Thermometers are used to measure heat. A thermometer should be chosen according to its application. Consider first the kind of installation—direct mounting or remote reading.
If remote readings are necessary, then the vapor tension thermometer is best. It has a closed, filled Bourdon tube. A bulb is at one end for temperature sensing. Changes in the temperature at the bulb result in pressure changes in the fill medium. Remote reading thermometers are equipped with 6 ft. of capillary tubing as standard. Other lengths are available on special order.
The location of direct or remote reading is important when choosing a thermometer. Four com- mon types of thermometers are used to measure temperature:
- Pocket thermometer
- Bimetallic thermometer
- Thermocouple thermometer
- Resistance thermometer
Pocket Thermometer
The pocket thermometer depends upon the even expansion of a liquid. The liquid may be mercury or colored alcohol. This type of thermometer is versatile. It can be used to measure temperatures of liquids, air, gas, and solids. It can be strapped to the suction line during a superheat measurement. For practical purposes, it can operate wet or dry. This type of thermometer can withstand extremely corrosive solutions and atmospheres.
When the glass thermometer is read in place, temperatures are accurate if proper contact is made between the stem and the medium being measured. Refrigeration service persons are familiar with the need to attach the thermometer firmly to the suction line when taking superheat readings. See Fig. 1-27A and B. Clamps are available for this purpose. One thing should be kept in mind, that is, the depth at which the thermometer is to be immersed in the medium being measured. Most instruction sheets point out that for liquid measurements the thermometer should be immersed so many inches. When used in a duct, a specified length of stem should be in the airflow. Dipping only the bulb into a glass of water does not give the same readings as immersing to the prescribed length.
Shielding is frequently overlooked in the application of the simple glass thermometer. The instrument should be shielded from radiated heat. Heating repairpersons often measure air temperature in the furnace bonnet. Do not place the thermometer in a position where it receives direct radiation from the heat exchanger surfaces. This causes erroneous readings.
Fig. 1-27 Thermometers used to measure superheat. (Marsh)
The greatest error in the use of the glass thermometer is that it is often not read in place. It is re- moved from the outlet grille of a packaged air conditioner. Then it is carried to eye level in the room at ambient temperatures. Here it is read a few seconds to a minute later. It is read in a temperature different from that in which it was measured.
A liquid bath temperature reading is taken with the bulb in the bath. It is then left for a few minutes, immersed, and raised so that it can be read.
A simple rule helps eliminate incorrect readings:
- Read glass thermometers while they are actually in contact with the medium being
- If a thermometer must be handled, do so with as little hand contact as Read the thermometer im- mediately!
A recurring problem with mercury-filled glass thermometers is separation of the mercury column. See Fig. 1-28. This results in what is frequently termed as a split thermometer. The cause of the column’s splitting
Fig. 1-28 Mercury thermometer. (Weksler)
is always rough handling. Such handling cannot be avoided at all times in service work. Splitting does not occur in thermometers that do not have a gas atmosphere over the mercury. Such thermometers allow the mercury to move back and forth by gravity, as well as temperature change. Such thermometers may not be used in other than vertical positions.
A split thermometer can be repaired. Most service thermometers have the mercury reservoir at the bottom of the tube. In this case, cool the thermometer bulb in shaved ice. This draws the mercury to the lower part of the reservoir. Add more ice or salt to lower the temperature, if necessary. With the thermometer in an up- right position, tap the bottom of the bulb on a padded piece of paper or cloth. The entrapped gas causing the split column should then rise to the top of the mercury. After the column has been joined, test the service thermometer against a standard thermometer. Do this at several service temperatures.
Bimetallic Thermometers
Dial thermometers are actuated by bimetallic coils, mercury, vapor pressure, or gas. They are available in varied forms that allow the dial to be used in a number of locations. See Fig. 1-29. The sensing portion of the instrument may be located somewhere else. The dial can be read in a convenient location.
Bimetallic thermometers have a linear dial face. There are equal increments throughout any given dial ranges. Dial ranges are also available to meet higher temperature measuring needs. Ranges up to 1000°F (537.8°C) are available. In four selected ranges, dials giving both Celsius and Fahrenheit readings are avail- able. Bimetallic thermometers are economical. There is no need for a machined movement or gearing. The temperature-sensitive bimetallic element is connected directly to the pointer. This type of thermometry is well adapted to measuring the temperature of a surface. Dome-mounted thermal protectors actually react to the surface temperature of the compressor skin. These thermometers are used where direct readings need to be taken, such as on:
- Pipelines
- Tanks
- Ovens
- Ducts
- Sterilizers
- Heat exchangers
- Laboratory temperature baths
Fig. 1-29 Dial-type thermometer. (Weksler)
The simplest type of dial thermometer is a stem. The stem is inserted into the medium to be measured. With the stem immersed 2 in. in liquids and 4 in. in gases, this thermometer gives reasonably accurate readings.
Although dial thermometers have many uses, there are some limitations. They are not as universally applicable as the simple glass thermometer. When ordering a dial thermometer, specify the stem length, scale range, and medium in which it will be used.
One of the advantages of bimetallic thermometry is that the thermometer can be applied directly to sur- faces. It can be designed to take temperatures of pipes from 0.5 through 2 in.
In operation, the bimetallic spiral is closely coupled to the heated surface that is to be measured. The thermometer is held fast by two permanent magnets. One manufacturer claims their type of thermometer reaches stability within 3 min. Its accuracy is said to be plus or minus 2 percent in working ranges.
A simple and inexpensive type of bimetallic thermometer scribes temperature travel on a load of food in
transit. It can be used also to check temperature variations in controlled industrial areas. The replacement chart gives a permanent record of temperature variations during the test period.
Bimetallic drives are also used in control devices. For example, thermal overload sensors for motors and other electrical devices use bimetallic elements. Other examples will be discussed later.
Thermocouple Thermometers
Thermocouples are made of two dissimilar metals. Once the metals are heated, they give off an EMF (electromotive force or voltage). This electrical energy can be measured with a standard type of meter de- signed to measure small amounts of current. The meter can be calibrated in degrees, instead of amperes, milliamperes, or microamperes.
In use, the thermocouples are placed in the medium that is to be measured. Extension wires run from the thermocouple to the meter. The meter then gives the temperature reading at the remote location.
The extension wires may be run outside closed chests and rooms. There is no difficulty in closing a door, and the wires will not be pinches. On air-conditioning work, one thermocouple may be placed in the supply grille and another in the return grille. Readings can be taken seconds apart without handling a thermometer.
Thermocouples are easily taped onto the surface of pipes to check the inside temperature. It is a good idea to insulate the thermocouple from ambient and radiated heat. Although this type of thermometer is rugged, it should be handled with care. It should not be handled roughly. Thermocouples should be protected form corrosive chemicals and fumes. Manufacturer’s instructions for protection and use are supplied with the instrument.
Resistance Thermometers
One of the newer ways to check temperature is with a thermometer that uses a resistance- sending element. An electrical sensing unit may be made of a thermistor. A thermistor is a piece of material that changes resistance rapidly when subjected to temperature changes. When heated, the thermistor lowers its resistance. This de- crease in resistance makes a circuit increase its current. A meter can be inserted in the circuit. The change in cur- rent can be calibrated against a standard thermometer. The scale can be marked to read temperature in degrees Celsius or degrees Fahrenheit.
Another type of resistance thermometer indicates the temperature by an indicating light. The resistance-sensing bulb is placed in the medium to be measured. The bridge circuit is adjusted until the light comes on. The knob that adjusts the bridge circuit is calibrated in degrees Celsius or degrees Fahrenheit. The knob then shows the temperature. The sensing element is just one of the resistors in the bridge circuit. The bridge circuit is described in detail in Chap. 3.
There is the possibility of having practical precision of ±1°F (0.5°C). In this type of measurement, the range covered is –325 to 250°F (-198 to 121°C). A unit may be used for deep freezer testing, for air-conditioning units, and for other work. Response is rapid. Special bulbs are available for use in rooms, outdoors, immersion, on surfaces, and in ducts.
