I use a Johnson controls a419. There about $60 on eBay. You can make a well for them out of 3/8 copper tube and a compression fitting. I have one for my fermentor that controls a fish tank heater. The heater is inside a piece of 2 inch copper pipe (pipe has water in it) that I mounted to the fermentor lid and hangs inside my fermentor too 4 inches above the bottom of the tank. The sensor is in a well made from 3/8 copper pipe located near the bottom of the fermentor. I have also used one in my power controller to monitor 4 temperature sensors. I used a 2 pole 4 position switch. This allows for switching the sensor input.
One way to go would be a single loop controller that will give you alarms and a control output if you need it. The Omega CN730 series is a good one, inexpensive and Omega gives good after purchase support. The one with a pulsed DC output will switch a SSR, avoid the relay output for heat control because they wear out. You can buy PT100 elements in lots of configurations, I'd suggest them over TC because of price and accuracy. http://www.omega.com/pptst/CN730_Series.html" onclick="window.open(this.href);return false;" rel="nofollow
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