Temperature measuring instruments

Custom Search

TEMPERATURE-MEASURING INSTRUMENTS

Temperature is the degree of hotness or coldness of a substance measured on a definite scale. Temperature is measured when a measuring instrument, such as a thermometer, is brought into contact with the medium being measured. All temperature-measuring instruments use some change in a material to indicate temperature. Some of the effects that are used to indicate temperature are changes in physical properties and altered physical dimensions. One of the more important physical properties used in temperature-measuring instruments is the change in the length of a material in the form of expansion and contraction.

Consider the uniform homogeneous bar illustrated in figure 8-11. If the bar has a given

Figure 8-11.—Expansion of a bar.

length (LO) at some temperature and is heated, it will expand (Lf). The amount of expansion   is a function of the original length and the temperature increase. The amount a material changes in length with temperature is called the linear coefficient of expansion.

The linear coefficient of expansion for a material is a physical property of that material and describes its behavior with respect to temperature.

BIMETALLIC EXPANSION THERMOMETER

If two materials with different linear coefficients are bonded together, as the temperature changes their rate of expansion will be different. This will cause the entire assembly to bend in an arc as shown in figure 8-12.

When the temperature is raised, an arc is formed around the material with the smaller      expansion coefficient. Since this assembly is formed by joining two dissimilar materials, it is known as a bimetallic element.

A modification of this bimetallic strip serves as the basis for one of the simplest and most commonly encountered temperature-measuring instruments, the bimetallic thermometer. Figure 8-13 shows a bimetallic thermometer. In it, a bimetallic strip is wound in the form of a long helix. One end of the helix is held rigid. As the temperature varies, the helix tries to wind or unwind. This causes the free end to rotate. The

Figure 8-12.—Effect of unequal expansion of a bimetallic strip.

free end is connected to a pointer. The pointer actually indicates angular rotation of the helix; however, since the rotation is linear and a function of temperature, the scale is marked in units of temperature.

Distant-reading dial thermometers are used when the indicating portion of the instrument must be placed at a distance from where the temperature is being measured. The distant-reading thermometer has a long capillary, some

Figure 8-13.—Bimetallic thermometer.

as long as 125 feet, which separates the sensing bulb from the Bourdon tube and dial (fig. 8-14). There are three basic types of distant-reading thermometers: the liquid filled, the gas filled, and the combination liquid-vapor filled. The thermometers are filled with fluid (liquid or gas) at some temperature and sealed. Almost the entire volume of the fluid is in the sensing bulb. As the temperature of the bulb changes, the volume of the fluid tries to change. Since the volume of the thermometer (sensing bulb, capillary, and Bourdon tube) is constant, a pressure change occurs within the thermometer. This pressure change causes the Bourdon tube to straighten out (with an increase in pressure), working a system of levers and gears, which causes the thermometer pointer to move over the dial and register temperature.

TEMPERATURE SWITCHES

Temperature switches operate from temperature changes occurring in an enclosure, or in the air surrounding the temperature-sensing element. The operation of the temperature switch is similar to the operation of the pressure switch shown in figure 8-9; both switches are operated by changes in pressure. The temperature element is arranged so a change in temperature causes a change in the internal pressure of a sealed-gas or air-filled bulb

or helix, which is connected to the actuating device by a small tube or pipe. Figure 8-15 shows a temperature switch and two types of sensing elements.

A temperature change causes a change in the volume of the sealed-in gas, which causes movement of a bellows. The movement is transmitted by a plunger to the switch arm. The moving contact is on the arm. A fixed contact may be arranged so the switch will open or close on a temperature rise. This allows the switch contacts to be arranged to close when the temperature drops to a predetermined value and to open when the temperature rises to the desired value. The reverse action can be obtained by a change in the contact positions.

GAUGE SNUBBERS

The irregularity of impulses applied to the fluid power system by some pumps or air compressors causes the gauge pointer to oscillate violently. This makes reading of the gauge not only difficult but often impossible. Pressure oscillations and other sudden pressure changes existing in fluid power systems will also affect the delicate internal mechanism of gauges and cause either damage to or complete destruction of the

Figure 8-15.—Temperature switch with two types of sensing elements. A. Bulb unit. B. Helix unit.

gauge. A pressure gauge snubber is therefore installed in the line that leads to the pressure gauge.

The purpose of the snubber is to dampen the oscillations and thus provide a steady reading and protection for the gauge. The basic components of a snubber are the housing, fitting assembly with a fixed orifice diameter, and a pin and plunger assembly (fig. 8-16). The snubbing action is obtained by metering fluid through the snubber. The fitting assembly orifice restricts the amount of fluid that flows to the gauge, thereby snubbing the force of a pressure surge. The pin is pushed and pulled through the orifice of the fitting assembly by the plunger, keeping it clean and at a uniform size.

Figure 8-16.—Pressure gauge snubber.