Temperature measurement
In industrial settings, temperature is one of the most commonly measured process variables. While various process variables like pressure, flow, speed, and pH are also critical to monitor, temperature is by far the most frequently measured. Some estimates suggest that temperature measurement alone accounts for up to 75% of all process measurements in industrial environments.
Historically, temperature has been measured in several ways, often based on the physical properties of materials. These methods include the use of gas or liquid expansion (for example, in traditional dial thermometers), where the change in volume of a gas or liquid corresponds to temperature changes.
As technology has advanced, electronic sensors have largely replaced traditional methods for measuring temperature. These sensors can be used to gather real-time temperature data and transmit it to remote control rooms or data acquisition systems for analysis. The main types of temperature sensors used in industry today are:
Thermocouple
A thermocouple is based on the fact that two different metals when welded together will generate a very low voltage (EMF) that is temperature dependant. Depending on the type of metals used this EMF is 15 to 50 microvolts. Because of the very low voltage used it is essential that all components of the measuring circuit comply with strict requirements. Also depending on the application it may be required that the wire is shielded with copper or stainless steel braiding to prevent outside noise.
Resistance Temperature Detectors (RTD)
RTDs are among the most widely used sensors in industrial temperature measurement. RTDs operate on the principle that the resistance of certain materials (typically platinum) changes predictably with temperature. The most common RTD is the PT100, which has a resistance of 100 ohms at 0°C and increases to 138.5 ohms at 100°C.
Thermistors or NTC
A thermistor is a type of semiconductor that behaves like a resistor but has a strongly temperature-dependent resistance. NTC stands for “Negative Temperature Coefficient,” meaning the resistance of the thermistor decreases as temperature increases. This makes thermistors ideal for temperature sensing in applications where temperature changes need to be monitored accurately within a specific range.
PTC
A PTC sensor is a resistor that exhibits a positive temperature coefficient, meaning that its resistance increases as the temperature rises. PTC sensors are used less frequently than thermistors but are still employed in certain applications where this characteristic is needed.
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