Temperature is measured using a resistance thermometer, which is a type of sensor. Examples include resistance temperature detectors (RTDs), platinum resistance thermometers (PRTs), and Pt100 sensors.
Temperatures between –200°C and 500°C are usually measured with these types of resistance thermometers. They work by predictably changing resistance as the temperature changes.
In this post, we’ll go over how resistance thermometers work and cover the main features of some common models.
How Does A Platinum Resistance Thermometer Work
Resistance temperature detectors (RTDs) such as the type made of platinum provide a linear output over an extensive temperature range.
They consist of a metal wire and a temperature-dependent resistor, also known as the thermistor. The thermistor is made of ceramic and glass, and its resistance changes with temperature. It is used to measure the temperature.
The resistance of a metal wire changes as it gets hotter, so this type of thermometer needs a way to distinguish the temperature-dependent change in resistance from the natural change in resistance as a metal wire heats up. That’s why platinum is often used – it has a high resistance that generates little error.
The metal wire is encased in a protective metal tube, and the two ends are attached to electrical connectors. One connector is attached to the thermistor, while the other is attached to a low-resistance reference. For example, this could be a length of platinum alloy with resistance very close to that of room temperature, such as platinum-rhodium.
The resistance of the reference should change minimally with temperature changes so it can be used to correct errors that might arise from the metal wire’s natural heating and cooling. One example is a length of constantan, which has a minimal temperature dependence and maintains its conductance over a wide range of temperatures.
The RTD is a useful device because its resistance output changes proportionally with the temperature. A differential amplifier compares the reference’s voltage to an electrical sensor measuring the metal wire’s resistance. With this type of circuit, a very small error in voltage from the reference can result in a much larger change in voltage from the sensor voltage, making the circuit more accurate.
6 Features of a Good Resistance Thermometer
1. Response Time
The time it takes for a resistance thermometer to record an accurate value of the measured object’s temperature is known as response time. Some examples include PRTs with lead wires requiring a 1-10 second response time and surface mount sensors with a 0.3 second response time.
2. Repeatability
Repeatability is how close each reading is to the previous one when measuring under unchanged conditions. In other words, how close each resistance thermometer reading is to the average of previous readings. This is because the reference junction does not cool instantly when taking a temperature reading, so it continues to contribute heat for some time afterwards.
3. Linearity
A resistance thermometer is linear if the change in temperature measurement is proportional to the change in temperature of the object being measured. This means that a resistance thermometer will provide an accurate reading as long as the temperature changes are minor/small or if it starts measuring at room temperature or close to it.
4. Short-Term Stability
Short-term stability is a resistance thermometer’s ability to maintain a constant value over short periods when used under unchanged environmental conditions. This means that factors such as vibration or shock will not affect the resistance thermometer’s accuracy or result in drift.
5. Long-Term Stability
Long-term stability is a resistance thermometer’s ability to maintain a constant value over long periods when used under unchanged environmental conditions. This means that factors such as vibration or shock will not affect the resistance thermometer’s accuracy or result in drift. The best way to test long-term stability is to take data over time while one factor is changed (focus on one factor at a time). Resistance thermometers that show little to no drift over this period are considered stable.
6. Temperature Coefficient of Resistivity (TCR)
The temperature coefficient of resistance is the change in resistance with respect to a change in temperature, which can be expressed in units °C-1 or K-1. Different materials that are used to construct PRTs have different TCRs. For example, bismuth has a TCR of -386 ppm/°C while nickel has a TCR of -2.5 ppm/°C, meaning that bismuth’s resistance will increase at a faster rate when the temperature increases by 1 °C. It is important to note that different PRTs are made up of different materials, making it difficult to compare their TCRs.
Resistance Thermometer Summary
Resistance thermometers are great devices for measuring the temperatures of objects with varying surfaces, be it above or below ground, whether it is hot or cold. They are typically not as fast as other devices but provide more accurate measurements than thermocouples and RTDs. You can always get them at your nearest supplies shop.
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