1 Pressure Measurement

1.1 Terminology and Conversions

1.1.1 Define the following instrument related terms: (1)

a. Absolute pressure: Pressure referenced from zero pressure rather than from atmospheric pressure – known as psia. By definition: psia = psig + atmospheric pressure.

b. Differential pressure: The difference between any two pressures (psid). Used in the airspeed indicator, this instrument measures the difference between ram air pressure that enters the pitot tube and static air pressure.

c. Gauge pressure: Is the pressure read directly from a gauge and represents the pressure in excess of barometric pressure as in the oil pressure gauge.

d. Hysteresis error: Hysteresis error is a lag in altitude indication due to the elastic properties of the material within the altimeter.  This  occurs  after  an aircraft  has maintained  a  constant  altitude for  an extended period of time and then makes a large, rapid altitude  change. After a rapid descent, altimeter readings are higher than actual. This error is negligible during climbs and descent at a slow rate or after maintaining a new altitude for a short period of time.

e. Parallax error: Parallax can be a problem when you are reading a meter. Since the pointer is slightly above the scale (to allow the pointer to move freely), you must look straight at the pointer to have a correct meter reading. In other words, you must be in line with the pointer and the scale.

f. Millibar: A unit of barometric pressure equal to approximately 0.75 mm of mercury. Atmospheric air pressure is often given in millibars where "standard" sea level pressure (1 atm) is defined as 1013.25 mbar (hPa), equal to 1.01325 bar. Despite millibars not being an SI unit, meteorologists and weather reporters worldwide have long measured air pressure in millibars. After the advent of SI units, some meteorologists began using hectopascals (symbol hPa) which are numerically equivalent to millibars.

1.1.2 Describe the methods of compensating instrument mechanisms for temperature variations and the reasons for hermetically sealing instruments. (2)

The methods adopted for temperature compensation are varied depending on the type of instrument to which they are applied. The oldest method of compensation is the bimetal-strip principle and is applied to such instruments as airspeed indicators, altimeters, vertical speed indicators, and exhaust-gas temperature indicators.

An application of the bimetal-strip principle to a typical rod-type mechanism is shown in Figure 1(a). In this case, the vertical ranging bar connected to the rocking shaft is bimetallic and bears against the arm coupled to the sector gear of the indicating element. The principal effect of temperature changes on this mechanism are expansion and contraction of the capsule, thus making the indicating element over-read or under-read.

Another compensation method adopts a thermo-resistor or thermistor connected in the indicator circuit. A thermistor, which is composed of a mixture of metallic oxides, has a very large temperature coefficient of resistance which is usually negative; i.e. its resistance decreases with increases in temperature. Assuming that the temperature of the indicator increases, the current flowing through the indicator will be reduced because copper or aluminium will characteristically increase in resistance; the indicator will therefore tend to under-read. The thermistor resistance will, on the other hand, decrease, so that for the same temperature change the resistance changes

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will balance out to maintain a constant current and therefore a constant indication of the quantity being measured.

Figure 1(a) Figure 1(b)

A thermo-magnetic shunt is a strip of nickel-iron alloy sensitive to temperature changes, which is clamped across the poles of the permanent magnet so that it diverts some of the air-gap magnetic flux through itself. As before, let us assume that the indicator temperature increases. The moving-coil resistance will increase thus opposing the current flowing through the coil, but, at the same time, the reluctance ('magnetic resistance') of the alloy strip will also increase so that less flux is diverted from the air-gap. Since the deflecting torque exerted on a moving coil is proportional to the product of current and flux, the increased air-gap flux counterbalances the reduction in current to maintain a constant torque and indicated reading. Depending on the size of the permanent magnet, a number of thermo-magnetic strips may be fitted to effect the required compensation.

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Figure 2: Bimetallic U-bracket (a strong spring) acts as a temperature compensating device by preventing the capsule from expanding and contracting. A decrease in temperature causes the capsule to increase making the air more dense. Lower temperatures cause the bracket to bend inward, stopping the expansion of the capsule.

In pressurized aircraft, the internal atmospheric pressure conditions are increased to a value greater than that prevailing at the altitude at which the aircraft is flying. Consequently, instruments using external atmospheric pressure as a datum, for example altimeters, vertical speed indicators and airspeed indicators, are liable to inaccuracies in their readings should air at cabin pressure enter their cases. The cases are therefore sealed to withstand external pressures higher than those normally encountered under pressurized conditions. The external pressure against which sealing is effective is normally 15 lbf/in2.

Direct-reading pressure measuring instruments of the Bourdon tube, or capsule type, connected to a pressure source outside the pressure cabin, are also liable to errors. Such errors are corrected by using sealed cases and venting them to outside atmospheric pressure. Many of the instruments in current use depend for their operation on sensitive electrical circuits and mechanisms which must be protected against the adverse effects of atmospheric temperature, pressure and humidity. This protection is afforded by filling the cases with an inert gas such as nitrogen or helium, and then hermetically sealing the cases.

1.2 Pressure Measuring Devices

1.2.1 Explain the construction, operation and functions of the following: a. Bellows (absolute and differential) b. Bourdon tubes c. Capsules (absolute and differential) d. Diaphragms (2)

a. Bellows:

Absolute bellows: For example, a sensitive altimeter is an aneroid barometer that measures the absolute pressure of the ambient air and displays it in terms of feet or meters above a selected pressure level. The sensitive element in a sensitive altimeter is a stack of evacuated, corrugated bronze aneroid capsules or wafers. [Figure 3 below] The air pressure acting on these aneroids tries to compress them against their natural springiness, which tries to expand them. The result is that their thickness changes as the air pressure changes. Stacking several aneroids increases the dimension change as the pressure varies over the usable range of the instrument. When altitude increases, the static pressure decreases, and the bellows expands, driving the indicating pointers through a series of gears.

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Figure 3: An indication of how a sensitive altimeter works – absolute bellows.

Differential bellows: A useful type of instrument which uses two enclosed bellows, each filled with the associated pressures to be measured. The bellows with the greatest pressure compresses the other bellows and moves the pointer mechanism. Dual-bellows element pressure indicators are used in the Navy as flow- measuring, level-indicating, or pressure-indicating devices. Differential bellows can be used to measure absolute (manifold pressure), differential (fuel pressure gauge) or gauge (airspeed indicator) pressure.

Figure 4: A differential bellows instrument where the pressure differential creates a linear movement of the bellows assembly. The indications are the result of a pressure differential and the mechanical linkage allows it to be presented on a round dial. Used as a MAP gauge.

A bellows type of element can be considered as an extension of the corrugated diaphragm principle, and in operation it bears some resemblance to a helical compression spring. It may be used for high, low or differential pressure measurement, and in some applications a spring may be employed (internally or externally) to increase what is termed the 'spring-rate' and to assist a bellows to return to its natural length when pressure is removed. The element is made from a length of seamless metal tube with suitable end fittings for connection to pressure sources or for hermetic sealing.

Another type of differential pressure instrument is shown in Figure 5 below.

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Figure 5: Difference in pressure between P1 and P2 cause the flexible wall to bend and move a pointer on a calibrated scale

b. Bourdon Tubes

To measure high pressure a Bourdon tube-type instrument is used. This instrument consists of a hollow or bronze elliptical-shaped tube formed into a semi-circle. One end of the tube is open and connected to the fluid to be measured: the opposite end is sealed. When pressure is applied, the elliptical tube changes shape - increased pressure tends to straighten the curve. This movement is transferred via the gear mechanism to move a pointer. The pointer moves across a scale thereby providing a direct reading of pressure. Materials used for the tube are selected for the pressure range being measured; these include phosphor bronze (0–1000 psi) and beryllium copper (0–10,000 psi).

Figure 6: As the pressure increases, the tube straightens and in doing so moves a mechanical linkage which in turn moves a pointer, creating a useful pressure measuring instrument. c. Capsules

Absolute capsules: Absolute pressure is often used on the aircraft in comparison to other pressures. In order to make this comparison, a device called an aneroid wafer (cell or capsule)

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was devised. This aneroid capsule (cell) is made from an alloy of beryllium and copper. The evacuated capsule (or usually more capsules) is prevented from collapsing by a strong spring (shown in the diagram below). Small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers such that the tiny movements of the capsule are amplified and displayed on the face of, for example, an altimeter.

Figure 7: An example of the use of an absolute capsule.

Depending on which reference is used, capsules are also said to be made up of two diaphragms placed together and joined at their edges to form a chamber which may be completely sealed or open to a source of pressure. Like single diaphragms they are also employed for the measurement of low pressure, but they are more sensitive to small pressure changes.

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Figure 8: The aneroid wafers shown opposite are stacked capsules that measure the difference in pressure between the vacuum inside the sealed chambers and the ambient pressure around it. Expansion and contraction of the pressure capsule, cell or chamber moves the needle on a gauge.

Differential capsules:

Capsules on the whole are just the same as wafers or bellows. They are sealed units evacuated of air to create a vacuum. How they work and operate is explained above.

d. Diaphragm:

Diaphragms in the form of corrugated circular metal discs, owing to their sensitivity, are usually employed for the measurement of low pressures. They are always arranged so that they are exposed at one side to the pressure to be measured, their deflections being transmitted to pointer mechanisms, or to a warning-light contact assembly. The materials used for their manufacture are generally the same as those used for Bourdon tubes. The purpose of the corrugations is to permit larger deflections, for given thicknesses, than would be obtained with a flat disc. Furthermore their number and depth control the response and sensitivity characteristics; the greater the number and depth the more nearly linear is its deflection and the greater is its sensitivity.

1.2.1 Compare linear and non-linear pressure gauge scales. (1)

Scale spacing is governed by physical laws related to the quantity to be measured. Hence, there cannot be complete uniformity between all quantitative displays. In general, there are two groups:

a. Linear – scales with evenly spaced marks, orb. Non - linear – non-evenly spaced marks such as the square law type (airspeed indicators

measure a differential pressure, which varies with the square of the airspeed), or the logarithmic type (rate of altitude changes). The non-linearity of some scales makes them difficult to read accurately.

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Figure 8: Here the diaphragm is shown as a bellows in a vertical speed indicator (VSI). Typically it is made of very thin, spongy springy metal.

Figure 9: Square-law characteristics. In Figure 9(a), the effect of linear deflection due to a response in pressure is shown graphically. In Figure 9(b), the effect is shown on the readout of the gauge.

1.2.3 Explain the functions and operation of direct reading pressure gauges in a light aircraft system. TEST QUESTION (2)

Pressure gauges are either direct or indirect-reading. Pressure gauges installed in hydraulic and pneumatic systems are used to indicate existing hydraulic and pneumatic pressures, and are calibrated in pounds per square inch. Aircraft can use both the direct reading gauges and the synchro (electric) type – the trend is away from the direct reading pressure gauges.

The direct-reading gauge shown in figure 10 consists essentially of a Bourdon tube, a pointer, a dial, and a rain tight case. The Bourdon tube and the pointer are interconnected by gearing to cause them to move together.

Figure 10: Pressure gauge where the system pressure comes from a hydraulic or pneumatic line

Pressure within the hydraulic system is admitted into the Bourdon tube through a connecting line. As hydraulic system pressure increases, it causes the Bourdon tube to straighten to a corresponding degree. The change in curvature of the tube reacts on the gearing, causing the

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pointer to move to a correspondingly higher reading on the dial. As hydraulic system pressure decreases, the Bourdon tube curls back toward its original shape by a corresponding amount. This causes the pointer to fall back to a correspondingly lower dial reading. Pressure gauge snubbers are usually used with hydraulic pressure gauges to dampen oscillations of the pointer.

1.2.4 Explain how hysteresis error affects the consistency of readings in a pressure gauge. (2)Hysteresis is the variation in pointer position, at any given point, between upscale and downscale indication after light tapping to minimize friction in the mechanism.  

Figure 11: The deviation illustrated is the hysteresis effect at 44 psig after tapping to minimize friction. The deviation is usually maximum at approximately mid-scale.

It is important to understand that hysteresis differs from friction in that when hysteresis is present in a gauge it cannot be eliminated. This condition will cause the indication to read high on decreasing pressure. Hysteresis is a function of elastic chamber repeatability. Since there is no mechanical correction which can be made, the only solution is to make a correction chart and attach it to the gauge.  In this way the user can make mental and visual corrections as necessary.

Hysteresis Error — Hysteresis error is a lag in altitude indication due to the elastic properties of the material within the altimeter. This occurs after an aircraft has maintained a constant altitude for an extended period of time and then makes a large, rapid altitude change. After a rapid descent, altimeter readings are higher than actual.  This error is negligible during climbs and descent at a slow rate or after maintaining a new altitude for a short period of time.

1.2.5 Explain the function and operation of a manifold pressure gauge. TQ (2)

Changes in manifold air pressure effect the amount of power an engine can produce for a given rpm. MAP readings are monitored by a gauge and provide a means of selecting power settings. MAP gauges indicate psia of the fuel/air mixture at a point just outside the cylinder intake port. Since MAP directly affects a cylinder’s mean effective pressure (mep), a pilot uses MAP gauge indications to set the engine power at a pressure level that will not damage the engine. This is especially true of turbocharged engines because it helps the pilot to avoid excessive manifold

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pressure. In normally aspirated engines at full power, the pressure will not exceed ambient pressure. If, for example, an engine is running in atmospheric conditions corresponding to the standard sea-level pressure of 14.7 lbf/in2, and the cylinder pressure is reduced to say, 2 ibf/in2, then the pressure difference is 12.7 lbf/in2, and it is this pressure difference which 'pushes' the charge into the cylinder. As one gets higher, say 10,000 feet, the pressure difference available to push the charge into the cylinder has dropped to a third.

A manifold pressure gauge consists of a sealed diaphragm constructed from 2 thin discs of concentrically corrugated metal soldered together at the edges to form a chamber. The chamber is evacuated to create a partial vacuum which is used as a reference point to measure absolute pressure.

Depending on the type of gauge, the engine manifold pressure is:a. Applied to the inside of the diaphragm orb. The outside of the diaphragm in which case the instrument case must be sealed. In either case

the diaphragm movement is transmitted to an indicator pointer through mechanical linkages.

Another MAP gauge uses a series of stacked diaphragms or bellows which are particularly useful for measuring low or negative pressures. This type has one of the bellows reading ambient pressure while the other measures the pressure in the intake manifold. Differential pressure causes motion which moves a pointer through a mechanical linkage.

1.2.6 Detail how the accuracy of a manifold pressure gauge may be checked. (2)

As a general rule, manifold pressure (inches) should be less than the rpm. A pilot can avoid conditions that overstress the cylinders by being constantly aware of the rpm, especially when increasing the manifold pressure. Conform to the manufacturer’s recommendations for power settings of a particular engine to maintain the proper relationship between manifold pressure and rpm.

Before starting an engine the manifold pressure gauge should indicate the local atmospheric pressure. Once started, the MAP gauge should drop. If this does not happen and the gauge continues to read atmospheric pressure, the sense line between the instrument and the induction manifold may be disconnected, broken or collapsed. When engine power is increased, the manifold pressure should increase evenly and in proportion to the engine power output. If it does not the restriction in the sense line is probably too large.

1.2.7 State specific gauge readings when the engine is stationary. (1)

When the engine is not running the MAP gauge should read ambient pressure.

1.2.8 Explain how manifold pressure gauges are protected from pressure surges caused by engine backfiring. TEST QUESTION (1)

The pressure line from the manifold to the instrument case must contain a restriction to prevent pressure surges from damaging the instrument. The restriction will also cause a slight delay in gauge response to changes in manifold pressure, preventing jumpy or erratic pointer movement on the instrument display.

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As an aside:

Reference from a pilot’s website which sites these two cases of manifold problems:a. Manifold gauges whose reading does not correlate with the altimeter pressure reading.b. Manifold gauges that are not temperature compensated.

A lot of manufacturers say their gauges are compensated for temperature, but no text references. Eric in ISD says all manifold pressure gauges are temperature compensated for ambient temperature.

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