How to Calculate Ohms/°C for an RTD

Introduction

All resistance elements (e.g. platinum, copper, gold, and aluminum) have a specific resistance value at a known temperature which changes in a predictable fashion according to the laws of nature. In this way, by measuring the resistance of an element, the temperature of the element can be determined from tables, calculations or instrumentation.

RTD

A resistance element is a critical material used in every RTD (Resistance Temperature Detector). A bare resistance element is too fragile and sensitive to be used in its raw form, so it must be protected by incorporating it into an RTD.

Resistance Temperature Detector is a general term for any device that senses temperature by measuring the change in resistance of a material. RTDs come in many forms but usually are sheathed.

An RTD probe is an assembly composed of a resistance element, a sheath, a lead wire, and a termination or connection. The sheath, a closed-end tube, immobilizes the element, protecting it against moisture and the surrounding environment. The sheath also provides protection and stability to the transition lead wires from the fragile element wires.

For more detailed information on PRIMUS RTDs, see DYK #3 – RTD Sensors.

Temperature Coefficient

The temperature coefficient of an element is a physical and electrical property of the material. It describes the average resistance change per unit of temperature from the ice point to the boiling point of pure water.

In 1983, the IEC (International Electrotechnical Commission) adopted the DIN (Deutsche Institute for Normung) standard of Platinum 100 ohm at 0ºC with a temperature coefficient of 0.385 ohms per degree centigrade. This is now the accepted industry standard in most countries.

• Resistance at the boiling point (100ºC) = 138.50 ohms
• Resistance at the ice point (0ºC) = 100.00 ohms
• Divide the difference (38.5 ohms) by the 100 ohms nominal value of the element
• The result is the mean temperature coefficient of 0.385 ohms per ºC

For practical purposes, the ohms per °C can be considered linear across the designed temperature range of the RTD.

Calculation Example 1

Using a DIN platinum RTD (all PRIMUS RTDs use platinum elements), what would the resistance be if the temperature was 65°C?

• 65°C x .385 ohms/°C = 25.03 ohms
• 25.03 ohms + 100.00 ohms nominal = 125.03 ohms
• Therefore, a reading of 65°C equates to a resistance of 125.03 ohms

Calculation Example 2

Using a DIN platinum RTD, what would the temperature be if the measured resistance was 115 ohms?

• 115.00 ohms – 100.00 ohms nominal = 15.00 ohms
• 15.00 ohms / .385 ohms/°C = 38.96 °C
• Therefore, a resistance reading of 115 ohms equates to a temperature of
• 38.96 °C