I am experiencing some Thermocouple reading problems. In a mini-scale reactor, under high pressures, the thermocouple measuring the temperature inside the vessel would start to fluctuate a lot at around 750C. Before that it would run smoothly. Any ideas on the problem would be greatly appreciated.

 

What type of thermocouple are you using?

If you are using a type K they are notorious for having difficulty between 1200-1400 deg F where carbon burn out occurs. You may want to try a Type N as it has the same temperature range, approximate cost and is less susceptible to hysteresis.

You may find the following article comparing the type N and type K to be of interest.

http://www.jms-se.com/catalog/TypeN_vs_TypeK_Thermocouple.pdf

 

I belive that you could use a thermowell to protect the thermocouple and reading against the high pressure. A type J thermocouple can do the job better than type k thermocouple. one of the component of type k tc, are made out of iron that allow the corrosion, giving in many cases false reading at high temperatures.

 

Type J is terrible for use in this application. As you can see, this person has a process at least to 750 C. That is the maximum range on Type J. Also, your comment about Type K having iron in it is completely off base since one of the legs of a Type J is iron. The pair is iron and constantan, a copper-nickel alloy. If you want to know more and need to make a custom app, use well documented tables to verify your problems. An oscilloscope isn't a bad idea either. Follow this link for Type J information. Spring loaded thermocouples work well to ensure thermowell contact, especially if thermal growth at higher temperatures is a problem. You must also consider the atmosphere when specifying a thermocouple. Read the links below as they will give you some guidance.

http://www.omega.com/temperature/Z/pdf/z203.pdf
http://www.omega.com/prodinfo/thermocouples.html

 

What kind of instrument are you using to read the t/c? Is it single or multiple channel? I have seen common mode voltage issues on multiple input devices show up at higher temperatures.