On Mon, 15 Apr 2013 19:04:54 -0400, Spehro Pefhany
wrote:

On Mon, 15 Apr 2013 20:02:01 +0000 (UTC), the renowned 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?

107 ohms is about 65°F on a Pt100 alpha = 0.00385 RTD. A bit cool for
room temperature, but okay..

There are other types of RTDs.. but the old "US Standard" alpha =
0.00392 is not very much different from the DIN standard at 40C (less
than 1 degree C). The old J Pt100 Japanese standard was similar to the
obsolete US standard. Everyone uses the Euro standard now, pretty
much.

What you are saying does not make a lot of sense. What is the
temperature of the developer? 100F is 37.8C, so very close to what you
say is the upper limit of the controller (but usually the standard
range is to 49.9C).

Is this thing sitting all apart? Are the sensor leads grounded at any
point? (Unplug the temperature controller, remove the sensor from the
bath and measure the leads to earth with an ohmmeter - power off).

It is possible they have grounded one of the sensor leads to reduce
bobble in the controller- the E5CS is/was an early switchmode supply
controller. Usually Pt100 probes are floating wrt to the shell.. is
the shell floating on the new probes? Any vigorous EMI sources kicking
around? Is the developer tank earthed?

Does it make a difference if the controller is calling for heat or
not?


One possiblity is that there is not much wrong, but the setup is not
properly wired so the controllers appear to be malfunctioning. Maybe a
heater is leaking AC current into the tank. If you take an AC
voltmeter and hold one lead in your hand and touch the other in the
tank do you read any voltage?

If you replace, be sure to use a decent auto-tune PID controller-
AFAIR, the C41 process is pretty fussy .. IIRC (frm a long time ago)
the spec is +/-0.1F, which is not easy.




Best regards,
Spehro Pefhany

If the spec is +/- 0.1F, then a 2-wire RTD ain't going to get it.
4-wire preferred, and that's pushing it, but I seem to recall the
Omrons taking 3. I always specified 4-wire and put two leads from one
end under the + terminal, the other two under the measurement and -
terminals. I also always used ungrounded sheathed elements. Don't
recall ever seeing a grounded RTD, but you could get t/c's that way.
A/C heater leakage played havoc with grounded t/c's.

Pete Keillor