Vacuum thermocouple integration to PLC

We have a small stand alone refrigerator that is gearing up for a device test. The lead engineer has decided it's time to get the 24 or so channel of insulating vacuum integrated into the APACS. Til now the vacuum has been read visually on an analog Televac chassis.
The assigned eng. came to me with a complicated scheme involving a laptop, Labview and realtime file transfers. I told him since we already have PLCs linked to APACS via an OPC service, He just needed analogs of the vacuum delivered to an A-D card in the PLC(all PLC hardware is autoDirect 205 series). Then I started pricing vacuum gauge transmitters-about $275/channel. x24 + PLC hardware+ new 120AC for the tansmitters and we're looking at $10K+ for the project.

Now a vacuum thermocouple gauge tube is a noble metal thermocouple attached to a heater filament. Run a fixed current through the filament and its temperature will vary inversely with the gas pressure. higher pressure-lower temperature. Scale the resulting thermocouple V and you have a vac reading in microns.

If I have a 4-20mA A-D card driving the filament at 20mA, why can't I run the thermocouple to an appropriate module?

My only real question is, Can anyone tell me why the filament excitation for a typical gauge is AC? Teledyne specs 21mA AC for the filament current of the DV-6M tube. What do you see as the potential wrench in the works?

I know nothing about the vacuum transmitters you describe, but your explanation makes it seem like here are two disctinct and electrically separate circuits. One is the power for the heater circuit. The other is the thermoncouple (T/C) circuit. If that is correct, you can indeed run the thermocouple leads into a T/C module on the PLC to read the mV signal.

Have you tried talking to your existing sensor supplier? I would be surprised if they didn't have a system for 4-20 mA or 0-5 VDC in place or that you can add to tie the sensor to a standard PLC or chart recorder - that to me would be an essential option.

Another option is to talk to guys like Rosemont or Foxboro to see if they have an alternate technology vaccum sensor that would meet your needs for less money.