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  1. #1

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    Why Is My Thermometer Inaccurate?

    After using just one thermometer all last year for my developing and printing, I decided to splurge and buy 2 more when I made my last Freestyle order. However, one of them gives high readings by about 4 degrees at what is supposed to be 68 degrees. These are the inexpensive glass thermometers, which I assume have mercury in them? Has anyone ever had one that was off? I suppose I could always mark it w/ a bit of paint to match my others at 68 degrees, but it seems odd that a glass thermometer would be inaccurate. I know why my expensive dial one is crazy...that drop on the floor has given it a permanent 10 degree optimism.

  2. #2

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    I went trough some nightmares on this very subject. If you have the big glass dial style you can calibrate them by turning the hex nut on the stem. At least I could with mine anyway. On the ones you bought being mercury? If the liquid is a pretty silver you have mercury. If it's red or blue you then have a alcohol filled one like most of the cheaper style ones are. If it's and all glass one with no metal housing I would say 4 degrees off is a little much and I'd find out which one is off and send it back or you'll be sorry in the future.

  3. #3

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    Calibration fault and most are alcohol as mercury is toxic.

  4. #4
    Roger Cole's Avatar
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    Quote Originally Posted by Xmas View Post
    Calibration fault and most are alcohol as mercury is toxic.
    As the above poster said, blue or red is dyed alcohol, silver is mercury.

    Mercury is toxic of course but not nearly AS toxic as some people apparently think. It seems common to confuse metallic mercury with toxic mercury salts like the one that famously killed someone when a drop penetrated her lab glove. Back not too many years before I was in high school it wasn't unusual for school labs to have metallic mercury, and for kids to even roll bits around in their hands. NOT safe, as it does penetrate the skin to some degree, but far from instantly drop-dead deadly either.

    Handle with care if you break one, but I wouldn't worry about them otherwise.

    But if you got it at Freestyle recently it will be alcohol. The old Kodak mercury thermometers are prized, sometimes sought after (and expensive to legally ship.) Fortunately these days accurate electronic thermometers mean you can get lab grade accuracy without mercury or the fragility of glass.

    I have a couple of the big dial type, but I consider them useless for anything except wash water temperature. You CAN calibrate them by turning the nut, but the calibration is only good in a relative small range. So calibrate for 68 or 75 or whatever you use and it will be good enough for verifying that. But if you then decide to run some color at 100F you'll need to re-calibrate etc.

  5. #5
    Steve Smith's Avatar
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    Something I read here a few years ago:

    "If you have one thermometer, you know what the temperature is. If you have two (or more) you never know".

    It's the same with clocks and watches.


    Steve.
    "People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.

  6. #6

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    Hello,
    I'm a chemist and work in the laboratory now for over 35 years. In this time I used many dozens of laboratory glas thermometers, filled either with mercury or with other fluids. For normal routine work the cheap glas thermometers have greater variations and two examples can differ sometimes by 2 or 3 degrees which is quite normal. For more exact work you have to order more expensive instruments whith greater accuracy. The best thermometers are officially certified with a calibration document which contains the readings at various temperatures. You can easily check the correct value of the zero point (° C, centigrade) which a mixture of snow or crushed ice and water, but only if the instrument has 0° C on its scale. Sometimes with mercury thermometers the mercury thread is mechanically interrupted, it can be re-combined by slowly and cautious heating in a vertical position until the mercury and the bubble reach the upper expansion bubble of the capillary. Another way is to compare your instruments with an certified one and mark the deviation. Mercury thermometers react faster than alcohol thermometers but if they break, you have a problem with the poisonous mercury vapour in your home. In a laboratory this problem is not so big as the air is renewed several times per hour. For exact measurements it is important to use the correct immersion length or to immerse the complete thermometer. It should be marked on the instrument whether it is for total immersion or for which immersion length. For most photographic processes you always need the same temperature (68° F or 20° C or the higher colour process temperatures). So it is enough to check your instruments at these temperatures. After my experience the cheap photo thermometers are not very accurate.

  7. #7
    Roger Cole's Avatar
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    I agree the cheap ones are often inaccurate. This doesn't matter much for black and white once you adjust your personal development times accordingly, as long as you always use the same thermometer.

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    Quote Originally Posted by Steve Smith View Post
    Something I read here a few years ago:

    "If you have one thermometer, you know what the temperature is. If you have two (or more) you never know".

    It's the same with clocks and watches.


    Steve.
    Exactly!

  9. #9

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    Thank you for the well informed responses. It was a $4 thermometer, but still. Interesting that it is alcohol in there, not mercury. In school science class we used to play w/ a blob of mercury, letting it roll around in out hands. I'm still alive. Like Roger said, I only use my dial thermometer for mixing up D76 at around 130 degrees, where I have a 10 degree flexibility. Sending the thing back won't work as it would probably cost what the thermometer is worth. I'll either buy another one or mark this one at 68 degrees based on the readings from the other two thermometers, which actually do agree w/ each other. I went through some bad film developing until I figured out what the problem was.

    Which brings me to another topic, just to get off topic for a moment. If someone is just setting out in film developing, make sure you get all your gradient cylinders from the same plastic. I bought mine in different places, and their differences in construction drive me batty. While my fixer and stop bath cool down very quickly due to the thin walls of their construction, my more thickly constructed developer gradient takes a lot longer to cool down. I always have to remember to grab the first two gradients out of the bath before they get way too cold, while the developer slowly cools down. It would have been simpler to have them made out of the same stuff. I run into the same problem in the darkroom when doing 8x10 prints, as two of my trays are thin walled and the others are thick walled. Setting them in an ice or cold water bath means things cool down at different times. In hindsight, it would have been wiser to have bought all of them together as I did the larger trays. Fortunately I don't need spot on temperatures for the darkroom printing.
    Last edited by momus; 02-25-2014 at 02:20 AM. Click to view previous post history.

  10. #10

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    Global warming.

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