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Thread: Temp measurement and accuracy

  1. #1
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    Temp measurement and accuracy

    Hi all,

    I have been stuggling with temp measurement. I started out with a cheaper ($200 - China) multimeter with temp probe. Then I borrowed a little solid temp probe from work that had a long straight metal shaft with the probe inside. This second probe read 2.3 degrees C different to the original. I then thought I'd buy a thermometer from B&H (the Patterson one) with as much accuracy as possible (0.1 degree graduations). Alongside that purchasing decision, I went and bought a calibrated multimeter as well (an expensive Extech 570A which is really 'accurate' ...) (This mulitmeter will replace the other Chinese one for all functions - it aint just a temp meter !)

    The Extech and the thermometer are about 1.5 degrees C different. The Extech and the original Chinese one are about 2.5 degrees C different. The Extech is reading similar to the probe from work.

    Which one do I trust ?? Just how accurate are the thermometers ?? I know the thermometers are certainly slower to repond to variations.

    Given that 1 degree matters in processing ...

    Cheers,

    Steve

  2. #2
    Vaughn's Avatar
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    Re: Temp measurement and accuracy

    It does not matter how accurate they are (at least within several degrees of "normal"), as long as they have good precision. In other words, as long as they give the same temperature under the same circumstances every time.

    Do all your work and testing with the one thermometer and you are set. If you get another thermometer, calibrate it to your original.

    Vaughn

  3. #3
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: Temp measurement and accuracy

    Hi Steve,

    I expect your multimeters are using Type K thermocouples, since those are most common and cheapest.
    Consumer-grade Type K thermocouples have a rated accuracy of ±2.5°C (~5°F).

    Even the best laboratory-grade Platinum thermocouples are only rated ±1.0°C (~2°F).

    ======

    I just looked through the offerings at McMaster-Carr. They have quite a selection.
    The one I would get is catalog nr 3569K58 at $53.92.
    It's rated accuracy is ±0.4°F (~0.2°C). It comes with an NIST certificate of calibration, so you know it's right.
    It looks like this: Click image for larger version. 

Name:	3569k58p1l.png 
Views:	22 
Size:	6.4 KB 
ID:	70763 The site is http://www.mcmaster.com
    Great folks to deal with. No minimum order and no handling charges, just actual shipping.

    ======

    You can calibrate any thermometer that has sufficient range (some have a very narrow range of readings).

    Mix up some ice water (preferably distilled water for both) in an insulated cup. Swirl it around to make sure
    it's reached equilibrium. The thermometer should read 0°C or 32°F.

    Put some distilled water into a pot and bring it to a good rolling boil. Your reading should be 100°C (212°F).

    While distilled water is not essential, any significant mineral content or contamination
    may change the actual temperatures from the ideal.

    - Leigh
    Last edited by Leigh; 23-Mar-2012 at 17:04. Reason: removed references to glass thermometers
    If you believe you can, or you believe you can't... you're right.

  4. #4

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    Re: Temp measurement and accuracy

    I've only recently re-acquired a thermometer, previously using my finger with no ill effects, but then I only "measured" the water, and not the mixed developer. Of all the tests people seem willing to perform, it's rare anyone tests the degree of precision necessary for their process. I'm speaking of myself here, and not being critical of anyone else. Now that I have a thermometer again, I use it.

  5. #5
    http://www.spiritsofsilver.com tgtaylor's Avatar
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    Re: Temp measurement and accuracy

    Steve,

    Leigh's suggestion of how to calibrate the thermometer seems correct to me - at least that is how I vaguely remember calibrating the thermometer in college. I currently use two Jobo thermometers that came with the CPA-2: one reading in degrees Centigrade and the other in degrees Fahrenheit and both read identical. I set my Patterson adjustable to the Jobos and all three read identical so I am comfortable with their readings. That said I always check the Patterson to the Jobo as it is fairly easy to knock the former out of adjustment - which is the reason for the included wrench.

    Thomas

  6. #6
    Joe O'Hara's Avatar
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    Re: Temp measurement and accuracy

    A couple of issues here. When a thermometer is said to be able to read to 0.1 deg F (or C), that is what is called "resolution". Resolution is just how precisely you can read it. It says nothing about how close the reading is to an accepted temperature standard. The latter is "accuracy". If you take a random thermocouple or thermistor-type probe and connect it to a multimeter, no matter how many digits it has to the right hand side of the decimal point, that will not guarantee an "accurate" reading if the probe is not calibrated (i.e., adjusted or measured against a standard). Your experience in getting readings to within a couple of degrees is what I would expect from devices like that. As Vaughn stated, for B/W work, reproducibility is more important than accuracy. Plus or minus a degree or so is OK, just tinker with the times until you get negatives you like. For color, it is a different story. To get a calibrated lab thermometer that will get you within 0.25 deg F will cost you about $85 the last time I checked. That was too much for me so I bought 5 different fever thermometers and calibrated my dial thermometer with the average of those at 100 deg F, which is within their specified range and exactly where C-41 wants to be. It is unlikely to be more than 0.5 deg F off unless I'm really unlucky. I'd be suspicious of the boiling-water or ice-water methods since (1) the boiling point of water is affected by atmospheric pressure and (2) unless you develop your film at 32 F or 212 F there's no assurance that you won't be off be a degree or more at your working temperature due to linearity errors in your thermometer (especially if it is one of the electrical kinds). Bottom line: the man who has one thermometer knows what the temperature is; the man with two is never sure.

  7. #7

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    Re: Temp measurement and accuracy

    Quote Originally Posted by Steve McLevie View Post
    Hi all,

    I have been stuggling with temp measurement. I started out with a cheaper ($200 - China) multimeter with temp probe. Then I borrowed a little solid temp probe from work that had a long straight metal shaft with the probe inside. This second probe read 2.3 degrees C different to the original. I then thought I'd buy a thermometer from B&H (the Patterson one) with as much accuracy as possible (0.1 degree graduations). Alongside that purchasing decision, I went and bought a calibrated multimeter as well (an expensive Extech 570A which is really 'accurate' ...) (This mulitmeter will replace the other Chinese one for all functions - it aint just a temp meter !)

    The Extech and the thermometer are about 1.5 degrees C different. The Extech and the original Chinese one are about 2.5 degrees C different. The Extech is reading similar to the probe from work.

    Which one do I trust ?? Just how accurate are the thermometers ?? I know the thermometers are certainly slower to repond to variations.

    Given that 1 degree matters in processing ...

    Cheers,

    Steve
    Ancient photographic saying --" Man with 2 thermometers is never sure what the temperature is, man with one thermometer is blissfully unaware what the real temperature may really be."

    For B&W Vaughn's advice is exactly correct.

  8. #8
    Land-Scapegrace Heroique's Avatar
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    Re: Temp measurement and accuracy

    Quote Originally Posted by Vaughn View Post
    It does not matter how accurate they are ... as long as they have good precision. In other words, as long as they give the same temperature under the same circumstances every time.
    I like what Vaughn says, so let’s give it give it an example:

    If you like results when your thermometer reads “68 F,” then you want to be sure that each time your thermometer says “68 F,” you're getting that very same temperature from job to job – even if it might be different than the “real” temperature.

    What makes inferior thermometers so exasperating is that w/ each indication of, say, “68 F,” you may actually be getting varying temperatures. Maybe 67 for one job, and 71 for another, even though your thermometer always says 68…

    I like dial (analog) thermometers, but I’ve noticed they have a tendency to vary w/ barometric pressure changes! Maybe the better dial thermometers don't do this.

  9. #9

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    Many good labs keep an expensive calibrated thermometer in a safe place to check other thermometers against occasionally and make adjustments.

    I've got an old Kodak Process Thermometer that I use for that purpose and a few of the Kodak color thermometers as well. They all seem to read within less than 0.5°C of each other.

    The analogue dial thermometers are another story. I've got stickers on them telling me the error at my processing temperature (e.g., "reads +2°F") for the ones with no adjustment. For the others that can be twisted to adjust the reading, I adjust against the process thermometer at processing temp. If it's wrong, at least I'm consistent.

    Acquiring a good, calibrated thermometer (the one Leigh suggests would be fine if it is that accurate through its entire range) and using that to calibrate other thermometers to.

    BTW, there are lots of other threads on this topic here. Do a search for even more than you wanted to know.

    Best,

    Doremus

  10. #10

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    Re: Temp measurement and accuracy

    Quote Originally Posted by Steve McLevie View Post
    Hi all,

    I have been stuggling with temp measurement. I started out with a cheaper ($200 - China) multimeter with temp probe. Then I borrowed a little solid temp probe from work that had a long straight metal shaft with the probe inside. This second probe read 2.3 degrees C different to the original. I then thought I'd buy a thermometer from B&H (the Patterson one) with as much accuracy as possible (0.1 degree graduations). Alongside that purchasing decision, I went and bought a calibrated multimeter as well (an expensive Extech 570A which is really 'accurate' ...) (This mulitmeter will replace the other Chinese one for all functions - it aint just a temp meter !)

    The Extech and the thermometer are about 1.5 degrees C different. The Extech and the original Chinese one are about 2.5 degrees C different. The Extech is reading similar to the probe from work.

    Which one do I trust ?? Just how accurate are the thermometers ?? I know the thermometers are certainly slower to repond to variations.

    Given that 1 degree matters in processing ...

    Cheers,

    Steve
    First, as long as the thermometer will repeat, it's good to go.

    Second, with your thermocouple probe, it's easy to check it at two points. A beaker of icewater will stabilise at 0C until all the ice melts. A pot of water boiling at sealevel will stabilise at 100C. If you are not at sealevel, find your altitude on a map and google the boiling point for that altitude (and your current barometric pressure).

    Virtually any thermometer can be checked at 0C, not all will go to boiling point.
    I have a certified calibrated mercury thermometer to check my other thermometers.
    One man's Mede is another man's Persian.

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