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  1. #21
    hrst's Avatar
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    Mercury/alcohol thermometers that have a separate scale usually have a small calibration line printed in the glass itself. It's usually at 20C. I've seen many meters that have the scale severely misaligned but are very accurate after this tiny line is aligned correctly to 20C at the scale. So, if the scale is separate, you can probably move the scale to calibrate it. If the scale is printed to the meter and is misaligned, you have to find the error and do the math every time.

    Digital thermometers can be very accurate but they must be calibrated and tend to drift, so calibration must be redone every now and then. The sensor has to be well insulated against water. If not, it will drift severely.

    Mercury/alcohol thermometers don't seem to drift at all or very little. If there is a drift, it may have received a physical shock and the mercury/alcohol is in many pieces. To fix it, just put it in a freezer so that all the mercury/alcohol goes to the "ball" where it comes from.

    If the meter does not have a calibration system on it's own, you just have to measure its error and add or subtract every time you read it.

    Fever thermometers (digital or mercury/alcohol) are usually very accurate but the range is small, however they are very good for color processing (37.8C/100F for C-41 or E6). Not good for BW, though.
    Last edited by hrst; 02-24-2010 at 02:23 AM. Click to view previous post history.

  2. #22

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    Many thermometers are inaccurate. My 100$ Kodak Therm is inaccurate... when I compare it to another thermometer. The secret is to stick to ONE thermometer, even if inaccurate, understand it and work from there, usually by adding a minute or two, depending on how thin the negs are so start with. My therm, even if inaccurate, is consistent thru time and that's all I want from it: consistence.

  3. #23
    Martin Aislabie's Avatar
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    Quote Originally Posted by Reinhold View Post
    The coefficient of expansion of mercury is based on the unvarying laws of God.

    Those impressive LED numbers on that hi-falootin elektronik gizmo are only a read-out from a collection of parts put together by monkeys.

    Get a good gla$$ thermometer and use it to confirm all your temperature dependent instruments and processes. Once everything is confirmed, put it away in a safe place as a standard reference. Do not use it routinely, it can be broken...

    Reinhold

    www.classicBWphoto.com
    if that doesn't work, try: http://mysite.verizon.net/res14rg7y/
    Reinhold is absolutely on the money with this

    Consistency in B&W is the key

    It doesn't actually matter is your reference thermometer is actually accurately reading 20C - what matters in B&W is that you always use the same indicated 20C consistently

    We all adjust our film and print processing time away from the manufacturers’ recommended starting point for time/temperature to give us the results we desire.

    So choose a good thermometer and calibrate all your others to it.

    Then use the temperature equivalent to 20C on your reference thermometer for all your other work.

    Keep your master/reference thermometer safe and don't use it for anything except calibrating work- as accidents sadly do all too frequently happen

    Martin

  4. #24

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    I use dial ones that are off a little, but I know it and compensate.

    They are checked against two Kodak process thermometers that hang only and are never used as working thermometers. One I used for 30 years. One was a hand me down from a year ago.

    These were $75 @ when new 30 years ago. Treat with care.

    Always remember a man with two watches never knows what time it is.

  5. #25

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    I believe that this is the reason that all developing times are suggestions and that you have to find the correct process time for you. If your negatives look consistantly "thin" with you would increase your developing time or aggitation. I found this out a few years ago when my trusted old mercury thermometer broke and had to be replaced with a "non-toxic" one (I assume alcohol based). It's probably calibrated differently from the old one (not available for comparison) but also takes much longer to equilibrate in use. So I had to adjust my processing times to the new "standard." People report the same happens when they move to a new location and the water is different. That's why they say "your milage may vary".

  6. #26
    Andrew Moxom's Avatar
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    After trying the new digital thermometer, I am happy to say neg densities are visibly (to the naked eye as I don't own a densitometer) up to about where they should be normally for the way I print. I tried Neopan 400 in HC-110 dilution H @ 68 Deg F for 10 minutes. Lovely tonal scale. Same for TMY-II in TMAX dev 1:4 for 7 minutes. Lots of good shadow detail and easy printable negs overall. I will be getting a good glass thermometer as the test one in the next few days to understand how much drift occurs over time to the new purchased digital one.
    Please check out my website www.amoxomphotography.com and APUG Portfolio .....

  7. #27
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    Last night I noticed that I had a pan of water that had been sitting in my trailer darkroom for a few days, and two thermometers - a small dial thermometer, and a mercury thermometer. I stuck them both in the pan, and wonder of wonders; while the dial had a lot less precision, they seemed to be right on at 59°F. It occurred to me that the most important comparison test would be the temperature that you actually use, especially if you use just one. Like 68°F/20°C. The range might possibly be different due to manufacturing standards, etc. between two thermometers, but if both give the same working temperature, it wouldn't matter. Would be important to repeat the test now and then, though.

  8. #28
    Martin Aislabie's Avatar
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    Quote Originally Posted by bowzart View Post
    Last night I noticed that I had a pan of water that had been sitting in my trailer darkroom for a few days, and two thermometers - a small dial thermometer, and a mercury thermometer. I stuck them both in the pan, and wonder of wonders; while the dial had a lot less precision, they seemed to be right on at 59°F. It occurred to me that the most important comparison test would be the temperature that you actually use, especially if you use just one. Like 68°F/20°C. The range might possibly be different due to manufacturing standards, etc. between two thermometers, but if both give the same working temperature, it wouldn't matter. Would be important to repeat the test now and then, though.
    Couldn't have put it better myself

    The only temperature range you are really interested in is 20C +/-3C or what ever your chosen processing temperature happens to be.

    Knowing accuracy at the freezing point and boiling point of water is of limited value.

    Personally, most of my very accurate thermometers don't have such a range, as I have sacrificed overall temperature range for an expanded scale around a chosen temperature range.

    Martin

  9. #29
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    Quote Originally Posted by Martin Aislabie View Post
    Reinhold is absolutely on the money with this

    Consistency in B&W is the key

    It doesn't actually matter is your reference thermometer is actually accurately reading 20C - what matters in B&W is that you always use the same indicated 20C consistently

    We all adjust our film and print processing time away from the manufacturers’ recommended starting point for time/temperature to give us the results we desire.

    So choose a good thermometer and calibrate all your others to it.

    Then use the temperature equivalent to 20C on your reference thermometer for all your other work.

    Keep your master/reference thermometer safe and don't use it for anything except calibrating work- as accidents sadly do all too frequently happen

    Martin
    Calibration can be as simple as taking your reference thermometer, sticking it in a pan or bowl of water adjusted to 20℃, putting another thermometer into the same pan or bowl of water, and seeing what matches up. It doesn't really matter if it says 20℃ or not, get a little bit of ladies nail polish and mark a line on it at your 20℃. Now it's calibrated. For a digital, if it can be adjusted, adjust it to read the same temperature. If it can't be, stick a label on it which tells you what 20℃ is equal to, include the calibration date.
    Paul Schmidt
    See my Blog at http://clickandspin.blogspot.com

    The greatest advance in photography in the last 100 years is not digital, it's odourless stop bath....

  10. #30

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    Quote Originally Posted by Reinhold View Post
    The coefficient of expansion of mercury is based on the unvarying laws of God.

    Those impressive LED numbers on that hi-falootin elektronik gizmo are only a read-out from a collection of parts put together by monkeys.

    Get a good gla$$ thermometer and use it to confirm all your temperature dependent instruments and processes. Once everything is confirmed, put it away in a safe place as a standard reference. Do not use it routinely, it can be broken...

    Reinhold

    www.classicBWphoto.com
    if that doesn't work, try: http://mysite.verizon.net/res14rg7y/
    I guarentee my Fluke thermometer is more accurate than any chem thermometer.
    I use a Fluke as reference for some Brannan 76mm Immersion Thermometers that sit in trays/tanks etc. The Brannans are reasonably accurate the Fluke is hyper accurate and calibrated professionally every year.

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