Pressure measurement

Pressure measurement

The measurement of pressure may take place from one of two possible datums, depending upon the type of instrument used. Absolute pressure is a total measurement using zero pressure as datum. Gauge pressure is a measurement above the atmospheric pressure which is used as a datum. To express gauge pressure as an absolute value it is therefore necessary to add the atmospheric pressure.

Manometer

A U-tube manometer is shown in Figure 15.1. One end is connected to the pressure source; the other is open to atmosphere. The liquid in the tube may be water or mercury and it will be positioned as shown. The excess of pressure above atmospheric will be shown as the difference in liquid levels; this instrument therefore measures gauge pressure. It is usually used for low value pressure readings such as air pressures. Where two different system pressures are applied, this instrument will Measure differential pressure.

Barometer

The mercury barometer is a straight tube type of manometer. A glass capillary tube is sealed at one end^ filled with mercury and then inverled in a small bath of mercury (Figure 15.2). Almost vacuum conditions; exist above the column of mercury, which is supported by atmospheric

pressure acting on the mercury in the container. An absolute reading of atmospheric pressure is thus given.The aneroid barometer uses an evacuated corrugated cylinder to detect changes in atmospheric pressure (Figure 15.3). The cylinder centre tends to collapse as atmospheric pressure increases or is lifted by the spring as atmospheric pressure falls. A series of linkages transfers the movement to a pointer moving over a scale.

Bourdon tube

This is probably the most commonly used gauge pressure measuring instrument and is shown in Figure 15.4. It is made up of an elliptical

section tube formed into a C-shape and sealed at one end. The sealed end, which is free to move, has a linkage arrangement which will move a pointer over a scale. The applied pressure acts within the tube entering through the open end, which is fixed in place. The pressure within the tube causes it to change in cross section and attempt to straighten out with a resultant movement of the free end, which registers as a needlemovement on the scale. Other arrangements of the tube in a helical or spiral form are sometimes used, with the operating principle being the same.

While the reference or zero value is usually atmospheric, to give gauge pressure readings, this gauge can be used to read vacuum pressure values.

Other devices

Diaphragms or bellows may be used for measuring gauge or differential pressures. Typical arrangements are shown in Figure 15.5. Movement of the diaphragm or bellows is transferred by a linkage to a needle or pointer display.

The piezoelectric pressure transducer is a crystal which, under pressure, produces an electric current which varies with the pressure. This current is then provided to a unit which displays it as a pressure value.

Temperature measurement by instruments will give a value in degrees Celsius (°C). This scale of measurement is normally used for all readings and temperature values required except when dealing with theoretical calculations involving the gas laws, when absolute values are required (see Appendix).

+Temperature Measurement

Liquid-in-glass thermometer

Various liquids are used in this type of instrument, depending upon the temperature range, e.g. mercury -35°C to +350°C, alcohol -80°C to 4-70°C. An increase in temperature causes the liquid to rise up the narrow glass stem and the reading is taken from a scale on the glass (Figure 15.6). High-temperature-measuring mercury liquid thermometers will have the space above the mercury filled with nitrogen under pressure.

Liquid-in-metal thermometer

The use of a metal bulb and capillary bourdon tube filled with liquid offers advantages of robustness and a wide temperature range. The useof mercury, for instance, provides a range from —39°C to +650°C. The bourdon tube may be spiral or helical and on increasing temperature it tends to straighten. The free end movement is transmitted through linkages to a pointer moving over a scale.

Bimetallic strip thermometers

A bimetallic strip is made up of two different metals firmly bonded together. When a temperature change occurs different amounts of expansion occur in the two metals, causing a bending or twisting of the strip. A helical coil of bimetallic material with one end fixed is used in one form of thermometer (Figure 15.7). The coiling or uncoiling of the

helix with temperature change will cause movement of a pointer fitted to the free end of the bimetallic strip. The choice of metals for the strip will determine the range, which can be from — 30°C to +550°C.

Thermocouple

The thermocouple is a type of electrical thermometer. When two different metals are joined to form a closed circuit and exposed to different temperatures at their junction a current will flow which can be used to measure temperature. The arrangement used is shown in Figure 15.8, where extra wires or compensating leads are introduced to complete the circuit and include the indicator. As long as the two ends A and B are at the same temperature the thermoelectric effect is not influenced. The appropriate choice of metals will enable temperature ranges from ~200°C to +1400°C.

Radiation pyrometer

A pyrometer is generally considered to be a high-temperature measuring thermometer. In the optical, or disappearing filament, type shown in Figure 15.9, radiation from the heat source is directed into the unit. The current through a heated filament lamp is adjusted until, when viewed through the telescope, it seems to disappear. The radiation from the lamp and from the heat source are therefore the same. The current through the lamp is a measure of the temperature of the heat source,

and the ammeter is calibrated in units of temperature. The absorption screen is used to absorb some of the radiant energy from the heat source and thus extend the measuring range of the instrument. The monochromatic filter produces single-colour, usually red, light to simplify filament radiation matching.

Thermistor

This is a type of electrical thermometer which uses resistance change to measure temperature. The thermistor is a semi-conducting material made up of finely divided copper to which is added cobalt, nickel and manganese oxides. The mixture is formed under pressure into various shapes, such as beads or rods, depending upon the application. They are usually glass coated or placed under a thin metal cap. A change in temperature causes a fall in the thermistor resistance which can be measured in an electric circuit and a reading relating to temperature can be given. Their small size and high sensitivity are particular advantages. A range of measurement from — 250°G to + 1500°C is possible.

+ Level measurement

Float operated

A float is usually a hollow ball or cylinder whose movement as the liquid surface rises or falls is transmitted to an indicator. A chain or wire usually provides the linkage to the indicator. Float switches may be used for high or low indication, pump starting, etc., where electrical contacts are made or broken, depending upon the liquid level.

Sight or gauge glasses

Various types of sightglass are used to display liquid level in storage tanks. The simple boiler gauge glass referred to in Chapter 4 is typical of such devices.

Pneumatic gauge

This is a device which uses a mercury manometer in conjunction with a hemispherical bell and piping to measure tank level. The arrangement is shown in Figure 15.10. A hemispherical bell is fitted near the bottom of the tank and connected by small bore piping to the mercury manometer.

A selector cock enables one manometer to be connected to a number of tanks, usually a pair. A three-way cock is fitted to air, gauge and vent positions. With the cock at the 'air' position the system is filled with compressed air. The cock is then turned to 'gauge' when the tank contents will further pressurise the air in the system and a reading will be given on the manometer which corresponds to the liquid level. The cock is turned to Vent' after the reading has been taken.

Flow measurement can be quantity measurement, where the amount of liquid which has passed in a particular time is given, or a flow velocity which, when multiplied by the pipe area, will give a rate of flow.

+ Quantity measurement

A rotating pair of intermeshing vanes may be used which are physically displaced by the volume of liquid passing through (Figure 15.1 l(a)). The

number of rotations will give a measure of the total quantity of liquid that has passed. The rotation transfer may be by mechanical means, such as gear wheels, or the use of a magnetic coupling. Another method is the use of a rotating element which is set in motion by the passing liquid (Figure 15.1 l(b)). A drive mechanism results in a reading on a scale of total quantity. The drive mechanism may be mechanical, using gear wheels or electrical where the rotating element contains magnets which generate a current in a pick-up coil outside the pipe.

+flow velocity measurement

The vmturi tube

This consists of a conical convergent entry tube, a cylindrical centre tube and a conical divergent outlet. The arrangement is shown in Figure 15.12. Pressure tappings led to a manometer will give a difference in

head related to the fluid flow velocity. The operating principle is one of pressure conversion to velocity which occurs in the venturitube and results in a lower pressure in the cylindrical centre tube.

The orifice plate

This consists of a plate with an axial hole placed in the path of the liquid. The hole edge is square facing the incoming liquid and bevelled on the

outlet side (Figure 15.13). Pressure tappings before and after the orifice plate will give a difference in head on a manometer which can be related to liquid flow velocity.

+ Other variables

Moving coil meter

Electrical measurements of current or voltage are usually made by a moving coil meter. The meter construction is the same for each but its arrangement in the circuit is different. A moving coil meter consists of a coil wound on a soft iron cylinder which is pivoted and free to rotate (Figure 15.14). Two hair springs are used, one above and one below, to provide a restraining force and also to

conduct the current to the coil. The moving coil assembly is surrounded by a permanent magnet which produces a radial magnetic field. Current passed through the coil will result in a force which moves the coil against the spring force to a position which, by a pointer on a scale, will read current or voltage.

The instrument is directional and must therefore be correctly connected in the circuit. As a result of the directional nature of alternating current it cannot be measured directly with this instrument, but the use of a rectifying circuit will overcome this problem.

Tachometers

A number of speed measuring devices are in use utilising either mechanical or electrical principles in their operation.

Mechanical

A simple portable device uses the governor principle to obtain a measurement of speed. Two masses are fixed on leaf springs which are fastened to the driven shaft at one end and a sliding collar at the other (Figure 15.15). The

sliding collar, through a link mechanism, moves a pointer over a scale. As the driven shaft increases in speed the weights move out under centrifugal force, causing an axial movement of the sliding collar. This in turn moves the pointer to give a reading of speed.