Patent ID: 12196642

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The drawings are schematic and not to scale. Several details known to the skilled person are omitted from the drawings for clarity of illustration.

The present invention concerns a gas leakage meter for use onsite in a process plant.FIG.12shows how a leakage meter100is connected to a unit to be tested3that is part of a process plant. When closed, the upstream isolation valve2isolates the unit to be tested3from the process plant without demounting it. A pressure source1220is connected to an upstream connector1020. The pressure source1220contains test gas and its pressure can be regulated. The leakage meter100is connected to the downstream connector1040.FIG.1shows the leakage meter100with an inlet101, an outlet102and a housing110. The inlet101is connected to the downstream connector1040. For illustration, the inlet101and outlet102have external threads for connection to external tubing (not shown). External threads is one of several standard connecting means. The outlet102does not need connection means to release test gas to the atmosphere.

The housing110has an inlet nozzle111in the lower left ofFIG.1. The inlet nozzle111has an orifice that is sufficiently wide to avoid significant drops in temperature and pressure during operation. A liquid level10illustrates that the housing110contains liquid during operation. Suitable liquids include water, some alcohols and mixtures thereof. For example, glycol added to water lowers the freezing point from 0° C. We describe heating as an alternative with reference toFIG.5. A gas volume over the liquid level10contains test gas, e.g. N2or air, with pressure p and temperature T at ambient atmospheric conditions.

The housing110contains brackets115supporting an inclined pipe120with a first, lower end121at the left of the drawing, and a second, upper end at the right. A small circle under the first end121and another small circle within the inclined pipe120at a bubble counter140illustrate gas bubbles entering and rising gently through the inclined pipe120during a test period.

Specifically, the inner walls of the inclined pipe120enhances coalescence of small gas bubbles into bubbles with diameter approximately equal to the inner pipe diameter. The inclination is a compromise between coalescence, which benefits from longer time and smaller inclinations, and adhesion of gas to the pipe walls, which may become a problem if the inclination is too small. In the drawings, the inclination of pipe120is 10° relative to the liquid level10. In real embodiments, the inclination of pipe120depends on the viscosity of the liquid in the housing110, and may be significantly different from the example 10°.

The upper end of the inclined pipe120is connected to a gas-collecting chamber122, hereinafter the chamber122for short. In use, a leak causes a volume V(t) of test gas with an overpressure Δp over ambient pressure p to collect in the chamber122.FIG.1shows an exaggerated gas volume V for illustration.

The shape of chamber122is arbitrary. However, the inclined pipe120is preferably connected to the bottom of the chamber122in order to minimize or avoid a dead volume of liquid without function. In use, the chamber122is fully submersed in all embodiments of the leakage meter such that liquid may replace test gas in the chamber122whenever needed.

A manometer comprising a U-shaped water trap123and a vertical pipe124is connected to the chamber122. An open, upper end of the vertical pipe124is exposed to gas pressure p over the liquid level10. Note that the liquid level in the vertical pipe124varies over a much shorter distance h(t) than the heights 53.5 cm and 13.4 cm from examples in the introduction.

The bubble counter140counts bubbles in the inclined pipe122as they pass. We consider bubbles/min at STP as a unit of measure equivalent to units of ml/min, ml/s, etc. In particular, the actual bubbles may be adapted to a desired resolution and the bubble count converted to units of ‘standard bubbles’/min, ml/min or some other unit of choice.

For a numerical example, a bubble with volume 1/16 ml=62.5 μl corresponds to a sphere with diameter 4.92 mm. This might suggest an inner diameter 4.92 mm of the inclined pipe120. However, a desired additional decimal of precision and available standard sizes for pipes might suggest an inner diameter 2.0 mm, which may contain a spherical bubble 4.2 μl. Corrections due to gas pressure p and gas temperature T will be described later.

A level indicator141measures a difference level Δh(ti)=h(ti)−h(tj) at discrete times ti and tj during each test period. The measurements are not necessarily performed at regular intervals iΔt. The first measurement h(tS) is at is when the test period starts. Subsequent measurements are relative to h(tS) rather than to the liquid level10. Thus, there is no need for an accurate liquid level10. The liquid just needs to cover the chamber122. In particular, the actual liquid level10may deviate at least a few mm from a nominal filling level without affecting measurements significantly.

An optical version of the level indicator141may comprise a light source such as a LED laser, a light detector, a collimating and/or magnifying lens142and a binary ruler143. For use with optics, the binary ruler143may be a sheet of cardboard, plastic or metal with a printed or cut out pattern to be described with reference toFIG.4. Ambient conditions do not significantly affect the binary ruler143.

Usually, ambient conditions do not affect the function of light sources, detectors or electronic circuits in optical versions of the bubble counter140or the level indicator141. In comparison, vapor may condensate on a lens such as the lens142, and reduce visibility and functionality if the lens is colder than the liquid within the housing110. For example, outdoor temperatures at or below 0° C. occur naturally in large parts of the world, and pure water in the housing110might be heated under such conditions. To avoid problems with condense, it is possible to keep the lens142at a temperature close to the liquid temperature, or use some principle other than optics in the bubble counter140or level indicator141. Low power commercial devices using electric conductivity, resistance or capacitance may be viable alternatives even in Ex-areas.

Smaller bubbles, e.g. 2 mm bubbles, increases resolution and reduces the risk for false positives caused by counting undersized bubbles. For example, a 2 mm bubble with volume 4.2 μl provide better resolution than a bubble with 1/16 ml=62.5 μl. N bubbles less than 2 mm cause an error much less than N·4.2 μl. For comparison, N bubbles less than 1/16 ml, each assumed to be 62.5 μl, cause larger deviations. Opposite, 2 mm bubbles increases the risk for bubble trains, for example a ‘bubble’ longer than 2 mm in an inclined pipe120with inner diameter 2.0 mm. A bubble train with ten 2 mm bubbles corresponds roughly to 20 mm liquid column in a pipe124with 2.0 mm inner diameter.

Before each test period, the gas release valve150at the top of chamber122may open fully to ensure that liquid completely replaces gas in the chamber122. During each test period, the gas release valve150permits single bubbles to escape from the chamber122, but delays a bubble train or burst of test gas long enough to estimate the amount of gas in the bubble train by means of the manometer123-124and the level indicator141-143.

It is possible to achieve this functionality by means of a simple control loop with an electronic pressure sensor and a solenoid valve. However, electronic pressure sensors have disadvantages discussed in the introduction.FIG.1illustrates an embodiment of the gas release valve150based on a buoyancy that disappears if or when the gas volume Vin chamber122becomes too large.FIG.3is a detailed view of the gas release valve150.

If the devices140and/or141are optical devices, at least part of the pipes120or124must be transparent. There are no large loads on the internal components120-124, so a transparent thermoplastic, for example PET or acrylic glass, may be a suitable material for one or more of these components. Further beneficial properties of thermoplastic polymers, e.g. mechanical and chemical properties as well as the recyclability of PET are available online and in literature.

An optional flow divider130is essentially a valve able to divide a flow or a pressure into a large and a small part. A first purpose is to prevent inadvertent expulsion of liquid from the inclined pipe120and/or the housing110. For this, the divider130initially leads the entire flow from the inlet101through a bypass pipe132. Before the test period starts, part or all of the flow from the inlet101is diverted to the inlet pipe131by means of a flow actuator133, in the drawings illustrated by a lever arm with a ball on a distal end. Carefully opening for flow through the inlet pipe131prevent the undesired expulsion of liquid. As mentioned in the introduction, the upstream pressure valve in Wilson's test device may be opened slowly and carefully for the same purpose.

A second purpose of the flow divider130is to estimate leaks larger than the maximum leaks required for approval or certification. Assume, for example, maximum allowed leaks as in previous examples and a flow significantly larger than 3.5 ml/min through the inlet101. We want to estimate the larger leak even if the chamber122is designed for leaks less than or equal to 3.5 ml/min. The flow divider130divides the incoming flow into an inlet flow through the inlet pipe131and a bypass flow through the bypass pipe132. The actuator133controls the ratio of inlet flow to bypass flow, preferably in predetermined steps. This will be further explained with reference toFIGS.9-11.

Before we continue with the description of the drawings, we introduce a few more definitions and make some general remarks.

We have already defined tSas the start time of a test period, and now introduce tEas the end time. The duration of a test period is Ttest=tE−tS. Formally, a parameter such as p(t), h(t) or T(t) during a test period might be denoted (·)(t−tS). We use the convention that t is reset to zero at the start of each test period such that (·)(t−tS)=(·)(t) for 0<t<Ttest.

All embodiments of the proposed leakage meter have a ‘pass or fail’ mode, in which ‘fail’ means that the amount of test gas collected during the test period Ttestexceeds a limit volume Vlimit. For example, Ttest=3 minutes times 3.50 ml/min give Vlimit=10.5 ml, and the unit to be tested fails the pressure test if V(3 min)>10.5 ml in this example.

Preferred embodiments perform additional measurements during each test period.

Ideal gas laws adequately describe conditions in real gases at pressures and temperatures occurring in gas pressure tests. A common form is, in SI units:
pV=nRT(1)
where p is pressure in Pa, Vis gas volume in m3, n is the number of mols, R=8.314 J/(mol·K) is the universal gas constant, and T is the temperature in K.

From (1), it follows immediately that the number of mols
n=pV/RT(2)

It may be convenient to use n from equation (2) in external systems because it facilitates comparison between measurements taken at different locations and/or at different times. In particular, n for a particular measurement ‘includes’ the gas ratio pV/T.

In some numerical examples below, we use resolutions 1 Pa and 1 K. Resolutions in units of hPa (mbar) and 1 K (° C.) are widely used in practice. Inexpensive barometers and thermometers with these resolutions are available for manual or automatic measurements. However, the skilled person knowing the problem at hand must determine the precision and resolution for p, V and T for use in real embodiments.

For illustration of equation (2), 4.5 ml gas at 1 atm=101325 Pa and 20° C.=293 K contains n=101325·4.5·10−6/(8.314·293) mol=187.18 μmol. Further, the ideal gas is a good approximation to real gases under all conditions of interest, so n is independent of test gas.

Storing and using n for comparisons does not exclude storing and using other parameters. For example, ambient temperature and/or icing may be a suspected cause of deviation for a particular type of valve to be tested. Accordingly, ‘(ambient) temperature’ may appear in an ‘Environment’ branch in an Ishikava diagram for the valve to be tested, and be mandatory in a test schema for this reason and/or because T is needed to adjust gas volumes.

A standard volume V0at STP can be computed as V0=nRT0/p0. However, ‘standard’ temperature and pressure depend on local definitions. Varieties include T0=273 K (0° C.) or 293 K (20° C.), and p0=1 bar=1000 hPa (105Pa) or 1 atm=1013.25 hPa=760.00 mmHg.

It is necessary to convert gas volumes V measured at p and T into V0or Vlimitat STP. For this, we rewrite equation (1) as:
V0=(p/T)(T0/p0)V(3)

To illustrate scale of volume corrections, we assume ambient pressure p in the range 920-1050 hPa, which is within recorded atmospheric extremes at 887 hPa and 1085 hPa. Further, we assume T in the range 0-50° C., which also may occur naturally, e.g. if equipment is exposed to sunshine for an extended period. Further, some pressure test standards specify specific temperatures or temperature ranges, e.g. 38° C. (100° F.) or 5-50° C.

In this example, we use T0/p0=293/1013.25 K/hPa. Using limits from the previous paragraph, p/T ranges from 920/323 to 1050/273 hPa/K. From (3), it follows that V0or Vlimitvaries from 0.82 V to 1.11V in this example. Regardless of whether these ranges and numbers are representative, they illustrate that corrections of measured volume V to ‘standard’ volume V0may approach 10-20%, and thus that volume corrections according to equation (3) are significant and necessary.

Next, we consider the vertical position of the bubble counter140. A bubble at the bubble counter has gas pressure p+Δp, where
Δp=ρgh(4)

Similar to parameters shown inFIG.1, Δp is an overpressure over ambient pressure p and h is a height relative to the liquid level10. As usual, p is liquid density and g≈9.8 m/s2. In this example, the bubble counter140is a fixed distance from the liquid level10, so the relative difference Δh between measurements at different times is not relevant.

The ratio V/V0=(p0/T0)T/(p+ρgh) is independent of bubble size, and thus a useful measure for the effect of counting bubbles at different depths h. Table 1 shows results with density ρ(T) for pure water at h=0, 10, 20 and 30 mm.

TABLE 1Effects of ambient p and T compared to depthof measurement within housing 110.h/mm01020304° C., 1050 hPa0.9790.9780.9770.9764° C., 920 hPa1.1171.1161.1151.11450° C., 1050 hPa1.1421.1411.1401.13950° C., 920 hPa1.3031.3021.3001.299

The first row in Table 1 contains values of h in mm, and the first column contains values for temperature and pressure in each row. Each cell in the rest of Table 1 contains values of V/V0=(p0/T0)T/(p+ρgh) to 3 decimals We have used p0/T0=101325/273 Pa/K, ρ=1000.00 kg/m3at 4° C. and ρ=988.05 kg/m3at 50° C. The acceleration of gravity g=9.81 m/s2in this example.

Starting with the column for h=0, it appears that V/V0increases from 0.979 to 1.303 with increasing T and decreasing p. Following each row, it appears that V/V0decreases with approximately 1/1000 per cm added depth in pure water. In practice, this means that a deviation plus/minus a few mm from a nominal liquid level over the bubble counter140has little effect on bubble sizes and fail criteria. A similar result applies to overpressure Δp in the chamber122. As before, values of p and T do affect gas volumes significantly.

Bubble sizes at the bubble counter140are expected to vary about a mean value, e.g. measured volume V=4.2 μl from a previous example. Adding bubble volumes reduces uncertainty because deviations from the mean in both directions tend to cancel each other. Sample mean and (signed) sample variance from basic statistics provide estimates for mean and variance if one desires concrete numbers. In such methods, ‘outliers’ such as bubble trains containing several bubbles may be replaced by a maximum bubble size, e.g. corresponding to 5 or 10 bubbles. The latter requires a bubble counter able to estimate bubble sizes to a certain degree, not necessarily accurately. Techniques using a camera may achieve this.

For a slightly more theoretical example, consider two abstract sensors. The first sensor corresponds to the bubble counter140, and adds 1 to a bubble count whenever it detects a bubble, regardless of bubble size. The second abstract sensor provides the gas volume in a bubble train whenever a bubble train arrives. The second sensor corresponds to the arrangement with gas release valve150, manometer123-124and level indicator141.

The Kalman filter (KF) mentioned in the introduction is a mathematical model outside the scope of the present invention. However, for the later description it is useful to know that a KF can provide accurate estimates from quite coarse measurements. In essence, whenever a measurement arrives, the KF multiplies the measurement with a weight K and a predicted value with a weight (1−K) to obtain a weighted sum. In a formal KF, the Kalman gain K is a matrix. Here and in a modular KF, the first abstract sensor has a scalar Kalman gain K1and the second abstract sensor a Kalman gain K2. The gains K1and K2change over time and quantify the confidence to put on measurements relative to predictions.

Incidentally, Poisson processes may conveniently model the first and second abstract sensors for a KF or SPC. We refer the interested reader to Kelly (2006) for a comprehensive description of a modular KF. Kelly (2006) also contains a collection of statistical formulas and derivations that may be used or useful in an external system

FIG.2shows a first end121of the inclined pipe120seen from below. During operation, a wide shallow spoon shape ensures that test gas is collected and fed into the inclined pipe120. Studs125enhance coalescence of tiny amounts of test gas into bubbles. The studs125may be omitted and/or replaced by other means known to promote coalescence.

FIG.3is a detailed view of the gas release valve150shown inFIG.1, and we describeFIG.3in the context ofFIG.1during operation.

Before each test period, liquid enters a pilot chamber151through a permanently open refill opening152. A liquid release orifice153in the bottom of the pilot chamber151provides a delay in a state described below. Before each test period, liquid enters the chamber122partly through the liquid release orifice153, but mainly through a filling pipe154leading to a first seat155. InFIGS.1and3, the first seat155has the shape of a truncated cone or funnel. Liquid entering through the filling pipe154provides buoyancy for the float156, which lifts a valve element157from a second seat158enabling gas in the top of chamber122to escape. This is the normal situation when bubbles arrive one by one.

If a burst of test gas arrives, the buoyancy from the float156becomes less than a threshold value. A level h1illustrate the minimum amount of liquid in the chamber122providing the buoyancy needed to keep the valve150open. If or when a bubble train or burst of test gas suddenly increases the gas volume, the buoyancy disappears such that the spherical float156falls into the cone or funnel, engages the conical wall and thereby temporarily prevents test gas from escaping through the filling pipe154. At the same time, the valve element157drops a short distance onto the second seat158to prevent test gas from escaping through the top. The shapes of the elements156and157and their respective seats155and158may be altered without inventive effort.

When the liquid level in chamber122is below h1, i.e. when the buoyancy is too small to keep the gas release valve150open, liquid slowly enters the gas volume in chamber122through the orifice153. Liquid entering through the refill opening152replaces liquid leaving through the liquid release orifice153.

The liquid release orifice153causes a delay that depends on several factors, e.g. orifice geometry, viscosity of the liquid and a depth h2from the liquid level10to the orifice. A practical way to determine a suitable delay is to test different orifice diameters d for a given liquid at an approximate depth h2. There is no need for an accurate delay. The delay just has to be long enough to perform the necessary measurements of Δh in pipe125, and short enough to allow gas from the bubble train to escape before the next (few) bubble(s) arrive. Recall that that some known external systems, e.g. a KF, may compensate for measurement errors. In the present context, the gas volume in chamber122is proportional to the difference Δh between liquid levels in the vertical pipe125before and after the bubble train arrived.

From the description in the past few paragraphs, it follows that a main purpose of the pilot chamber151is to provide walls for the liquid release orifice153and the pipe154. Thus, the pilot chamber151may have any shape, and the refill opening152may be an open top of a cylinder or simply be a hole in the wall of chamber122.

The bottom of the pilot chamber151is preferably slightly conical with apex up to avoid a gas trap under the pilot chamber151. Dashed lines extending downward from either side of the orifice153illustrate such a conical bottom of the pilot chamber151.

Further, since the buoyancy from float156must carry the weight mg of the valve element157, the mass m of element157is preferably small. Since the valve element157is submersed during operation, it should have a density slightly greater than the liquid density. For example, acrylic glass has density 1.16-1.18 g/cm3, which is slightly greater than the density of water with or without additives around 1.0 g/cm3. It follows that reducing the mass of the valve element157amounts to reducing its size. Since small spheres are cheaper to make and less sensitive to orientation than small, truncated cones, a sphere may obviously replace the frustoconical valve element157shown inFIG.3.

InFIG.3, a stiff rod transfers lifting force from the float156to the valve element157. The rod is shown as a straight line inclined 5° to the vertical inserted into cylindrical bores in the float156and the valve element157. The bores are wide relative to the rod to reduce the need for accurate horizontal alignment of these elements and their respective seats.

Preferably, the stiff rod is unconnected or flexibly connected to at least one of the elements156and157. In use, this allows the float156and the valve element151to sink or fall into their respective seats155and158at different speeds.

FIG.4illustrates a binary ruler143. Binary rulers for use with optical readers usually have a pattern printed on a suitable material such as cardboard, metal or plastic. Alternatively, binary rulers for use with optical readers may comprise an opaque material with punched out, transparent fields. The example inFIG.4has four columns, where white means ‘0’ and black means ‘1’. A level h(t) crosses a unique pattern, inFIG.4‘black, white, black, white’, which for h(t) inFIG.4is interpreted as binary 1010=1·23+0·22+1·21+0·20=10 (=1·101+0·100) in decimal numbers.

The rightmost column defines the resolution, e.g. by alternating white and black rectangles each 1.0 mm high. To achieve better resolution, a column with alternating white and black rectangles each 2−1=½ mm high would be added to the right hand side of the binary ruler in this example. Similarly, it is obviously possible to add columns to the left hand side of the binary ruler143to represent 24, etc.

Later during the test period, we assume that a new level h(t+iΔt) is detected at the pattern ‘black, black, white, black’ corresponding to binary 1101=13 decimally. At 1 mm resolution, the difference Δh=h(t+iΔt)−h(t)=(13−10) mm measures a pressure change in the chamber122independent of the liquid level10. A 3.0 mm column of pure water at 4° C. corresponds to a change Δ(Δp)=103kg/m3·9.8 m/s2·3.0·10−3m=29.4 Pa. In general, a resolution 1 mm water column corresponds to a fine pressure scale about 10 Pa=0.1 hPa.

Incidentally, the binary ruler143is similar to a binary search from left to right, and the principle is by no means new. The ‘binary principle’ is widely used in non-optical sensors.

FIG.5illustrates an alternative embodiment of the leakage meter100that utilizes environment control and a lever principle. We do not repeat the description of components101-111and130-133explained with reference toFIG.1.

An auxiliary chamber103attached to the housing110illustrates one or more spaces fit to contain equipment and components as required. Such a chamber without reference numeral contains the upper end of the vertical pipe124inFIG.1.

An internal environment conditioner104represents elements to alter conditions within the housing110, for example, a heating element and/or a pressure regulator. In terms from control theory, the conditioner104is an actuator in a control loop. A temperature sensor112and/or a pressure sensor113may provide an associated feed forward or feedback.

In general, internal conditions such as temperature, pressure, humidity etc. may be different from corresponding conditions in the surrounding atmosphere. This approach minimizes adverse effects of atmospheric conditions rather than correcting for them. Insulation, tightness and other features of the housing110should of course be adapted accordingly.

Regardless of internal temperature, the pressure p may be equal to or very close to atmospheric pressure. For example, a gas tight housing110may comprise a bellow, a flexible membrane or other gas tight means to equalize internal and atmospheric pressure p. Further, internal pressures p (+ ·p) and temperature T will be different from STP in most cases.

The temperature sensor112represents sensors to measure any temperature of interest, for example, gas and liquid temperatures within the housing110and/or the temperature of the atmosphere surrounding the leakage meter100. The temperature sensor112supplies data to downstream information systems automatically as opposed to manual measurements entered by means of a keyboard.

For example, a control system may comprise a heater that changes liquid temperature faster than atmospheric temperature. The control system may need an automatic temperature sensor112, whereas manual measurements may suffice for atmospheric temperature.

A major benefit of environment control in the housing110is an ability to use sensors calibrated for a narrow range of conditions. For example, the electronic pressure sensor113may be accurate to within 0.1 hPa (0.1 mbar) within a controlled, narrow temperature range.

As inFIG.1, the inclined pipe120is attached to the bottom of the chamber122. However, the chamber122inFIG.5may be considerably larger than needed to contain a bubble train, for instance permitted leaks 1.5 ml/min×3 min=4.5 ml or 3.5 ml/min×3 min=10.5 ml compared to 10/16=0.625 ml plus a small safety margin.

Contrary to the embodiment inFIG.1, the inclined pipe120inFIG.5can tilt about a pivot116. A stopper126under the chamber122ensures that the inclined pipe120leads test gas toward the chamber122at all times, i.e. that an angle θ between the inclined pipe120and a horizontal plane is always greater than zero. As before, a minimum angle θ reduces the risk for adhesion of test gas to the inner wall of pipe120, especially at small leaks.

A lever127, e.g. a thin beam or sheet with its smallest dimension along the pivot axis, may support the assembly120-122if the assembly, in particular the inclined pipe120, lacks sufficient rigidity. The inclined pipe120and the lever127may be the same mechanical component, soFIGS.6and7do not show the lever127explicitly. Either way, the pivot116divides the lever127into a short arm with length a extending toward the first end121, and a long arm with length b extending toward the chamber122.

A gas release valve170in the top of chamber122replaces the gas release valve150shown inFIG.1. If the chamber122is designed to collect test gas during the entire test period, it may suffice to open the gas release valve170only between test periods in order to replace test gas with liquid in the chamber122. The gas release valve170may be any suitable valve, for example a commercially available solenoid valve certified for Ex-areas or a manually operated valve. We present an energy efficient alternative with reference toFIGS.8aand8b.

During operation, a buoyancy force F(t) increases as the volume of test gas in the chamber122displaces liquid. The buoyancy force of interest equals the weight of liquid displaced since time tS. With conventions described previously:
F(t)=ρgV(t)−F(0); 0<t<Ttest(5)

F(0) is a downward force exerted on the stopper126at tS, e.g. when the chamber122is filled with liquid and the gas release valve170is fully closed. In words, equation (5) states that F(t) becomes less negative as the gas volume V(t) increases. Once the buoyancy from V(t) overcomes F(0), F(t) becomes positive, i.e. directed upwards. For accuracy, F(0) should be as small as practically possible.

By the lever principle, an upward force F at distance b from the pivot116exerts a downward force −(b/a)F measured at distance a on the opposite side of the pivot. For example, (b/a)=10 implies that the downward force is ten times the buoyancy force at the chamber122. This may add a decimal of precision. However, measurement errors will also be multiplied by b/a.

As mentioned in the introduction, some electronic pressure and force sensors are relatively sensitive to temperature. This is not necessarily a concern in a housing110heated to a stable temperature, for instance a typical lab temperature near 20-22° C.

FIG.6essentially shows the lever inFIG.5acting with a ‘large’ force FS=−(b/a)F on a spring161. At equilibrium, the spring161acts on the lever at a with a force −FS=kx where k is a spring constant and x is a downward extension of the spring in this example. A small downward extension x at a corresponds to a ‘large’ upward displacement X=−(b/a)x at b. Measuring and adjusting ‘large’ quantities is a widely used strategy to reduce relative errors.

A dashed line around the box112illustrates that the temperature sensor112is not necessarily part of the leakage meter100. As noted, instruments outside the leakage meter may measure the ambient temperature and pressure required for correction of gas volumes.

The spring161is a specific embodiment of the force sensor160inFIG.5. It is used as an example inFIG.6because the spring extension x has the approximate size of a, b and other typical dimensions in the housing110, e.g. to within an order or two of magnitude. It is irrelevant that the measured quantity is actually a displacement as long as the displacement represents the force F. For comparison, the element sensing force in another force sensor160might be an elastic beam with displacement or strain several orders of magnitude smaller than a, b etc. Here, it is irrelevant that force sensors using strain actually may measure stress, i.e. force per unit area, as long as the output from the force sensor160represents force.

An adjustment nut162connects the spring161to the housing110. Turning the adjustment nut162causes a vertical displacement z, which should not to be confused with the spring extension x. For example, the spring161may be a tension spring unable to compress from equilibrium. Then, the entire spring161may move downward a distance z1until it exerts a force −(b/a)F(0) on the lever at a, thereby opposing F(0) in equation (5) to within acceptable limits. In another example, the adjustment nut162might cause a downward spring bias FS0=kz1when the adjustment nut162moves downward the distance z1. FS0=(b/a)F(0) would also cancel F(0) in equation (5) to within predefined limits.

Either way, we may set F(0)=0 in equation (5) and solve for V(t):
V(t)=(k/ρg)X(t) 0<t<Ttest(6)
where X is a displacement at b adjusted such that X(0)=0 at the start of each test period.

Summarized and neglecting opposite signs on opposite sides of the pivot116, at a, x(t)=(a/b)X(t) cancels FS=(b/a)F=kx, so the lever ratio b/a does not appear in equation (6). Still, measuring displacement at b does reduce uncertainty by the factor a/b, so the lever ratio is implicit in the apparatus inFIG.6. Further, the spring constant k determines the ratio V(t)/X(t) and thereby amplification through the factor k/ρg in eq. (6).

In order to illustrate measurement of vertical displacement at b, we assume a binary ruler143as inFIG.4where the white fields are transparent and the black fields are opaque. The binary ruler143is attached to the chamber122at right angles to the inclined pipe120. A light source144and a focusing lens145produce a fine focused line across the binary ruler143. In this example, a light detector146has four vertical photosensitive strips such that light passing through a transparent field produces a voltage. Thus, a unique horizontal pattern of opaque and transparent fields produces a unique pattern of voltages from the four strips. A sequence ‘0101’ in Courier normal font illustrates a binary output on line147. Further treatment of binary and electrical signals is beyond the scope of the present invention, but still well known to those skilled in the art.

Since the binary ruler143forms the angle θ with the vertical in this example, the measured height is X(t)cos θ. In practice, θ may be reduced by an angle corresponding to the start position where the chamber122touches the stopper126. This corresponds to a vertical ruler143in the start position, i.e. not 90° from the pipe122as shown inFIG.6. Since cos θ≈1 for small angles θ, angle corrections may be superfluous in real embodiments. For example, 15° is relatively large for a ‘small’ angle, and causes a correction [1−cos(15°)]=0.034 mm for a 1 mm slit in the binary ruler143. This cosine correction is less than 10% of a height about 0.5 mm of the horizontal line of light produced by a typical focusing lens145.

FIG.7shows an alternative embodiment in which the spring161is attached to the chamber122at b. There is no need to repeat explanations of reference numerals 10-161 and symbols p, T, θ, a, b and x. As before, ambient pressure p and temperature T are required for volume corrections, but the associated temperature and pressure sensors112,113are not necessarily parts of the leakage meter100.

InFIG.7, a counter weight163can rotate on fine threads164on the short arm of the lever, and replaces the adjustment nut162shown inFIG.6. Rotating the counter weight163causes an axial displacement that, in effect, alters the lever ratio b/a to exactly oppose the weight of the liquid filled chamber122at the start of each test period. In terms defined above, a counter weight163with mass m at an adjusted distance a provides a weight mg=(b/a)F(0) that opposes F(0) from equation (5) to within predefined limits.

The skilled person understands that there is no need for both an adjustment nut162and a counter weight163, and further that a lock screw, a jam nut or equivalent means may prevent undesired rotation of either element162or163. Furthermore, any force sensor160may replace the spring161without inventive effort.

The counter weight163and the liquid filled chamber122are exposed to the same local acceleration of gravity g, so g cancels in a force equation relating gravity acting on the chamber122and the counter weight163. This principle distinguishes a traditional balance from a traditional scale. As is well known, traditional ‘reference weights’ for a balance are usually labelled in units of mass, e.g. kg or gram, rather than in units of force such as Newton.

The mass of liquid displaced by a gas volume V(t) within the chamber122is pV(t). Moreover, equation (6) indicates that V(t) is proportional to the vertical displacement X at b. As discussed, we may safely neglect the cosine correction at small angles θ. Perhaps more important, equation (6) follows from assumed linear relationships between buoyancy, volume and displacement in the sense that second order effects become insignificant. For the spring161or an elastic beam, the linear assumption is equivalent to elastic deformation and Hooke's law. For the volume V(t), a linear relationship to X implies that it suffices to ‘measure’ the chamber122filled with gas and filled with liquid in order to determine a maximum and minimum X. All values in between are proportional:
V(t)/(Vmax−Vmin)=X(t)/(Xmax−Xmin)  (7)

The liquid filled chamber122in previous examples corresponds to Vmin=0. However, Vmin≠0 is possible in equation (7). Similarly, any Xmin≠0 is equivalent provided the difference Xmax−Xminremains constant. Setting Xmin=0 is just a convention.

Measuring mass is generally easier than measuring volume, so the exact volume Vmaxmay be determined by filling the chamber122with liquid at a known density and measure the mass before and after filling. For a numerical example, we assume a lab and 22° C. at which pure water has density ρ=997.76 kg/m3. Further, we assume that a lab balance measures the difference between masses before and after filling to Δm=5.678 gram independent of local gravity. The internal volume Vmax=(5.678±0.001)/0.99776=(5.691±0.001) ml. In this example, mg precision on a lab balance corresponds to μl precision in volume. This is comparable to the 4.2 μl resolution of a 2 mm bubble.

FIGS.8aand8billustrate a bi-stable embodiment of the gas release valve170in an open and a closed state, respectively. The valve170has a shell171mounted in the top of chamber122. The shell171has a vertical bore and contains a rotatable valve element172, e.g. a ball or a cylinder. The valve element172has a secondary bore, which opens for flow through the valve170when aligned with the vertical bore as inFIG.8a.

FIG.8ashows the valve170in its open state before each test period. Dash-dot lines cross at the valve element's axis of rotation. A spring attached to an eccentric pin173acts on the rotatable valve element172with a torque a1F1. As long as the eccentric pin173remains below the valve element's axis of rotation, the torque a1F1keeps the secondary bore in a vertical position such that the valve170remains open.

FIG.8bshows the valve170in its closed state during test periods. In this state, the eccentric pin173is above the valve element's axis of rotation, and the spring attached to the eccentric pin173acts on the rotatable valve element172with a torque a2F2. As long as the eccentric pin173remains above the valve element's axis of rotation, the torque a2F2. keeps the secondary bore in a horizontal position such that the valve170remains closed.

It is understood that the same technical effect is achieved whenever the spring forces F1and F2are applied to opposite sides of the valve element's axis of rotation, not necessarily over and under the valve element's axis of rotation.

A lever174connected to the valve element172is able to rotate the valve element172in the shell171. InFIG.8a, the lever174is directed 45° from a horizontal axis, i.e. upward to the right, when the valve170is open. InFIG.8b, the lever174is directed 135° from the horizontal axis, i.e. upward to the left, when the valve element172is rotated 90° in order to close the valve. Suitable stoppers (not shown) limit the rotation.

The spring attached to the eccentric pin173may be a tension spring attached to the wall of the chamber122to the left of components shown inFIGS.8aand8b. Alternatively, a compression spring or a torsion spring may provide similar torques a1F1and a2F2. Either way, the spring attached to the eccentric pin173provides the force required to keep the valve170open or closed. Thus, the only energy required to operate the valve170is the work needed to overcome a limited spring force over a short distance separating the two stable states.

In terms of force and distance, factors a2/b1and a1/b1substantially reduce the force required to open or close the valve if b1is a lever arm substantially longer than a1and a2. A longer lever arm b1along the lever174increases the distance over which an actuator has to act on the lever. In terms of power and time, smaller power implies longer time to supply the required energy. Thus, it is possible to operate the bi-stable valve170inFIGS.8aand8bat low power, e.g. supplied from a battery through a low-power inductive actuator. Note that wires from the housing to the gas release valve170might affect weight measurements, and that. inductive couplings do not require wires.

FIG.9is a vertical section through the flow divider130. As explained with reference toFIG.1, the inlet pipe131leads to the inlet nozzle111, the bypass pipe132leads test gas away from the housing110and the lever133represents an actuator.

The lever133is attached to a rotatable dividing disc134, which inFIG.9separates an upper compartment from a lower manifold. In use, the upper compartment receives test gas through the inlet101. The lower manifold has at least two manifold chambers1310and1320. One or more first manifold chamber(s)1310is/are fluidly connected to the inlet pipe131, and one or more second manifold chamber(s)1320is/are fluidly connected to the bypass pipe132.

The orientation of the flow divider130inFIG.9is not limiting. Real embodiments may have the manifold chambers1310,1320on a side rather than in the lower part, etc.

The dividing disc134may be set in one of several discrete angular orientations by means of protrusions, e.g. pins or balls, fitting in grooves in a raceway135. A compression spring136urges the protrusions into the grooves and the dividing disc134against vertical walls within the manifold. A gasket137under part of the dividing disc134seals for gas with a small overpressure.

A central shaft138is attached to the dividing disc134and inserted in a sleeve139attached to the shell of the flow divider130. The shaft138in the sleeve139prevents that the dividing disc134tilts relative to the raceway135. This facilitates using loose balls instead of pins between the dividing disc134and the raceway135.

FIG.10is a section along plane X-X inFIG.9, and illustrates the dividing disc134viewed from above. The dividing disc134has a gas tight closing part1341and an open part1342through which some internal walls in the lower manifold are visible. A thin rectangle illustrates a mandatory division between the at least two chambers1310and1320. Radial lines illustrate optional internal walls in the lower manifold. In real embodiments, the walls represented by radial lines and by a thin rectangle may have similar designs.

In use, the angular orientation of the dividing disc134determines the ratio of amount of test gas entering the chamber1310to the amount of test gas entering the chamber1320. In FIG.10, this ratio is 5/3 corresponding to five sectors visible through the opening1342to the left of the thin rectangle, and three visible sectors to the right of the thin rectangle.

In general, the flow divider130diverts a precisely known fraction of a ‘large’ leak arriving through the inlet101to the inlet pipe131such that the ‘large’ leak may be estimated from measurements produced by the leakage meter100and the precisely known fraction.

FIG.11is a top view of a raceway135for the dividing disc134. Grooves1350may conveniently be drilled holes with a diameter slightly less than protrusions, e.g. pins or balls, located between the dividing disc134and the raceway135. As best seen inFIG.9, the spring136urges the protrusions into the set of grooves1350, thereby creating the set of angular orientations.

While the invention has been described by means of examples, the scope of the invention is determined by the following set of claims.

REFERENCES

Alonzo Kelly: “A 3D State Space Formulation of a Navigation Kalman Filter for Autonomous Vehicles”, Carnegie Mellon University, Rev 2 2006, original from 1994