I've been working on a large C-41 film developing machine that has vats of chemicals which must be held at exact temperates of 100F.
Each tank had 100 ohm platinum junction probes and the controllers are the digiwheel style Omron E5CS-somethingsomething units.
Most have failed and other methods are used to maintain chemistry temperature, but the critical one for developer is acting strange.
We got a new probe for it, a two lead Pt probe and wired it up into the cable harness going back to the controller of the machine using the original three leads.
When exposed to air the proble and Omron unit work fine. If you warm it with your hand, it works fine. If you dip it in a jug of any liquid it measures the temp corrected.
If the probe goes into any part of the developer, the temp reads as out of range (39.9C or around 106F).
Ok, maybe it's electrical leakage. Putting the probe in an electrically insulated sheath doesn't work either. Temps go out of limits or read really high.
Ok, maybe the probe isn't grounded, or there's a weird electrical leakage issue, so I ground the probe to the original thermistor shield. No dice, temps our of range.
I even attached a lead to the metal probe and dumped it into the developer tank to see if it's electrical noise or leakage. No problems with temp readings in air or liquid if the probe housing is wired to the developer tank.
It just never works when immersed, which is baffling.
I tried the other "spare" controllers and they all seem to behave the same way, or are just dead. Tried other new probes, they all behave the same way too. They all measure 107ish ohms at room temp and the controllers are the correct ones for platinum probes.
The only and next move is just replace the temp controller. The omron stuff has crappy docs (good luck finding old data sheets) and is overly complex, so I'm steering towards something from Panasonic. No crazy hysteresis loops or fuzzy logic are needed. If the heater has to cycle every 10 seconds, that's fine as the racks going in an out of the tanks can shock the temperatures, and fast recovery is needed.
The machine itself has no docs, is no longer made and there are no original parts available. Having custom parts made for these things is becoming the norm, and pretty expensive. The original probes were sealed in plastic tubes, but the reason why isn't known. There are no spares of even duds left to take apart. I tried replacing the cable of about 10 feet between the controller and probe, but it behaved about the same way but still registered a temp too high.
Has anybody come across something this strange, or know what may be causing it?