Thermometers are used to measure temperatures for a variety of HVAC service tasks. The thermometer is frequently used to determine the suction line or liquid line temperatures when charging cooling systems. These measurements are important because the proper calculation and adjustment of the superheat or subcooling temperature is critical to determining the correct system charge. Another common use for the thermometer is to measure return air and supply air temperatures to determine the temperature difference for the purpose of determining the airflow rate in a system.

Shown in Figure 1-5 are the various types of thermometers commonly in use. Included are simple pocket-style bimetal dial-indicator and digital thermometers and two versions of electronic thermometers. In addition to the examples shown, many other special-purpose thermometers are also available. Thermometers come in a variety of temperature ranges based on the ir intended use. For heating and air conditioning work, thermometers covering the temperature range of -40° F to 180° F or 0° F to 220° F are commonly used. Because pocket-style bimetal thermometers can be inaccurate and difficult to read, the use of electronic thermometers is advised for obtaining the most accurate temperature readings.

Electronic thermometers may h ave several different sensor probes to cover different temperature ranges. Sensor probes used with electronic thermometers are normally thermocouple, thermistor, or resistance temperature detector (RTD) probes. Thermocouple probes convert heat into low-level DC voltages that produce the temperature reading on the electronic thermometer. Thermocouple probes tend to be rugged and inexpensive.

Thermistor and RTD probes have sensing elements in which the resistance varies depending on the applied heat. The thermometer circuit translates the resistance into a temperature value, which is displayed on the meter. Thermistor probes tend to respond more quickly and produce higher outputs than RTD probes. On the other hand, RTD probes usually provide more stable , accurate, and linear readings than either the thermocouple or thermistor probes.

Digital multi meters (Item 11, page 11) equipped with thermocouple and/or thermistor probe accessories can also be used to make temperature measurements. Infrared temperature sensor accessories that can be used with digital multimeters are also available. These sensors can read the surface temperature or refrigerant line temperature just by pointing the sensor at the surface or line.

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