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  1. #1
    Andrew Moxom's Avatar
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    Note to self on digital thermometers

    Never take for granted what your digital thermometer is telling you..... The last few film developing sessions have been a little frustrating. No matter what film I tried, and developers used, I thought my negs were a little thinner than usual. This used to happen on the odd occasion, but became more regular. Exposure wise, I know I am bang on accurate. So this weekend I noticed that ALL of my 5 rolls that were developed, were thinner than I really like. They are all printable, but not as robust as I normally like them. To that end, I bought a new digital thermometer and I found my original digital thermometer I use was over stating the temperatures by 2 degrees!! That means I'm developing film at 66 deg F!!

    As a test, I put the old and new thermometer probes into the hot water kettle while boiling for a test and the old one is consistently showing higher temps than it should. I placed new batteries into the old unit... No change.

    So I plan to test in the next week on my film and belive this will cure the thinner negs.

    So note to everyone... Never take anything for granted thermometer wise if you are using a digital thermometer. They can and do drift!!! :-)
    Please check out my website www.amoxomphotography.com and APUG Portfolio .....

  2. #2
    Thomas Bertilsson's Avatar
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    I'm glad you were able to figure out what went wrong.
    "Often moments come looking for us". - Robert Frank

    "Make good art!" - Neil Gaiman

    "...the heart and mind are the true lens of the camera". - Yousuf Karsh

  3. #3
    df cardwell's Avatar
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    "ever take for granted what your digital thermometer is telling you"

    Never take for granted what ANY thermometer is telling you.
    "One of the painful things about our time is that those who feel certainty are stupid,
    and those with any imagination and understanding are filled with doubt and indecision"

    -Bertrand Russell

  4. #4

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    Good reminder. Anything that relies on a sensor can drift. Even with a mercury thermometer the mercury column can separate (but fixable).

  5. #5
    Worker 11811's Avatar
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    A container full of ice water is often a better standard to calibrate a thermometer with. The freezing point of water is more constant than the boiling point. The boiling point of water can vary depending on altitude. But for extremes of altitude/pressure or the addition of salt or other substances, the freezing point of water does not vary. At least not by an amount that can be measured without laboratory equipment.

    I have a digital thermometer which can be recalibrated. Its instructions call for a container of crushed ice and water to be used as a calibration standard.

    Quibbling aside... Yes! I agree. You should not take your thermometer for granted. If your thermometer can be calibrated, by all means do so!

  6. #6

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    Never have just one thermometer

    Probably many folk know this 'cos I read it here and in one of the better darkroom books, but I think the advice to never have just one thermometer is quite good. The idea is that all thermometers are likely off a little, and after a while you get used to your own private temperature scale. Inevitably when it breaks, your new one will show different readings.

    What one should do is to have more than one and note how they vary. First it will likely tell you if one has absolute errors, and second it will tell you their relative difference which you can use to adjust your readings when one breaks

  7. #7
    DWThomas's Avatar
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    I would vote for the ice/water calibration for another reason. One of the errors in digital (or analog!) stuff is non-linearity. The freezing point is much closer to the normal use temperature than the boiling point and likely to better represent the calibration for temperatures of interest. Alas, it never hurts to do a cross check every now and then

  8. #8
    Reinhold's Avatar
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    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/

  9. #9

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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/
    ...and that's exactly how I deal with the cheap kitchen thermometers. I have one glass analog thermometer of known accuracy from which I calibrate the el-cheapo ones. The dial thermometers I've collected over the years are horrible and useless. They're inaccurate and drift over the course of a single darkroom session. They look impressive, but they're not at all reliable.
    Frank Schifano

  10. #10
    bowzart'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/
    Right!

    A long time ago, I worked in a color lab in San Francisco. The boss was the biggest jerk on the planet (he'd suddenly open the door to your printroom to see whether you were working or ?). If you ever worked there, you know who I mean. However, I did learn some amazing things from him. One day, the UPS truck showed up with a box. "Oh, good, I've been waiting for this!"

    He opened the box. It was full of mercury/glass lab thermometers. He took one out, put it in his desk drawer, and handed the box to me with the instruction to put it in the stock room. I asked why he put one in the drawer.

    "That's the one that's right." He calibrated his processes with it, and checked the others against it.

    How many thermometers are there? Quite a few. How many agree?

    I have often used the Wing-Lynch water temp controller at 20°C as my standard. Even so, calibration can slip. The are still problems with the ice water standard, if you are going to be picky. Use distilled water. Finding 0°C isn't the whole picture, either. For a full calibration, you'd need the freezing AND boiling points, since the range can be off as well, and for the boiling point, the barometric pressure would have to be accounted. I suspect this would show up more often with digital, but glass/fluid types are manufactured at different quality standards. What if the column varied in the diameter of it's tube?

    Now, for my kitchen thermometer, I will test it with boiling water. So the turkey is two degrees too hot?

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