Temperature Probe for Zone VI Compensating Developing Timer

Discussion in 'Darkroom Equipment' started by Doremus Scudder, Jun 9, 2012.

  1. Doremus Scudder

    Doremus Scudder Member

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    I have a couple Zone VI Compensating Developing Timers, one now that is in need of a replacement temperature probe. Since I like this tool, I'd like to get a couple of probes in order to have one on hand for the future. I have a quote from Calumet, who will make them for me (no original parts left in stock apparently) for $75 each.

    While this is a reasonable offer, I'm hoping for a cheaper solution. I have no idea of the probe specs., voltage, etc. and was hoping someone here might be able to either provide me with enough info for me to cobble together my own probe or possibly point me to another source.

    Thanks in advance.

    Doremus

    www.DoremusScudder.com
     
    Last edited by a moderator: Jun 9, 2012
  2. Neal

    Neal Subscriber

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    Dear Doremus,

    Post a photo of the probe. the color of the wires could be a clue if it uses thermocouples. If it does, simply get some thermocouple wire of the same type, crimp the wires at one end and you have a probe.

    Neal Wydra
     
  3. Doremus Scudder

    Doremus Scudder Member

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    Neal,

    Unfortunately, I'm not near my darkroom for a month or so, so I can't post a picture. Google images is no help either.

    The Zone VI compensating developing timer came with two kinds of probes, one is a big, c-clamp kind of thing (see attached photo). The ones I have, however, are simply an 8-inch long metal probe, about 3/16-inch in diameter (oblong really). It attaches to the unit with standard phone wire and an RJ type plug, so, without disassembling, I can't see any wires/colors, etc.

    Best,

    Doremus
     

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  4. Neal

    Neal Subscriber

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    Dear Doremus,

    Guessing wildly, based on the connector it seems that the probe is more likely a thermistor than a thermocouple (with luck, someone else will jump in with better information). If that is the case, I don't see how you could determine the correct thermistor/resistor set to buy as a replacement without a working probe. If it is a thermocouple after all, you might be able to work up a cheap replacement.

    In any case, a visit to the Wikipedia pages on both thermocouples and thermistors as well as to omega.com should help you decide how far you want to take this.

    Good luck,

    Neal Wydra
     
  5. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    The chances are 99.9999%+ that what has failed is either the wire connecting the sensor clamp to the timer or one of the connectors.

    It looks like a telephone cable, if you can unplug both ends just replace it with a known good cable and see if that fixes it. Examine the ends of the cable where it goes into the connector/probe - if it is lumpy then there is a good chance the copper wire inside the cable has broken.

    Telephone connectors become noisy with time. A little spritz of contact cleaner on the connectors may also do the trick.

    The last thing you should be expecting to replace is the sensor element itself. I'd take a WAG and say it is an RTD (thermistor) or a solid-state temperature sensor.

    http://www.omega.com/temperature/z/ResistanceElements.html
    http://www.ladyada.net/learn/sensors/tmp36.html
     
  6. Doremus Scudder

    Doremus Scudder Member

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    Thanks Nicholas,

    I'll try cleaning connections/rewiring before I spring for a new probe and see if that helps.

    Best,

    Doremus
     
  7. Todd Foster

    Todd Foster Member

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    Simple mod increases probe life.

    Those of us persisting in using the evergreen Zone VI Compensating Timer need to add some strain relief to the end of the probe where the wire goes in. I found some vinyl covered cable that when stripped, fits the stainless probe tube exactly.

    Better would be heat shrink tubing, maybe 2 layers of it, about 2 1/2 inches long. I'll try Radio Shack next time I'm by for heat shrink. Think I'll put the probe in a glass of water to prevent any heat from getting to the sensor near the probe tip while shrinking the tubing. You're supposed to have a heat gun to shrink it, good luck using a propane torch at a considerable distance. It can be done but is not recommended by me. I got a cheap heat gun at Harbor Freight for less than $10 a couple years ago. Still new in box. Maybe a little silicone sealant under the plastic tubing ends just before heat shrinking is a good idea, and next time I have some high strength water resistant (that is the slow setting one, not the 5 minute) epoxy mixed, a little coat on the end of the sensor to reseal the epoxy plug in there just for good measure.

    Idea is to make the wire/probe joint rigid so the wires don't break from flexing right at the end of the metal probe tube. This is where all the work was done in the 29 years since my timer was made! Flex marks clearly visible there. Strain relief stops that flexing and moves it further out on the cable where presumably a break can be found and fixed in another 29 years, maybe. And yes, those phone plug connectors, RJ-11 I think, go bad. They are crimped onto the little wires and this tiny crimp can get loose. New connectors can be crimped onto the original wire for a fresh start. Home Depot used to have the connectors and the tool to crimp them on.

    This dang timer is unique and a great simplifier for both film and paper. Hope it runs for the rest of my photography run. Still some out there on ebay and the prices are down on them. I'd much rather have this thing than boot and run a computer just to use as a compensating timer. Not sure if that product is around anymore anyway. Last time I tried the site it was there, but scrambled, and the email address bounced as full.

    Best, Todd F.