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Wondering if anyone has suggestions on a type of integrated sensor to use in either a mica or ceramic heater.  The heater will maintain a temp of 400°C, and I need a sensor to work consistently at that level.  Will an RTD work for this, or is the temp too high?

Yes, an RTD is still an option at this temperature.  Standard temperature tables for RTDs cover from -260°C to 962°C.  The trick is finding an insulation for the sensor that is good up to 400°C.  Perhaps you could use an integrated thin film element or ceramic element.

The limiting factor for an RTD is the insulation. A wirewound ceramic element can achieve 850 deg C. Themocouples can go much higher, but require special leadwire and cold junction compensation in control instrumentation (translation: expensive). Will the heater be in a vibration environment? If not, a simple ceramic potting compound can hold the sensor in place.

The heater and sensor will experience some vibration (its on plastic processing equip).  Will the ceramic compund hold?  I was thinking of manually mounting the sensor along on the heatsink.  Any suggestions?

Also, would an RTD or thermocouple be more accurate at these temps?

I am not clear as to what measurement accuracy you need at 400 C. Thermocouples would be appropriate and you may choose the right one for high temperature. Alternatively if Mica and Ceramic heaters are accessible you may use non contact IR thermometer generally used for high temperature measurement in furnaces. Ceramic IR heaters are also available with built in Thermocoples for controls  and you can use them.

Any time that you are making a temperature measurement there are several factors that need to be considered. One of the first considerations should be exactly what is it that you want to measure. Do you really want to measure the heater temperature or more likely you want to be able to measure some medium under the mounted heater. This will have a lot of influence on the type of sensor choice and the accuracy of your measurement. For example, using IR sensors over the heater will not likely tell you much about the temperature of the medium below the heaters, at least not with any accuracy. The heaters temperature will fluctuate rapidly and be much hotter than the medium below, especially as the watt density rises.

It may very well be that you want a sensor embedded into the metal beneath the heater, or maybe even a probe whose tip enters the stream of material being heated.

The range of temperature operation and the need for fast response will have impact on the design options. At 400 degree C you will find both thermocouples and RTDs constructions that are reliable, even with vibration. RTD's of thin film or wire on ceramic construction can give you reliable performance, possibly meet your time response requirements and are considered the most accurate of the sensing choices. Thermocouples come in many junction metals and different packages  with grounded or ungrounded junctions, or even exposed junctions for the fastest time response. Thermocouple selection should be made such that the junctions are least likely to corrode in the environment and temperature range. Corrosion is slow death for thermocouples and causes shifting readings that impact your process without any indication that this drift has occurred.

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