What is a resistance thermometer?
A resistance thermometer or resistance temperature detector (RTD) is a device which measures temperature through the resistance of a conductor. Resistance of the conductor can vary with time. It is this property of the conductor that is used for industrial temperature measurement. The RTD’s primary purpose is to produce a resistance alteration in response to temperature.
Metals usually exhibit high temperature coefficients, indicating resistance increases as temperature rises. On the other hand, carbon and germanium typically demonstrate low temperature coefficients, thereby showing a resistance decrease with increasing temperature.
The material used in a resistance thermometer
The resistance thermometer has a sensitive element that is made from the purest metals such as platinum, copper, or nickel. There is a direct connection between the resistance of the metal and temperature. In most cases, platinum is used in resistance thermometers due to its high accuracy, stability, and its ability to withstand extremely high temperatures.
Metals like gold and silver are not used for RTD heat trace because they don’t exhibit the high resistance that is needed, they have low resistivity. Whereas a material like tungsten has high resistance but is very brittle. Copper is often used to make the RTD element as it has low resistivity and is a cheaper metal. The only downside to copper is that is has less linearity.
The maximum temperature of the copper is around 120C. An RTD material is made from either platinum, nickel, or nickel alloys. Nickel wires work well within a certain temperature range but are not linear. The RTD sensor requires a conductor with a high resistivity so a small amount of conductor volume can be used. The resistance should vary as much as possible with temperature.
Construction of a resistance thermometer
The resistance thermometer is put inside a protective tube to keep it from becoming damaged. Platinum wire is wound around a ceramic bobbin to form the resistive element, which is placed inside a stainless or copper steel tube. Lead wire is attached to the element and external lead and covered with an insulated tube to protect against short circuits. Ceramic is an insulator for high-temperature elements, while glass or fibre is used for low-temperature.
How a resistance thermometer works
The tip of the resistance thermometer is positioned near the heat source and heat is evenly distributed across the resistive element. Changes in the resistance vary the temperature of the element and the final resistance is measured.
Linear approximation
Linear estimation is predicting the resistance-temperature correlation using a linear equation. Quadratic approximation is a precise guess of the resistance-temperature connection expressed as a quadratic equation.
Quadratic approximation
Quadratic approximation is a precise guess of the resistance-temperature connection expressed as a quadratic equation.
The resistance thermometer is less responsive, and the material used to make the element is cheaper.
How can TRM help?
If your business manufacturing processes need heat management then temperature monitoring and control will be extremely important to you. Our high-temperature cable and measurement sensors can suit practically any environment in any industry.
The team at TRM are experts in heat management and industrial temperature measurement, offering services relating to design, supply, and installation as well as providing a wide selection of temperature-related products. Contact us today to discuss the specific requirements of your operations and find out how we can help you.