[BillA]

I have an environmental chamber which has an RTD thermostat which will maintain the temp at 22°C
- with a toggle switch I can select 'manual control', and then by means of a pot set the temp manually
- I substituted a 10 turn pot and relocated it, works fine

the problem:
I would like to be able to manually change the set point to something other than 22°, and then have the thermostat maintain that temp
2 apparent means to do this:
1) modify the control circuit to be able to externally change the (now fixed) set point, or
2) using a PID controller set up in LabVIEW, read the temp and provide a (variable) resistance output in place of the present pot

1) am talking to Forma Scientific now, but this is not how the chamber was designed to be used
- and I have only an interconnect wiring diagram, not the pc board schematic

2) what do I use to obtain a variable resistance output ?
(a Valhalla GPIB controlled resistance unit went for over $400 on eBay - and that is the ONLY such I've seen)

any ideas ?