Can a process thermometer go off?

Discussion in 'Darkroom Equipment' started by erikg, Sep 24, 2013.

  1. erikg

    erikg Member

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    I have a Kodak process thermometer that I have trusted for many years that now seems to be at least 8 degrees off from my other thermometers. Can that happen? and if so how? I developed some film last night and it was telling me that the water from the tap was well below what the thermometer on the water panel was telling me. At first I started to adjust to conform to what the Kodak thermometer was giving me and then I realized that I never have water from the tap below 70F this time of year. Something had to be wrong, I checked the water with two other thermometers that I trust and they agreed with the water panel. The developed film looks right on so that seems to be confirmed. Am I crazy? This thermometer could not have been off that much all these years, so what happens to them?
     
  2. E. von Hoegh

    E. von Hoegh Member

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    Is it mercury or alcohol? Has the thread separated? Are the graduations on the stick itself, or a separate scale that may have shifted?

    I have a small collection of thermometers going back to the 1860s, there is no correlation between the age and accuracy of the ones I have; the one from the 1860s is good within a degree over most of the scale.
     
  3. erikg

    erikg Member

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    It is glass with blue fluid, not really old enough to be mercury I don't think. the markings are inside the tube, no way they can move. No breaks in the fluid at all.
     
  4. E. von Hoegh

    E. von Hoegh Member

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    Well if it was accurate, and now it's not, something had to change. There isn't much to change in a hermetically sealed glass tube though!
     
  5. Gerald C Koch

    Gerald C Koch Member

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    Certainly any thermometer can go off. Column separation due to rough handling is a common problem. Take a good magnifying lens or jeweler's loupe and check the entire column. If you can get all the liquid into the bulb you can correct this problem.
     
  6. E. von Hoegh

    E. von Hoegh Member

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    Column separation is the only problem that can occur. Anything else involves breakage of the tube. The easiest way to fix it is to cool the thermometer until all the fluid is in the ball, but it can be problematic when the thing goes down to minus 40.
    An error of 8 degrees would involve a single very apparent separation, or a less apparent series of separations.
     
  7. rjs003

    rjs003 Subscriber

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    Get together a container of ice water ( ice and water with the ice no longer melting) this is 32 degrees
    After performing the ice water test then a pan of boiling water with vapor raising from the liquid this is 212 degrees
    Take the two reading and extrapolate the difference this is how you check for accuracey
     
  8. erikg

    erikg Member

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    Thank you everyone. I'll take a close look tonight, there must be a break that I didn't see. I will try the cooling method and see how that goes. Apug as always a wealth of knowledge.
     
  9. E. von Hoegh

    E. von Hoegh Member

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    You need to make your ice with distlled water, then make a slushy with the ice and distilled water.
    Pure water boils at 212 degrees at a pressure of 30 inches of mercury- that is sea level, normal barometric pressure. You need to correct for altitude and barometric pressure, and again use distilled water.
    The difference between these two points is then divided into 180 degrees for Fahrenheit and 100 degrees for Celsius (centigrade) thermometers.
     
  10. erikg

    erikg Member

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    I'm in Rhode Island so I'm basically at sea level so no trouble there! Looks like I need to do some preparation. Thanks again for the information, I've learned something new.
     
  11. Ken Nadvornick

    Ken Nadvornick Subscriber

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    If the thermometer has an expansion chamber at the top of the glass column, then careful heating may be an easier way to go.

    I once had an alcohol field thermometer that was subject to rough handling and frequently sustained bubbles (column separation gaps). Carefully holding it above the vents of my Coleman gas lantern for a few moments back at camp always did the trick.

    Ken
     
  12. E. von Hoegh

    E. von Hoegh Member

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    You're right. I keep thinking mercury and old, but it would be easier for a mercury instrment too, if it has the space at top. It's also usually easier to find enough heat, rather than extreme cold.
     
  13. Etr420

    Etr420 Member

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    Here are the instructions for the Kodak Type 3 Process thermometer (the mercury version), with advice on how to fix column separation.


    ProcessType3Instructions.jpg
     
  14. Ken Nadvornick

    Ken Nadvornick Subscriber

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    This is the thermometer that is used as my darkroom standard. I even calibrate my Hass Intellifaucet against it. It's a beautiful instrument.

    Ken
     
  15. Gerald C Koch

    Gerald C Koch Member

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    I didn't mention another problem as it is rather rare. There is a small chamber or expansion spacer at the top of most thermometers. I have had liquid become trapped here. Some might lump this with column separation but it is not quite the same and results in a different error from true column separation. The thermometer will read lower than the actual temperature.
     
    Last edited by a moderator: Sep 24, 2013
  16. erikg

    erikg Member

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    I took a closer look at the thermometer last night and as you probably would have guessed there was a break, some fluid was stuck up at the very top of the tube. I heated it up and reconnected that bit, now it is as it should be. Thank you everyone.
     
  17. AgX

    AgX Member

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    Column seperation is not the only issue with capillary Thermometers though:

    The height to which the thermometer has to inserted is of importance for precision metering.
    With this in mind the ambient temperature is of importance too.
    Precicision thermometers have a mark or such to indicate the level of immersion at room temperature.
     
  18. sepiareverb

    sepiareverb Subscriber

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    Well, if it occurs again you can schedule a trip to the top of Mt. Washington NH this February. -40 ought to be easy enough to come by.