Temperature Measurement Basics

What Is Temperature?

Qualitatively, the temperature of an object determines the sensation of warmth or coldness felt by touching it. More specifically, temperature is a measure of the average kinetic energy of the particles in a sample of matter, expressed in units of degrees on a standard scale.

For more information on temperature, read the temperature tutorial.

There are numerous technologies and sensors for measuring temperature, however, two of the most common types of sensors are thermocouples and RTDs. See how to measure temperature with a thermocouple and an RTD below.

Temperature Measurement with a Thermocouple

You cannot simply connect the thermocouple to a voltmeter or other measurement system to measure a thermocouple Seebeck voltage. This is because connecting the thermocouple wires to the measurement system creates additional thermoelectric circuits.

Consider the circuit illustrated in Figure 1. A J-type thermocouple is in a candle flame that has a temperature you want to measure. The two thermocouple wires are connected to the copper leads of a DAQ board. Notice that the circuit contains three dissimilar metal junctions -- J1, J2, and J3. J1, the thermocouple junction, generates a Seebeck voltage proportional to the temperature of the candle flame. J2 and J3 each have their own Seebeck coefficient and generate their own thermoelectric voltage proportional to the temperature at the DAQ terminals. To determine the voltage contribution from J1, you need to know the temperatures of junctions J2 and J3 as well as the voltage-to-temperature relationships for these junctions. You can then subtract the contributions of the parasitic junctions at J2 and J3 from the measured voltage at junction J1.

Thermocouples require some form of temperature reference to compensate for these unwanted parasitic "cold" junctions. The most common method of cold junction compensation is to measure the temperature at the reference junction with a direct-reading temperature sensor and subtract the parasitic junction voltage contributions. This process is called cold-junction compensation. You can simplify computing cold-junction compensation by taking advantage of some thermocouple characteristics.

For more information on measuring temperature with thermocouples, read the thermocouple tutorial.

Temperature Measurement with an RTD or Thermistor

Because RTDs and thermistors are resistive devices, you must supply them with an excitation current and then read the voltage across their terminals. If extra heat cannot be dissipated, I2R heating caused by the excitation current can raise the temperature of the sensing element above that of the ambient temperature. Self-heating will actually change the resistance of the RTD or thermistor, causing error in the measurement. The effects of self-heating can be minimized by supplying lower excitation current.

RTD and thermistor output signals are typically in the millivolt range, making them susceptible to noise. Low-pass filters are commonly available in RTD and thermistor data acquisition systems, and they can effectively eliminate high-frequency noise in RTD and thermistor measurements. For instance, low-pass filters are useful for removing the 60 Hz power line noise that is prevalent in most laboratory and plant settings.

You can also significantly improve the noise performance of your system by amplifying the low-level RTD and thermistor voltages near the signal source. Because RTD and thermistor output voltage levels are very low, you should choose a gain that optimizes the input limits of the analog-to-digital converter (ADC).

The easiest way to connect an RTD or thermistor to a measurement device is with a 2-wire connection.

With this method, the two wires that provide the RTD or thermistor with its excitation current are also used to measure the voltage across the sensor. Because of the low nominal resistance of RTDs, measurement accuracy can be drastically affected by lead wire resistance. For example, lead wires with a resistance of 1 ? connected to a 100 ? platinum RTD cause a 1 percent measurement error.

For more information on measuring temperature with RTDs read the RTD tutorial.