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).

The installed omron controllers max out at 39.9C, which really doesn't
give much wiggle room. The operating procedure for the machine is ignore
what the controllers say, but trim the reset pot on the front panel and
fiddle with the numbers until the the temp measured in the tank with a
normal thermometer is correct.

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).

The machine is in use, so and can only be played with at slow times.

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?

The original probes would have been what was cool in West Germany the mid
to late 1980s. It's not clear if the Omron controllers are for the
international Pt100 or Japanese version. They are all fairly old so it's
not out of the question they're all bad.

I've not tested if the new probes are floating or attached to the red or
white lead. Flipping polarity doesn't seem to have any effect. I've not
examined how and if the tank is directly grounded. There is a chain driven
mechanism that raises and lowers rack of film into the tanks. The machine
is mostly single phase 208. Motors and pumps are run with contactors and
heaters and fans with "advanced" solid state relays.

The highest tech thing in those machines are the omron controllers.
There's no know EMI sources, just other similar machines.

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

It does not. None of this machine makes sense. The rest are straight
forward- replace/rebuild the bad part and they work again.


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?

Will have to try that. Leakage makes sense as a cause, but again, a wire
wrapped around the probe's metal sheath dunked into the developer tank
doesn't cause weird readings of air temp, hand temp or even temp of a
container of developer. Just thought about it now, but the test of is this
an electrolytic reaction problem is bridge the container of developer with
probe to the main tank with a wire. Maybe the wire itself isn't reactive.

The cable running from the operator panel to the back of the machine is
three conductor, white, brown and green with a braided shield. I believe
the shield is chassis grounded, but I'd have to look at it again. The last
working but not OEM probe was somehow lost while trying to source
replacements, so it's not clear how or what was even connected internally.

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.

Somehow the size of the machine and all the adjacent tanks and the
constant pumping somehow make it possible.