On Mon, 15 Apr 2013 20:02:01 +0000 (UTC), Cydrome Leader
wrote:

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?

Have you simply considered putting in a balancing resistor/pot?

Sometimes screwing the unscrutable is a pain in the ass..so find a
work around to make it work....is much simpler

Gunner