RTD Pt100 probe resistance versus temperature

[richiem]

Pt100 probes are temperature variable resistors. By design and definition, they have a resistance of 100 ohms at 0° Centigrade.

A quick check of the working condition of your Pt100 temperature probe is to immerse the probe in a slurry of ice and water (a big glass or a quart jar full) and connect a hand-held digital multimeter in low-ohms mode (200 ohms or 2k ohms full scale) and check the resistance of the probe.

Most Pt100 probes have three wires, two of the same color. Connect the ohmmeter between one of the two wires of the same color and the wire with a different color. The resistance should be very close to 100 ohms, give or take a couple of ohms AT MOST.

A further check is to put the probe into rapidly boiling water -- if you're within 500 feet of sea level, the resistance should read near 139 ohms, again give or take a couple of ohms AT MOST.

Pt100 probes are actually pretty linear, so the average slope of the curve of resistance versus temperature can be used for intermediate (or somewhat higher than 100°C -- say up to 200°C) temperatures. The formula for the resistance versus temperature for a Pt100 probe is:

R = (0.385 * Temp °C) + 100

R = (0.214 * Temp °F) + 93.15

Conversely,

Temp °C = (R/0.385) - 259.74

Temp °F = (R/0.214) - 435.29

I hope this helps.