A gauge carrier comprises a main body capable of incorporation into a suitable drill string, and a separate instrument housing including a gauge, the instrument housing being securable to the main body. The main body includes a handling region suitable for being handled by rig tongs, such that the instrument housing may be attached substantially along that handling region subsequently to its being handled. The handling region is located substantially centrally along the length of the main body. The main body includes at least one protective region having a larger cross section than the majority of the main body.

FIELD OF THE INVENTION 
     This invention relates to carrier assemblies, that is, assemblies for carrying instrumentation down boreholes and the like. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawing: 
     FIG. 1 is a prior-art pressure-gauge carrier in perspective view with a cutaway portion; 
     FIG. 2 is a longitudinal section of a preferred embodiment of a pressure-gauge carrier; 
     FIG. 3 is the instrument housing and narrowed region of the pressure-gauge carrier in cross section; 
     FIG. 4 is a perspective view of the pressure-gauge carrier; and 
     FIG. 5 is a partly broken-away view taken in the direction of arrow V of the instrument housing. 
    
    
     BACKGROUND OF THE INVENTION 
     Pressure-gauge carriers are often incorporated into drill strings so that the pressure within the drill string may be monitored. A conventional pressure-gauge carrier is generally tubular, having a bore similar to that of the drill string. 
     A known way of making such a pressure-gauge carrier is to machine the shape from a solid bar of metal. Such a gauge is shown in FIG.  1 . An inner bore  11  corresponding to the drill string bore is drilled axially through it. 
     The bar is progressively machined at each end region to produce a central saddle  10  (that is, central when considering the pressure-gauge carrier lengthways), the outer boundary of the cross section of the saddle being defined by a lower curve and an upper curve, joined by two flat sides opposite and parallel to each other. 
     The two curves have a radius of curvature limited by the well casing. The axial bore  11  runs close to the lower curve, so that the thickness between the bore  11  and the lower curved surface corresponds approximately to the wall thickness of the drill string. 
     At either end of this central saddle  10 , the pressure-gauge carrier extends axially as tubules  18  and  20 , the end regions of the central saddle  10  being chamfered down to meet the diameter of the tubes  18  and  20  which have the same thickness as the thickness between the bore  11  and the lower curved surface of the central saddle  10 . At each end of the pressure-gauge carrier, the outer surface of the device is threaded to fit with a drill pipe. 
     An axial groove  12  is machined from the upper curve of the saddle  10 , the depth of the groove  12 , considered in cross section, descending radially towards the center of the axial bore  11 . 
     The pressure gauge  14 , comprising a pressure-gauge sensor and its associated electronics, is arranged in a generally linear fashion. The pressure gauge is encased in a sheath to protect the components from the hostile borehole environment. The electronics thus sheathed are laid in the groove  12  in the saddle  10 , the length of the groove  12  being such that the electronics run its full length. The pressure gauge sensor is situated at the end of the line of electronics (that is, the linear arrangement of circuitry associated with the pressure-gauge sensor), at the closed end of the groove  12 . A channel  16  radially communicates between the axial bore  11  and the groove  12  at the closed end. 
     At the one end of the groove  12  the electronics and sheath terminates in a connector which is attached to a cable  13  that supplies the pressure gauge  14  with any power or commands it needs, and allows the transmission of data back to the surface. Somewhat short of this end of the groove  12 , a plate  15  is clamped over the groove  12  to secure the pressure gauge  14 . The central saddle  10  may be formed with a recess to accommodate the plate flush with its surface. 
     To incorporate the pressure-gauge carrier into the drill string, rig tongs grip the tubes  18  and  20  of the pressure-gauge carrier, and then carry the pressure-gauge carrier to the drill string, and introduce one end of the pressure-gauge carrier into the open end of the drill pipe at the top end of the drill string. The pressure-gauge carrier is rotated by the rig tongs so that the pressure-gauge carrier and the drill pipe are securely joined by their respective threads. The rig tongs are designed to grip a standard circumference (corresponding to the diameter of the drill pipe sections) with the required force. 
     The tubes  18  and  20  of the pressure gauge provide regions of standard circumference on the pressure gauge which can withstand the grip of the rig tongs. 
     The pressure-gauge carrier is an expensive piece of equipment. It involves a lot of machining from the original solid block of metal. 
     OBJECTS OF THE INVENTION 
     It is an object of the present invention to provide a pressure-gauge carrier which is economic to manufacture, which has good mechanical properties, and which is convenient to install. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention there is provided a gauge carrier comprising a main body capable of incorporation into a suitable drill string, and a separate instrument housing including a gauge, the instrument housing being securable to the main body, and the main body including a handling region suitable for being handled by rig tongs and the like. The instrument housing is attached substantially along that handling region subsequently to its being handled, that is after the body is mounted on the drill string. 
     Preferably the handling region is located substantially centrally along the length of the main body. In addition the main body includes at least one protective region having a larger cross section than the majority of the main body, and preferably two flanking the handling region. Preferably the cross section of the protective region or regions correspond to the thickness of the instrument housing. 
     According to another aspect of the present invention, there is provided a method of incorporating a gauge carrier as herein defined into a drill string comprising the steps of: 
     joining an end of the main body to a drill pipe section, and 
     thereafter attaching the instrument housing to the main body. 
     According to a further aspect of the present invention, there is provided a gauge carrier capable of incorporation into a suitable drill string including a gauge whose components are generally arranged in series, the gauge being housed in at least two substantially parallel chambers, and the gauge including a 180° bend in its arrangement. Preferably the chambers are axial bores. 
     According to a still further aspect of the present invention, there is provided a main body, an instrument housing, or a gauge as herein defined. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 2 to  4 , the pressure-gauge carrier embodying the present invention comprises a main body  60  including a narrowed handling region  45 , an instrument housing  30 , and a pressure gauge  40 . 
     Considered in cross section, the main body  60  is generally tubular, having a circular bore  22 , and an generally cylindrical outer surface which is of generally similar diameter to the production tubing P with which it is intended to be joined. Internal screwthreads  24  and  25  are provided on the inner surfaces of the bore  22  corresponding to the thread on the male ends of the drill pipes  6  with which the carrier is to be attached. 
     Somewhat in from each end, the main body  60  widens to form two protective shoulders  34  and  35 . At the shoulders' greatest extent, the cross section comprises the lower curve of the main body  60 , two parallel sides, and an upper curve, which may, as shown here, have a greater radius of curvature than the curvature of the outer diameter of the tubular parts of the main body  60 . The shoulders  34  and  35  have a chamfered flat surfaces  37  and  38  extending down to its respective end of the main body  60 . The shoulders  34  and  35  then extend a short distance along the length of the main body  60 , before ending with respective flat surfaces  42  and  43  perpendicular to the axis A of the main body  60 . 
     The outer diameter of some or all the region between the faces  42  and  43  of the shoulders  34  and  35  may narrow somewhat as shown in the drawing. The diameter over this narrowed region  45  is smaller than that of the rest of the main body  60 , so that the curvature of at this region  45  is more similar to the outer curvature of the drill pipe P. In this way, the region  45  is better adapted to being gripped by rig tongs as described below in more detail. 
     To aid introduction of the drill pipe ends into the main body  60 , one or both ends of the main body  60  may extend outward in the form of tubular portions  47  and  48  whose wall thickness tapers somewhat, as shown in the drawing. As the tubular portions  47  and  48  are not necessary for the handling of the main body  60  by rig tongs, they need not be so long as in the case of the prior-art pressure-gauge carrier. Such tubular portions  47  and  48  could even be externally threaded to be accepted in a male fashion by female drill pipe sections, or indeed the main body  60  could have one male connecting end and one female connecting end. 
     An instrument housing  30  sits on a surface  32  of the main body  60  between the two shoulders  34  and  35 . Referring to FIG. 3, the instrument housing  30  has a uniform cross section having a curved base face  52  which rest upon the outer surface  32  of the narrowed region  45  of the main body  60 , two flat parallel sides  54  and  55 , and an upper curved surface  50 . The radius of curvature of this upper surface  50  is similar to that of the upper curved surfaces of the shoulder  34  and  35 . When placed on top of the main body  60  between the shoulders  34  and  35 , the instrument housing's upper curved surface  50  and two sides  54  and  55  lie inside or at least flush with the shoulders' upper curved surfaces and sides. The instrument housing  30  is therefore hidden behind the shoulders  34  and  35  when the carrier is viewed end on, so that the shoulders  34  and  35  protect the instrument housing  30  as the drill string advances through the borehole. 
     The instrument housing  30  also includes two axial bores  70  and  72  running side by side in the instrument housing  30 , each an equal distance from the top surface  32  of the main body  60 . These bores  70  and  72  are drilled out along the entire length of the instrument housing  30 , and at one end of the instrument housing  30 , the wall between the bores  70  and  72  is removed so that they communicate. 
     The instrument housing  30  is fitted in the region between the shoulders  34  and  35  and fastened to the main body  60 , for example by four screws  94 ,  95 ,  96 , and  97  at each corner  90 ,  91 ,  92 , and  93 . The instrument housing  30  is somewhat shorter than the distance between the opposing faces  42  and  43  of the shoulders  34  and  35 , so that there is enough room to fit a cable connector to a row  41  of electronic components including the pressure sensor  40 . Fitting the row  41  of electronic components in a linear arrangement is very convenient, first as circular bores are easier to form than other shapes, and second because the electronic components of various instruments are usually supplied in a standard linear form to enable interchangeability between instruments and their carriers. 
     The pressure gauge's row  41  of electronic components is threaded through the first bore  70 , turned back on itself and then threaded through the second bore  72 , so that the row  41  of electronic components doubles back at the communicating region of the instrument holder where a wall between the bores  70  and  72  has been removed as shown in FIG.  5 . At this end (hereinafter termed the switch-back end) of the instrument holder  30 , the axial bores  70  and  72  are sealed so that they are no longer open to the outside. 
     At one end of the row  41  of electronic components, (at the end of the instrument housing  30  opposite the switch-back end and hereinafter termed the connector end) a connector (not here visible) is detachably attached. It is connected with a cable  77  that supplies any necessary power to the pressure gauge  40 , and carries the data that the gauge collects back to the surface. The socket which accepts the connector fits firmly in one of the axial bores  70  and  72  at the connector end of the instrument housing  30 , sealing the aperture. The shoulder  34  at the connector end includes a hole  80  to allow the cable  77  to pass through. 
     The row of electronic components also ends with a plug  78  at the connector end of the instrument housing  30 , extending somewhat through the other aperture. This plug  78  seals the aperture, so that the chamber containing the electronic components is completely sealed off from the outside environment. A small channel  82  connects the main body's bore, via the plug  78 , to the pressure gauge sensor  40 . 
     In order to install the pressure-gauge carrier, the main body  60  (without the instrument housing  30  attached) is picked up by rig tongs, which grip the main body  60  at the narrow region  45  between the two shoulders  34  and  35 , and place the main body  60  upon a drill pipe P so that a free end of the drill pipe  10  is inserted into the bore of the main body  60 . The rig tongs then rotate the main body  60  so that it is securely joined to the drill pipe P. Once securely joined, the rig tongs are removed from the main body  60 . 
     The instrument housing  30 , with the pressure gauge  40  installed, is now attached to the main body  60 . The cable  77  is threaded through the hole  80  in the uppermost shoulder  34 , and joined by its connector to the pressure gauge  40 . 
     The next drill pipe section P is introduced into the free end of the pressure-gauge carrier body  60  and tightened in the conventional manner using rig tongs. If desired the fitting of the instrument housing  30  may be postponed until subsequent drill pipe has been fitted. 
     Since the instrument housing  30  and pressure gauge  40  are only fitted after the main body  60  is attached to the pipes P of the drill string, the danger of the pressure gauge  40  being damaged is greatly reduced. 
     The main body  60  may also be handled by the region  45  between the shoulders  34  and  35  prior to being added to the drill string, for example in the workshop, and when being brought to the rig. 
     The pressure-gauge carrier body  60  may be a much shorter length, and therefore require far less machining, than a conventional pressure-gauge carrier. There are several reasons for this. Since the pressure gauge  40  is not fitted to the main body  60  until after the main body  60  has been installed in the drill string, the rig tongs may grip the main body  60  at any point between the shoulders  34  and  35 . The tubular portions extending from the prior art pressure-gauge carrier, by which the pressure-gauge carrier is carried, are thus not necessary. For this reason the main body  60  ideally fits in a female fashion to introduced drill pipe sections. 
     A length saving also results from turning the row  41  of electronic components back upon itself. The instrument housing  30  is about half the length of the electronic components disposed along the groove of the prior-art pressure-gauge carrier. 
     The instrument housing  30  is stronger than known instrument housings of the same diameter, as the double bore ensures a greater wall thickness between the bores  70  and  72  and the upper curve  50  than would be the case for a single bore. 
     These savings in length allow a smaller block of metal to be used to form the pressure-gauge carrier body  60 , and consequently much less machining is required. This represents a considerable cost saving over machining a longer pressure-gauge carrier. 
     The pressure gauge  40  itself is much less likely to be damaged, as it is fitted only after the main body  60  has been incorporated into the drill string. The pressure gauge  40  is also protected to a greater extent by the greater structural strength of the instrument housing  30  over the corresponding portion of the prior-art pressure-gauge carrier. By sacrificing some of this additional structural strength, the pressure-gauge carrier body  60  could be made to a smaller outer diameter. 
     Specific features disclosed herein could be combined with other features of prior-art gauge carriers in many permutations. For example, the parallel bores  70  and  72  of the instrument housing  30  could be bored directly into an integral pressure-gauge carrier of the prior-art type. Rather than bores, the gauge  40  could be housed in two parallel channels machined out of the pressure-gauge carrier body  60 . Those skilled in the art will realize that the principles disclosed herein could be applied to any similar instrument carriers with the necessary adaptations, and it is intended that such alternatives are included within the scope of the invention, the scope of the invention being limited only by the following claims.