Heating of the Rtd Wire Leads

It should be OK with a 4-wire RTD. The excitation will be driven by a constant current source, so changes in the resistance of those wires won't change the current through the sensing element, and there will only be a minute current flowing in the sensing wires, so again, changes in the resistance shouldn't affect the result significantly.

What's the instrument you're using to monitor the RTD? Transmitters are available to convert the signal to a 4-20 for instance. Just a thought.

I'm currently in the design phase and don't have any equipment in mind. However, I do have another questions.

The RTD is not fixed to the surface that it is measuring; contact will be done with slight pressure and no thermal conductive epoxy between the RTD and measured surface will be used. Can I expect accurate results?

Thanks for your help.

For good thermal contact, I would recomend Omegatherm 201. Its a paste that stays soft and is easily removed after you are done. Have you considered doing a calibration of the system once you have it up and running? There are super accurate infrared type no contact meauring devices that can verify what your RTD is telling you.

JGEngr,

The temp of 300F is well within the service range of the RTD alloys. You will want to ensure that any insulation on the wires is not deteriorated by this temperature. That is about 150°C, so look for a suitable insulation. I would be very skeptical about claims for accurate infrared temperature measurements, because of how dependent they are on the surface emissivity. The suggestions by others about a little insulation over the point of contact and possibly using a conductive paste are all valid. If you need speed and are using an automatic means of collecting the data, it is possible to do a prediction of the ultimate equilibrium temperature based on the rate of temperature change of the RTD. (This is the method used for rapid measurement of the temperature of humans using a sensor pointing into the ear for a few seconds.)

--John M.