an important amount of the degradation of the wire  30  may occur in high-degradation area  200 , it is possible to detect much of the degradation by testing only this part of the wire. For linguistic convenience, this high-degradation area  200  will be referred to as the end  200  of the wire  30 . It will be appreciated, however, that this usage of the term “end” means more than just the end of the wire, in the normal sense, but includes the part of the wire between the equipment (Unit A, for example) and the point of inaccessibility  110 .  
           [0040]    It will be appreciated, however, that the degradation of wire can occur anywhere along the length of the cable. The degradation of wire is often associated with locations in which the wire is exposed and, thus, more vulnerable to damage. Similarly, the degradation of wire may occur in situations in which the wire is improperly installed. For example, when a wire is improperly installed in an airplane, it may be subjected to excessive vibration or rubbing. Wire may be secured in place improperly, thus subjecting the insulation to damage by the device that secures the wire in place.  
           [0041]    For the sake of explanation, the focus of the following description will be on degradation that occurs at or near the end of the wire, but it will be appreciated that the explanation and techniques described herein may apply along the entire length of the wire. Furthermore, the description will be targeted to detecting degradation using a jacket. Afterward, the discussion will reach ways to test for degradation without a jacket. Also, as a preliminary matter, the term, “conductive medium” will be defined for the purposes of this disclosure.  
           [0042]    A conductive medium is a substance which, in fluid form, is electrically conductive. Normal water is an excellent example of a conductive medium, and is used as an example throughout this discussion. Water is advantageous as a conductive medium in that it is plentiful, safe, relatively easy to work with, and, typically, sufficiently conductive. Distilled water, of course, lacks any minerals and does not constitute an effective conductive medium. Water evaporates so that, if some remains on the wiring being tested after the test, it will rapidly disappear without further human intervention.  
           [0043]    Water, of course, may be used as a solvent to dissolve a solute, like salt, to form a more conductive medium than regular water. Salt water is thus an example of a conductive medium. Salt water has excellent conduction of electric current. An advantage of salt water is that, when it dries, all that typically remains is dried salt. Dried salt is not an effective conductor, and may be said to have important insulative properties.  
           [0044]    Substances like salt water, which are conductive in fluid form and yet dry, leaving behind a substantially non-conductive substance, are preferred for use as a conductive medium. As used herein, for the remainder of the discussion, and unless otherwise specified, water means water which is not just distilled water and which may contain some salt or other minerals.  
           [0045]    Another example of a substance which may be useful as a conductive medium is an ionized noble gas, such as Argon.  
           [0046]    [0046]FIG. 7 depicts a cable  10  in which one of the wires  30  includes an area of degraded insulation  210 . Although the degradation  210  is shown as a large, visible break in the insulation, the degradation of insulation might not be manifested in such a manner. The degradation of insulation can appear as cracks visible only under magnification. These micro-cracks are large enough, however, to present a potentially serious problem.  
           [0047]    [0047]FIG. 8 shows only two of the wires  30  that were depicted in FIG. 7. Also shown is crack  210 , which is representative of the different kinds of degradations that can occur in a wire.  
           [0048]    [0048]FIG. 9 shows the same two wires that were shown in FIG. 8, but also shows a jacket  300 . Jacket  300  includes an introduction port  310  and an evacuation port  320 . Introduction port  310  is for introducing into the jacket a conductive medium, such as water. The evacuation port  320  is for removing the conductive medium from the jacket.  
           [0049]    [0049]FIG. 10 shows a perspective view of jacket  300 , introduction port  310 , and evacuation port  320 . FIG. 10 also shows one of the two ends  330  of the jacket  300 . As a practical matter, the jacket may be constructed in a wide variety of ways. For example, the jacket may include a tube through which the part of the wire to be tested is inserted. Similarly, the jacket may include a tube which may be opened along an edge so that the tube may be positioned around the part of the wire to be tested. Another way to produce jacket  300  is to fabricate it from tape used as a wrap. In particular, latex rubber tape, silicon rubber tape, or self-bonding polymer tapes, all commercially available, may be used as a wrap to form a substantially watertight jacket.  
           [0050]    In particular, the part of the wire to be tested is wrapped using overlapping turns, and the introduction and evacuation ports are positioned so as to communicate with the inside of the tube formed by the plastic wrap. Then, more tape may be used to make the jacket have a substantially watertight seal around the ports, at the two jacket ends  330 , and all up and down the length of the jacket. One important goal in making the jacket is that the jacket hold, without much leaking, the conductive medium. A small amount of leaking is acceptable, provided that the leaking conductive medium can be absorbed or captured before it has the chance to cause any damage to the surrounding area. Of course, it is wise also to cover sensitive equipment.  
           [0051]    [0051]FIG. 11 shows a container for introducing the conductive medium CM, and a reservoir R for receiving evacuated conductive medium. The conductive medium is caused to enter the introduction port  310  from the container CM by virtue of and inducing force, which may be air pressure, gravity feed, or the like. The conductive medium fills up the space in jacket  300 , and, at the desired time, may be evacuated from evacuation port  320  into reservoir R by virtue of another force, which may be a vacuum or the like. Valves may be placed at the introduction port  310  and the evacuation port  320 . Without such valves, it may be necessary to provide a constant flow of conductive medium from container CM through jacket  300  to reservoir R.  
           [0052]    [0052]FIG. 12 is similar to FIG. 11, except that the container CM and the reservoir R have been removed to simplify the drawing, the degradation  210  in the insulation  50  of one of the wires is shown, and the test unit  400  also is shown. A first test element  410  is connected to test unit  400  and is electrically coupled to one of the conductors  40  of one of the wires.  
           [0053]    Here, the terms “electrically coupled” and “connected” typically mean that there may be any number of intervening connectors and other connecting wires between the test element and the conductor, so long as there is an electrical path to the conductor. When a direct connection between two components is required, without any intervening components, the term “directly connected” will be used.  
           [0054]    A second test element  420  is connected or coupled to test unit  400  and is electrically coupled through the evacuation port  320  to the conductive medium inside of the jacket  300 . The second test element  420  has an end  425  that is contained within the jacket  300 .  
           [0055]    Referring now to FIGS. 6, 10,  11 , and  12 , the manner in which the high-degradation part  200  of the wire  30  is tested according to the DelTest system will now be described. A jacket  300  is disposed around a substantial part of the high-degradation part  200  of the wire  30 . The jacket ends  330  are made substantially watertight. A container CM of conductive medium is connected to introduction port  310 . The end  425  of the second test element  420  is disposed inside the jacket  300  via the evacuation port  320 . A reservoir R for receiving the conductive medium from the jacket  300  is connected to the evacuation port  320 . Conductive medium from container CM is used to fill up the jacket  300 . The conductive medium surrounds the part of the wire  30  disposed inside the jacket  300  (or, if a cable is being tested, the conductive medium surrounds the parts of all of the wires  30  disposed inside the jacket  300 ).  
           [0056]    The second test element  420  is connected to test unit  400 . The first test element  410  is electrically coupled to the conductor  40  of the wire  30 . An attempt is made to induce a current within the wire  30 . If the insulation  50  of a wire  30  being tested is degraded (i.e., has a degradation  210 ), then a current will flow from the test unit, through the first test element  410 , through the conductor  40 , through the degradation  210 , through the conductive medium, to the end  425  of the second test element  420  and back to the test unit  400 . On the other hand, if the insulation  50  of a wire  30  being tested is not degraded, no current will flow back to the test unit  400 , owing to the prevention of such a current by the insulation  50  surrounding the conductor  40 .  
           [0057]    The test unit  400  may be provided with a receptor for receiving a cable connector (see FIGS. 16, 17, and  18 , for example). For example, a socket to receive the cable  10  as shown in FIG. 2 may be included in the test unit. In such a case, the test unit will include a switch that selectively connects the first test element  410  with the pins  45  (and, ultimately, with each of the conductors  40  of the wires  30 ) of the cable  10 . By way of such a receptor, the testing of each of the wires  30  of a cable  10  may be facilitated.  
           [0058]    One of the challenges of testing wiring for degradation in situ is that it might be undesirable for the conductive medium to remain after the test is completed. Therefore, according to one embodiment of the DelTest system, the conductive medium is be removed. For example, if the conductive medium is water, it may easily be removed from the jacket  300  to the reservoir R by vacuuming it out. The provision of the jacket  300  makes it possible to test the wires in the high-degradation part without getting conductive medium in the undesired areas of the surrounding environment. In the case of water, trace amounts of the conductive medium do not pose a problem because the water can evaporate. In the case of salt water, when the conductive medium dries, it leaves behind only a dry substance (namely, salt) that is substantially insulative in nature.  
           [0059]    The foregoing discussion has focused on one embodiment of the DelTest system in which a jacket is used. It is not always possible to use a jacket. FIG. 13 shows an example of a testing method according to the DelTest system which does not use a jacket.  
           [0060]    In FIG. 13, a bundle of wires  500  is to be tested. More particularly, only a portion of the bundle of wires  500  between the positions marked by reference numeral  510  and reference numeral  520  is to be tested. To effect the testing, a conductive medium is sprayed from a source  530  onto the bundle  500 . Under the influence of gravity and capillary-like action of the interstices between the wires, the conductive medium flows to a low point of the wire indicated as reference numeral  535 . At the low point  535  of the bundle of wires  500  there is provided a suction unit  540 . Suction unit  540  may be a rubber boot or the like, and may optionally be tapered toward a suction hose. To catch any conductive medium that falls off of the bundle of wires  500  prior to reaching the suction unit  540 , there may be provided a pan  550  or the like.  
           [0061]    By spraying the conductive medium from source  530  constantly onto the bundle of wires  500 , taking into account the rate at which the conductive medium flows down the bundle  500 , the conductive medium is disposed around the wires in sufficient quantity so that testing may be undertaken. The first test element  410  (not shown in this figure) is connected selectively to each conductor of the wire bundle  500  (or to all of them at once, optionally, as described further below). The second test element  420  may be disposed in various different locations, but is preferably placed near suction unit  540 , as shown in FIG. 13, but not so close to the suction as to dry out the test element.  
           [0062]    [0062]FIG. 14 shows another example of a testing method according to the DelTest system which does not use a jacket. FIG. 14 is similar to FIG. 13, except that it shows a close-up area near the suction unit  540  and also shows a barrier  110 . The location of barrier  110  presents a particular problem because the low part  535  of the wire bundle is not disposed on the accessible side of the barrier. This is an awkward arrangement of the wires. In such a situation, testing according to the DelTest system may still be implemented without causing damage to the environment around wire bundle  500 . According to one embodiment of the DelTest system, there is provided a source of high-pressure air  595 . The source of high-pressure air  595  encourages, or urges, any of the conductive medium that is not extracted by the suction unit  540  back in the direction of that suction unit so that it can be removed. To put it another way, the high-pressure air source  595  is strong enough to create a barrier of air which prevents or at least substantially retards the progression of the conductive medium beyond the suction unit  540 . A pan  550  may of course be provided under this area as necessary.  
           [0063]    Now some details relating to the electrical aspects of testing the wires for degradation in the related application will be provided. In particular, there are three different testing circuits which are preferably used, depending upon the situation.  
           [0064]    With reference to FIG. 15, there is shown a decision diagram that indicates which of the three different testing circuits is to be used. The circuits themselves will be described briefly below. In FIG. 15, an initial check of the load resistance is made using circuit No. 2 (see FIG. 17). From this initial check, it can be determined whether most of the wires being tested in the cable have high resistance loads. At step S 10 , a decision is made as to whether most of the wires do have high resistance loads.  
           [0065]    When it is determined in step S 10  that most of the wires do have high resistance loads, a further decision is made in step S 20 . That decision, in step S 20 , is whether the highest sensitivity to degradation is a priority. If the highest sensitivity is an important priority in the present situation, then circuit No. 1 (see FIG. 16) is employed at step S 30 . If not, then circuit No. 2 (see FIG. 17) is employed at step S 40 .  
           [0066]    Returning now to step S 10 , if most of the wires do not have high resistance loads, then a further decision is required at step S 50 . At step S 50 , an investigation is made as to whether the cables being tested include shielded or metal braided cables. If there are shielded or metal braided cables in the harness, then it may be necessary (see step S 60 ) to disconnect the load from as many wires as possible, and start again from the initial check in step S 00 . If it is not the case that there are shielded or metal braided cables in the harness, then circuit No. 3 is used at step S 70  (see FIG. 18).  
           [0067]    A discussion of circuit No. 1 will now be given with reference to FIG. 16. Circuit No. 1 is the most sensitive of the three circuits, and can be used to find very fine cracks in the insulation. Circuit No. 1 can detect cracks that have a resistance of up to 10 12  Ω. It compares the voltage decay of wires as they are with the voltage decay after a conductive medium has been applied to the wire. The impedance at the opposite end of the wire (i.e. the end of the wire that is not connected to the test unit  400 ) must be large. In particular, that impedance must be greater than 10 12  Ω.  
           [0068]    Circuit No. 1 includes a voltage selection switch  600 , a momentary pushbutton test switch  610 , a high impedance voltmeter  620  (high impedance being, for example, around 10 17  Ω), an oscilloscope  630 , and a voltage polarity switch  640 . The voltage polarity switch  640  is operated as necessary to ensure the correct polarity for circuit No. 1 for testing circuits that have diodes. A switch box  650  is optionally provided so as to conveniently connect the different wires of the cable being tested to the circuit.  
           [0069]    The voltage selection switch  600  has positions for no voltage, 1.5 V, and 28 V.  
           [0070]    First, the voltage selection switch  600  is set to 1.5 V and the momentary pushbutton test switch  610  is closed. The test switch  610  is released, and the voltage decay measured by the high impedance voltmeter  620  is monitored using the oscilloscope  630 . There should not be much decay for at least one minute after releasing the test switch  610 . The test switch is then re-closed and the conductive medium is applied. Then, the test switch  610  is opened and the voltage decay is noted. If the decay is approximately the same as before, then there is no breach in the insulation under test. If the decay is rapid, however, (i.e., less than one second) then there is a breach in the insulation. In circuit No. 1, rapid decay indicates an insulation breach.  
           [0071]    It will be noted that, in circuit No. 1, the switch box  650  is capable of connecting all of the different wires of the cable being tested to the circuit at the same time. Likewise, each wire may individually be switched on or off. In one embodiment of the DelTest system, all of the wires are tested at once for the above-identified rapid decay. If a rapid decay is detected, this indicates that at least one of the wires under test has an insulation breach, and then each wire is individually tested to determine which one has the breach.  
           [0072]    The sensitivity of circuit No. 1 is limited by the load impedance (i.e. impedance at the end of the wire that is not connected to the test unit  400 ). For the test to successfully detect a crack, the change in the decay time after the conductive medium is applied must be easily observed. Without more sophisticated equipment than discussed here, this means that the resistance of a detectable crack is limited to about a factor of 10 lower than the load impedance. For example, if the load impedance was 10 12  Ω, a crack with resistance of up to 10 11  Ω could be detected. If the load impedance was 10 7  Ω, a crack with resistance of up to about 10 6  ω could be detected.  
           [0073]    As a practical matter, however, if the resistance of the crack is too high the measurement time needed to detect a change in the decay time may become too long. For typical values of wire capacitance found on aircraft (10 −10  to 10 −9  Farads), the decay time for a crack of 10 11  Ω is  10  to 100 seconds.  
           [0074]    Unless test time is not an issue, this will limit the useful sensitivity of circuit No. 1 to detection of a crack with 10 11  Ω or less, regardless of the load resistance.  
           [0075]    It should be noted, for clarity, that, in the foregoing discussion, a value given for the resistance of a crack assumes the crack has been treated with the conducting medium and is not dry. The resistance of a typical dry crack may be as high as 10 20  Ω.  
           [0076]    Circuit No. 2 includes several features that are substantially the same as circuit No. 1, and these are indicated by like reference numerals. In addition, circuit No. 2 includes a voltmeter  720  and a micro-ammeter  730 .  
           [0077]    Circuit No. 2 is the most used circuit. It requires the load at the opposite end of the wire to be only 10 MΩ or greater. It is sensitive enough to find almost any cracks that have reached through to the conductor. It compares the current in the wire under test without the conductive medium applied, with the current after the conductive medium is applied. If an insulation breach exists, the later current will increase significantly. As with each of the circuits, care must be taken to insure that the conducting medium does not get into undesirable locations. As will become apparent, this circuit may be more convenient to use than No. 1. Circuit No. 1 requires the observation of a test signal for a decay, but circuit No. 2 involves just the comparison between two different currents. Also, the load at the end of the wire does not have to be to great for the circuit No. 2 as was the case with circuit No. 1.  
           [0078]    Circuit No. 2 has a 100 k and a 1 k resistor in parallel with the voltmeter. These resistors are used to measure the current coming from the battery. When the switch in series with the 1 k resistor is closed, the equivalent resistance is 1 k (1 μA per mV measured on the voltmeter). When the switch is open, the equivalent resistance is 100 k (0.01 μA per mV measured on the voltmeter). Thus, the switch in series with the 1 kΩ resistor is for selecting the sensitivity of the battery current measurement circuit.  
           [0079]    When using circuit No. 2, the voltage selection switch  600  is set to 28 V or 1.5 V depending on the delicacy of the load circuits of the wires under test. The current is noted. Then, the conducting medium is applied and any increase in the current is noted. If a jump in the current is detected, this indicates an insulation breach. Subsequently, the conductive medium can be reapplied to verify and to pinpoint the location of the insulation breach.  
           [0080]    With circuit No. 2, as was the case with circuit No. 1, it is advantageous to initially connect all the wires to the test switch. If the above-identified jump in current is detected, an insulation breach is present. To isolate the individual wire which has the breach, the individual wire switches of the switch box  650  are turned off one by one until the current goes away. The wire thus detected may then be further investigated.  
           [0081]    A description of circuit No. 3 will now be given with reference to FIG. 18. The individual components of circuit No. 3 are substantially the same as circuit No. 2, but the arrangement is different. Circuit No. 3 can be used when there is a low impedance load on the wires to be tested and it is impractical to remove the load. It is a great advantage not to need to consider the wire loads. A disadvantage of the circuit is that hidden current leakage paths to an airframe, to clamps, to wire shields, and the like, will give false positive readings. Therefore, circuit No. 3 can be used to prove that there are no insulation breaches; an indication of a wire breach, however, should be verified using another method. As already mentioned, circuit No. 3 is similar to circuit No. 2, but in circuit No. 3 the test voltage is applied to the second test element  420  and leakage current is measured through the first test element  410  and the switch box  650 .  
           [0082]    When using circuit No. 3, the voltage selection switch  600  is set to 28 V or 1.5 V depending on the delicacy of the load circuits of the wires under test. The current is noted. The conductive medium is then applied to the wire or wires under test, and any increase in the current is noted. If a jump in current is found, this indicates an insulation breach. The wire with the breach is found by turning off individual ones of the wire switches on the switch box, one by one, until the current goes away. Once a wire is isolated, the conductive medium can be reapplied to verify and to pinpoint the location of the insulation breach.  
           [0083]    Problem to be Solved in the Present Invention  
           [0084]    The wire testing method in the related application and described herein is a system which can be subject to noise. The noise in question is stray DC offset currents that can vary between 0 and 20 μA. These currents can make detection of small breaches difficult, because the magnitude of the signal current that constitutes a leakage current may be of the same order as the background current (i.e., as the stray current). This can also increase the time it takes to perform a test with the DelTest system in a given zone because time can be wasted investigating spurious noise signals.  
           [0085]    The origin of the stray DC offset current has not been fully determined but likely sources include:  
           [0086]    battery effects causes by the conductive fluid mixing with dirt and salts on the wire thus becoming electrolytic;  
           [0087]    stray ground currents caused by operation of equipment at the facility (both DC and AC partially rectified by dissimilar metals);  
           [0088]    ground loops picking up power AC signals (i.e. radar) which are partially rectified by dissimilar metals; and  
           [0089]    other sources as yet to be determined.  
           [0090]    In the DelTest system, one way to attempt to detect and compensate for a stray current is to manually switch the voltage applied to the conductor of the wire under test between high (e.g., 28 VDC) and ground.  
           [0091]    If there is an actual breach in the insulation, then the measured current will follow the applied voltage switching between high and zero. If the measured current is due to a stray current then it will stay relatively constant even when the voltage is switched back and forth.  
           [0092]    Often in practice the result in somewhere in between as the measured current changes between a high value and a low value but because there is also a stray offset current the low value is not zero (i.e. the measured current switches between 20 and 15 μA). Because each of the above listed potential sources for the stray current can change over the time of testing a particular bundle, it can difficult to decide whether a particular signal is due to a breach or not.  
           [0093]    For example, an airplane may be tested at an airport. Suppose equipment at the airport emanates energy that is not at a constant level. The stray currents caused by such energy might vary considerably, and an operator might become easily confused as to whether an actual leakage current is being detected. A decision must be made as to whether to open up a bundle of wires because there may be a breach, or to leave the bundle alone and possibly ignore a breach. Stray currents make this decision difficult, and thus the successful employment of the testing method in the related application requires a high level of training/expertise. The requirement for such a high level of expertise can be disadvantageous.  
           [0094]    Moreover, due to the above-identified sources of stray currents, the amount of current can change dramatically when the testing process is actively undertaken. That is to say, when a wire bundle under test is bumped or pressed, this may affect the stray current level. Testing the wire bundle, however, especially in cramped quarters, may involve quite a bit of bumping or pressing. The manual setting and resetting of the voltage between high and low each time a possible leakage current is detected, and the attendant observation of the difference in reading, is quite laborious. It slows the testing process. Furthermore, it is quite hard to interpret the reading, given the frequent changes in the stray currents.  
         SUMMARY OF THE INVENTION  
         [0095]    An object of the invention is to simplify the interpretation of data obtained using an in-situ testing method such as, but not limited to, the system described in the related application. The simplification of the interpretation is achieved by providing a system that indicates stray current effects, and provides an indication of a measured current in which the effects of the stray current are compensated.  
           [0096]    The object of the invention is attained in the achievement in a method and apparatus for wire testing.  
           [0097]    The object of the invention is also attained in the provision of an apparatus for showing and compensating the effects of stray current, and a resulting system that avoids the need for highly experienced and knowledgeable technicians.  
           [0098]    To achieve the above and other objects, the method and apparatus for wire testing include a circuit that automatically switches the applied voltage between high and ground at a predetermined switching rate that permits a comparison between the current measured when the voltage is high and when the voltage is grounded. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES  
       [0099]    FIGS.  1 - 18 , which have already been mentioned above, are explanatory figures describing wire testing according to the DelTest system. FIGS.  19 - 21  show specific examples which help explain, in a non-limiting manner, the method and apparatus according to the invention. In particular:  
         [0100]    [0100]FIG. 1 is a sectional view of a cable.  
         [0101]    [0101]FIG. 2 is a perspective view of a cable connector.  
         [0102]    [0102]FIG. 3 shows a wire connecting two pieces of equipment.  
         [0103]    [0103]FIG. 4 shows the approximate location of these two pieces of equipment in an airplane.  
         [0104]    [0104]FIG. 5 shows an exploded view of the airplane, and shows the two pieces of equipment connected by a wire.  
         [0105]    [0105]FIG. 6 shows the two pieces of equipment and part of the wire in a simplified schematic representation.  
         [0106]    [0106]FIG. 7 shows a perspective view of the wires of a cable, with one of the wires having a degradation in its insulation.  
         [0107]    [0107]FIG. 8 shows a simplified schematic representation of two wires and a degradation in the insulation of one of the two wires.  
         [0108]    [0108]FIG. 9 shows a simplified schematic representation of the same two wires enclosed by a jacket.  
         [0109]    [0109]FIG. 10 shows a perspective view of the wires of the cable enclosed, in part, by a jacket.  
         [0110]    [0110]FIG. 11 shows a simplified schematic representation of how conductive medium is introduced into the jacket, and how it is evacuated.  
         [0111]    [0111]FIG. 12 shows a simplified schematic representation of how a wire is tested for degradation using the jacket, a test unit, and two test elements.  
         [0112]    [0112]FIG. 13 shows how a wire may be tested for degradation without use of a jacket, and with the effective control of the conductive medium.  
         [0113]    [0113]FIG. 14 shows how a wire may be tested for degradation without use of a jacket when the wire to be tested is in an awkward position.  
         [0114]    [0114]FIG. 15 shows a decision flow diagram relating to the selection of test circuits.  
         [0115]    [0115]FIG. 16 shows one test circuit, referred to as circuit no. 1.  
         [0116]    [0116]FIG. 17 shows another test circuit, referred to as circuit no. 2.  
         [0117]    [0117]FIG. 18 shows yet another test circuit, referred to as circuit no. 3.  
         [0118]    [0118]FIG. 19 shows an embodiment of the present invention, related to the circuit no. 2 of FIG. 17.  
         [0119]    [0119]FIG. 20 shows another embodiment of the present invention, related to the circuit no. 3 of FIG. 18.  
         [0120]    [0120]FIG. 21 shows a timing chart relating to an exemplary operation of the circuit of FIG. 19. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0121]    In FIG. 19, 810 indicates a function generator (also referred to as a control signal source) outputting a square wave signal  820 ;  650  indicates a switch box as already described above;  410  indicates a first test element as already described above;  420  indicates a second test element (also referred to herein as a collection electrode);  500  indicates a bundle of wires being tested;  540  indicates a rubber boot or the like, as described above;  640  indicates a voltage polarity switch;  721  indicates a circuit for detecting electrical conditions prior to testing bundle of wires  500  for leakage current;  731  indicates a circuit for detecting leakage current during the testing of bundle of wires  500 ; R11 and R12 are current limiting resistors.  
         [0122]    For linguistic convenience only,  721  may be referred to as a “first measurement circuit” and  731  may be referred to as a “second measurement circuit”. They may be thought of, as well, as a preliminary measurement circuit and a primary measurement circuit, respectively. Together with the ammeter, they may be thought of as a measurement circuit for rectifying detected readings.  
         [0123]    The test circuit of FIG. 19 is similar in several ways to the test circuit shown in FIG. 17 (i.e., test circuit no. 2), and similar parts are identically numbered. The description of their operation being already given, the differences between FIGS. 17 and 19 will be highlighted.  
         [0124]    First measurement circuit  721  has been substituted for the voltmeter  720 . It should also be noted that the second measurement circuit  731  has been added to the output of micro-ammeter  730 . The voltage source shows only 28 volts, but the source shown in FIG. 17 could equally well be used. The voltage polarity switch and voltage source may be thought of, in general, as a voltage supply circuit.  
         [0125]    Between the switch box  650  and the voltage polarity switch  640  there is a switch S1. First measurement circuit  721  includes a switch S2. Second measurement circuit  731  includes a switch S3. As shown in FIG. 19, signal  820  is provided by function generator  810  as a control signal for all of the switches S1, S2, and S3. In other words, all three of these switches are controlled in synchronism by the signal output from function generator  810 . This can be accomplished by any appropriate means. As shown in FIG. 19, all of the switches are in the position in which they are connected to the pole on the right hand side of the figure. When the state of output signal  820  changes from a low state to a high state, all three of the switches will change, at the same time, so that they are connected to the pole on the left-hand side of the figure.  
         [0126]    The three switches, together, may be thought of as a switching circuit for synchronously switching the voltage supply and the output measurement circuit. It will be appreciated that the switches operate under the control of the square wave signal (viz., control signal)  820 .  
         [0127]    First measurement circuit  721  is substantially similar to second measurement circuit  731 . The values of the resistors and capacitors in the two circuits do not need to be precisely identical in every way, but for the purposes of this discussion it will be assumed that they are. Given the similarities between circuits  721  and  731 , the operation of this embodiment will be described with detail with respect to circuit  721 , and it will be appreciated that the functionality is not different in circuit  731 . It will be appreciated that the input to the first measurement circuit  721  is a voltage measurement across R12 and the input to the second measurement circuit  731  is an output from the micro-ammeter  730 .  
         [0128]    The purpose of S1 is to switch the battery in and out of the circuit and thus switching between applying 28 V and 0 V. When the battery is out of the circuit any ground loop, which may be contributing to stray current, is maintained by the connection for S1 to R11. The result of this switching is that any signal current that is generated by current flowing through a breach will be modulated by S1. The stray current will not be modulated by S1.  
         [0129]    The purpose of circuits  721  and  731  is to correctly measure the generated currents and separate the currents resulting from the excitation voltage from the stray currents. The block  721  measures the current output of the battery. This has two purposes. The first is during an initial dry test before the conducting fluid is applied to the sample. This determines what loads are connected to the wires under test. This corresponds roughly to box S 00  of FIG. 15.  
         [0130]    The second purpose of this circuit is to monitor the current output of the battery during testing. It is possible during the course of testing that the conducting medium may make unexpected connections. For example, the backshell of a connector may be temporarily “contaminated” and shorting to a grounded pin may occur. This would make detection of a real breach difficult because, due to the presence of the current limiting resistors (R11 &amp; R12), the applied voltage could be drastically reduced.  
         [0131]    A second scenario is a leakage path from a breach to a ground plane (i.e. a metal clamp with damaged rubber boot) that may be of lower impedance than the path to the collection electrode (i.e., second test element  420 ). In these cases the current output of the battery will be relatively high but the current in the collection electrode will be low. The operator needs to be alerted to both of these cases so he can either clear any shorting or move the collection electrode to a better location to find the breach.  
         [0132]    As shown, the circuit in block  721  uses the voltage drop on resistor R12 to monitor the current output of the battery. This is convenient because the resistor already functions as a current limiter but, if desired for sensitivity, the output current of the battery could be measured by a micro-ammeter such as is used to measure the collection electrode current by putting it in series with R11 and sending its output to block  721 .  
         [0133]    Block  721  performs in the following way. When S1 switches to the left, putting the battery in the circuit, S2 simultaneously switches to the left side circuit. Any voltage that is developed across resistor R12 due to current output of the battery begins to charge C3 through R7 and also a ratio of the voltage on C3 is measured from B to A. When S1 and S2 switch back to the position on the right C3 will begin to discharge through R13 and R8. The RC time constant of C3 and R13+R8 is chosen to be long compared to the switching frequency (i.e., the frequency of the square wave  820  produced by function generator  810 ) so the voltage does not drop much before S1 and S2 return to the position on the left and C3 continues to charge if the current output remains high or discharge if the current output of the battery is lower. In this way the voltage measured across B to A will follow the current while the battery is in the circuit and will not be affected by the current when the battery is out of the circuit.  
         [0134]    In view thereof, it will be understood that R7, C3, R13, and R8 constitute an in circuit detector of the circuit  721  (i.e., it detects only when the voltage supply is in circuit).  
         [0135]    The voltage across C to D follows the current when the battery is not in the circuit. The circuit on the right works in the same manner as the left side circuit with C4 being charged through R9 when S1 &amp; S2 are in the right position and slowly discharging through R14 &amp; R10 when S1 &amp; S2 are in the left position. Hence the voltage across C to D is proportional to the current when the battery is out of the circuit and will not be affected by the current when the battery is in the circuit.  
         [0136]    Thus, R9, C4, R14, and R10 constitute an out of circuit detector of the circuit  721  (i.e., it detects only when the voltage supply is out of circuit).  
         [0137]    In practice, the voltage from B to A will be the main value of interest when evaluating the loads during the initial check. However, if the stray currents are high, the voltage from C to D may be needed to determine whether there are unexpected connections being made by the conducting fluid.  
         [0138]    The block  731  works in a way similar to block  721  with the output of the micro-ammeter being switched by S3 which is also synchronized with S1 &amp; S2. The in circuit detector of circuit  731  is constituted by R1, C1, R3, and R5; the out of circuit detector of circuit  731  is constituted by R2, C2, R4, and R6. Here the voltage difference F to E follows the collection electrode current measured with the battery in the circuit and the voltage difference G to H follows the current when the battery is out of the circuit. The difference between F and G is the main value of interest during testing with the conductive medium as it is the difference between the currents from a breach or leakage current and a stray current.  
         [0139]    That is to say, when the stray current (i.e., current detected with no voltage applied) is subtracted from the detected current (i.e., current detected with voltage applied), the difference represents the amount of current due to only a leakage current.  
         [0140]    One familiar with this field will recognize that resistors R3 and R4 are optional, and may be provided so as to insure that the voltage being measured across resistors R5 and R6, respectively, is not too high for the test equipment being used. Mutatis mutandis, the same is true for R13 and R14.  
         [0141]    In FIG. 19, 830 refers to a test readout unit. As part of test readout unit  830 , there is a dry test readout unit  840  and a wet test readout unit  850 . These names are for linguistic convenience only and are not meant to indicate that the testing performed and values read out must be dry or wet testing. In particular, if the conductive medium is a gas, the term “wet” hardly seems applicable. Nevertheless, the terms provide a convenient way to name the circuits.  
         [0142]    The dry test readout unit  840  includes three readings: a low value reading reflecting the voltage drop across R10 (i.e., CD); a high value reading reflecting the voltage drop across R8 (i.e., BA); and a difference value reading that reflects the difference between the two (i.e., BC).  
         [0143]    The wet test readout unit  850  includes three readings: a low value reading that reflects the voltage drop across R6 (i.e., GH); a high value reading that reflects the voltage drop across R5 (i.e., FE); and a difference value reading that reflects the difference between the two (i.e., FG).  
         [0144]    It is preferred that the readouts in the test readout unit  830  are digital readouts, but analog readouts may be used. It is possible that, for operator convenience, certain additional lights such as LEDs be added to indicate certain unusual conditions or to indicate a difference value exceeding a certain predetermined threshold.  
         [0145]    In the same manner that FIG. 19 is related to FIG. 17, so too is FIG. 20 related to FIG. 18. That is, the first test element  420  connects to switch S1 in FIG. 20, and the second test element  410  connects (indirectly, via switch box  650 ) to micro ammeter  730 . The operation of the circuit in FIG. 20 is thus basically the same as FIG. 19, except that it is used in the same situations as circuit no. 3 as described above (see also FIG. 15).  
         [0146]    [0146]FIG. 21 shows a simplified and exemplary timing and output diagram. It is assumed that the circuit of FIG. 19 is being used. The testing of a wire bundle is in progress, and the part of the wire bundle  500  under test is disposed in a conductive medium (i.e., has a conductive medium applied to it). Therefore, circuit  721  has already been previously used and the focus of the test is on circuit  731  at this point.  
         [0147]    A control signal  820  (“square wave”) is generated at 5 Hz. At time 0, the control signal goes from low to high. At the same time, switches S1, S2, and S3 move from the right hand pole to the left, thus switching the voltage supply into the circuit.  
         [0148]    In FIG. 21, 900 indicates an instantaneous output of the micro-ammeter;  910  indicates the voltage measured across FE (i.e., the high current); and  920  indicates the voltage measured across GH (i.e., the low current). To put it another way,  900  shows the instantaneous current,  910  shows the measurement of the in circuit detector, and  920  shows the measurement of the out of circuit detector. For simplicity, the difference between high current  910  and low current  920  is not illustrated in FIG. 21.  
         [0149]    The square wave frequency (frequency of signal  820 ) is chosen based on practical limitations. If the frequency is too low the response of the equipment will be slow and the rate at which the DelTest is performed will be reduced. If the frequency is too high, the response time of the test equipment and the time constant of the breach itself will cause inaccuracies in the measurement. Both of these aspects are discussed below.  
         [0150]    The limit on the low side of the frequency is a practical effect of the speed at which the testing is performed. The purpose of the RC filter component of circuit  731  for both the high and low circuits is to provide a stable output that can easily be read from a test readout unit. This requires that the time constant for these circuits be at about 5 times the period of the square wave. If the time constant becomes comparable to the period of the square wave, the high side read will follow the square wave, oscillating between the correct high value and zero. This would be confusing to the operator and difficult to read. Therefore the frequency of the square wave should, as a practical matter, be at least 5 times the rate at which the operator may read off data from the test readout unit.  
         [0151]    For practical reasons the frequency at which data should be collected is 1 second or less. This time is directly related to the speed that the operator can move along the harness under test. The location of a breach is determined by where the operator is applying the conducting fluid when a positive signal is seen. If there is too much delay between application of the fluid and receipt of the signal, the operator will have to go back and search for the location of the signal. This is time consuming and frustrating. While slowing down rate the operator proceeds along the harness may mitigate this problem, it does not eliminate it because the speed the conducting fluid spreads through the bundle is not always controllable by the operator. In any case, slowing down the process is not a desirable solution to this issue. Thus, the readout rate should be at least 1 Hz.  
         [0152]    Considering a 1 Hz minimum practical readout rate, the square wave frequency should be at least five times that, giving a practical lower limit on the square wave frequency of 5 Hz. Clearly, a higher rate is desirable.  
         [0153]    Consideration must be given to both the response time of the measuring device and, more fundamentally, the time constant of the breach resistance harness capacitance.  
         [0154]    In both of these cases, the effect of increasing the square wave frequency too much is that the signal for the high side of the measurement will “bleed” into the low side measurement and vice versa. This will cause the high side measurement to read lower than it should and the low side measurement to read higher than it should. The prevent this effect from distorting the data, the frequency of response for the current measuring device and the breach/harness should be at least 5 times that of the square wave.  
         [0155]    A Keithley electrometer is a concrete example of a micro-ammeter that may be used in accordance of the invention. The Keithley can generally be used with a square wave frequency of about 10 Hz. There are other current measuring instruments with faster response time that would be suitable for this application and may improve operation by allowing a high square wave frequency.  
         [0156]    Even with a faster current measuring device, however, there is yet an upper limit on the frequency of the square wave. This is due to the fact that, for the signal to reach the collection electrode, it must first charge up any capacitance formed by the conducting fluid and harness between the breech and the conducting electrode. This charging is done by current passing through the breach itself. A reasonable approximation for the resistance of a small breach has been found to be 1 MΩ (10 6  Ω). The capacitance of the conducting fluid harness combination is 200 picofarads (i.e., 2×10 −10  F). This gives a time constant of 2×10 −4  seconds, corresponding to 5 kHz. Taking the factor of 5 to allow the RC circuit to charge or decay, the effective upper limit is about 1 kHz for the square wave frequency. Operating near (but below) this frequency would allow the operator to see an almost instantaneous response with the high and low currents separated, which would improve ease of use and accuracy.  
         [0157]    The various embodiments above are provided in terms of concrete circuits with many specificities that can be varied. The scope of the invention is not to be limited to these exemplary embodiments but, rather, should be understood in the light of the appended claims.