# RTD Returns Wrong Values in LabVIEW

Updated Feb 9, 2018

## Reported In

• NI-9217
• NI-9216
• NI-9226

### Software

• Measurement & Automation Explorer (MAX)
• LabVIEW

• NI-DAQmx

## Issue Details

• I have an RTD and am using the conversion function in LabVIEW. When I use the Convert RTD Reading.vi, I do not get the values I am expecting. How can I get the right readings?
• I'm using an RTD to measure temperature with NI-DAQmx. The readings are very different depending on which RTD Type I use. How does the RTD type affect my measurements?

## Solution

The Analog Input RTD Temperature channel configuration, as well as the Convert RTD Reading.vi function, are only suitable for platinum RTDs. If your RTD is not made of platinum, the voltage temperature relation will be different and the RTD convert function cannot be used. This VI converts the voltage from the RTD to temperature in Celsius.

The RTD conversion functions in DAQmx are described in detail in the DAQmx help document: Platinum RTD Types.

An RTD, or Resistance Temperature Detector, is a temperature sensing device with resistance that increases with temperature. An RTD is usually constructed with wire coil or deposited film of pure metal. RTDs can be made of different metals and have different nominal resistances, but the most popular RTD is platinum and has a nominal resistance of 100 Ω at 0 °C.

Numerous types of RTDs exist, and they are typically defined by their material, their nominal resistance, and their temperature coefficient of resistance (TCR). The TCR, or α, of an RTD is the average temperature coefficient of resistance of the RTD from 0 to 100 °C and is the most common method of specifying the behavior of an RTD. The TCR for platinum RTDs is determined by the Callendar-Van Dusen equation.

The Callendar-Van Dusen equation is as follows:

Temperatures below 0 °C:

RT = R0[1 + A × T + B × T2 + C × T3 × (T – 100 °C)]

Temperatures above 0 °C:

RT = R0[1 + A × T + B × T2]

T = temperature in degrees Celsius

RT = RTD resistance at temperature T

R0 = RTD nominal resistance at 0 °C

A, B, and C = coefficients are listed in the help document: Platinum RTD Types.