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Thread: Thermometers

  1. #11
    yeknom02's Avatar
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    This might be a stupid question, but are thermocouples an impractical option?
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  2. #12
    Nicholas Lindan's Avatar
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    Quote Originally Posted by yeknom02 View Post
    This might be a stupid question, but are thermocouples an impractical option?
    A thermocouple is a differential temperature measurement. In order to find the thermocouple's temperature you need to know the 'cold-junction' temperature - the temperature of the connection of the voltmeter to the thermocouple.

    Because you need two temperature measurements - the thermocouple and the RTD (a temperature dependent resistor) used to measure the cold junction temperature - and the problem of maintaining an isothermal connection to the thermocouple, thermocouples aren't normally used for precision temperature measurement. They are best suited to measuring temperatures that are far from ambient - a few hundred degrees to a few thousand. That doesn't mean a thermocouple can't be used for ambient temperature measurements, but there are better ways.

    For electric measurement of temperature at ambient temperatures (-55 to +150C) a platinum RTD is the instrument of choice. The platinum RTD will need calibration - a triple point cell is the normal calibration standard.

    The Omega corporation maintains an information database on temperature measurement http://www.omega.com/temperature/z/zsection.asp
    Last edited by Nicholas Lindan; 02-14-2011 at 11:51 PM. Click to view previous post history.
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  3. #13
    Greg Davis's Avatar
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    I have both a Kodak Process Thermometer, Type 3 and a Paterson process thermometer. They both read the exact same and I was able to get them both for $25 each. Quite frankly, the Paterson is easier to read and responds to changes faster.
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  4. #14
    BetterSense's Avatar
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    This might be a stupid question, but are thermocouples an impractical option?
    Thermocouples suck. They are just a pain. The only reason to use a thermocouple is if the temperature you need to measure is too high for anything else (like greater than 800C) or you need the absolute cheapest thing. A lot of people make the mistake of noting the ubiquity of thermocouples and coming to the conclusion that that means they are good. Even a thermistor would be better for photo chemistry.

    And many platinum resistance thermometers will be pre-calibrated to within photographic tolerances. The behavior of platinum is very well known.
    f/22 and be there.

  5. #15
    flatulent1's Avatar
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    I'm partial to this one.

    Economical stainless steel waterproof dial thermometer for color and black and white processing. Accurate within a 1/2% over the entire range . Its adjustable clip allows it to be used with tanks or trays. Chemical resistant and rust proof, easy to read. Can also use for barbecuing
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  6. #16
    holmburgers's Avatar
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    I have a Jobo Color Thermometer (3321) and I know there's one currently on eBay for very cheap. Glass, mercury, high resolution, very accurate.

    It has incredibly fast response, and a good range, all in Celcius. I've never used a higher end thermometer, but I don't know how it could be better. Compared to a cheap glass one I have, it responds orders of magnitudes faster. Reliably sits on a temperature in 5 seconds or less.

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  7. #17
    jp498's Avatar
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    My weston dial thermometer has been very reliable for me for 20+ years and hasn't been babied either. Whether it's accurate or not I don't know for sure, but it is very precise in that it helps me develop consistent negatives and I'd notice if I were a few degrees off in the results.

    I shy away from the mercury/liquid thermometers; I've accidently rinsed them in hot water before and *snap*. Didn't even have to drop it to bust it. I suppose having one for reference is fine, but I would not choose to use one on a regular basis.

  8. #18
    holmburgers's Avatar
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    I've often wondered about exceeding the thermometers scale. Luckily, it's 55° C so it's possible but I've always been careful.

    It's speed is just really a huge bonus for me; compared to my meat/dial thermometer... well honestly there's no comparison (granted it's a cheap one). The dial takes 30 seconds or more to settle, and since I don't have temp. control on my faucet, getting immediate readings is a must for dialing in 20°C.
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  9. #19
    MattKing's Avatar
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    Here is what I've done:

    1) I found a good, consistent thermometer (in my case a Kodak Process thermometer, but something like a Jobo Colour thermometer would be great as well). I use that thermometer for calibration purposes only;
    2) I use on a regular basis a cheap digital kitchen thermometer with a probe at the end of a cord.

    The trick is that I regularly check the digital thermometer against the Process thermometer. If the digital thermometer wanders over time, I can easily compensate for that. In the case of my current digital thermometer, it has been really consistent.

    In my experience, the digital thermometers suffer more with problems with water ingress than they do from erratic behaviour. The design of my current thermometer seems to prevent that problem.
    Matt

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  10. #20

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    I have had for very many years and still use two Weston dial thermometers. They read room temperature at the same as our digital thermostat and both read the same also at 68o solutions. So if they are off they are both equally wrong and have no apparent effect on my negatives.

    http://www.jeffreyglasser.com/

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