Thermometers

[photonsoup]

I decided to start doing some testing, so I thought I would compare my thermometers. Off course they all give different readings of the same container of water.

I have a Paterson Colour Thermometer, A Fieldpiece stick meter with dual temp head callibrated to 32 degrees f. in ice water, and a Raytec infared digital unit.

They read as follows:
Paterson 68.0
Fieldpiece 65.1
Raytec 65.5

In hot tap water
Paterson 115.0
Fieldpiece 110.7
Raytec 105.5

How have you dealt with this issue?
I do plan on doing some color negatives and slides so accurate temperature is critical.

[neil poulsen]

Year ago, I purchased two electronic thermometers and mounted them in my darkroom. Each has a metal tube that can be inserted into baths to determine temperatures. They have a red readout that can be easily seen in the dark. They can also be calibrated by adjusting a knob. I keep a Kodak, long mercury thermometer that's accurate to a half degree on hand for calibration. As long as the Kodak doesn't break, I know that I can have consistent readings over the long term.

[Tin Can]

All measurement tools need yearly calibration.

Or pick one device as your master calibration and use it for 'your' standard.

I use a VOM with thermocouples.

http://www.astm.org/Standards/E220.htm

[Heroique]

Before you choose your thermometer, be sure you fully understand the difference between "accuracy" and "precision" on all levels, plain and subtle.

[Arne Croell]

Calibration is essential, as Randy and Neil pointed out. It is also important to distinguish between a fixed error and drift of an instrument. The three units you have all use different measurement principles with different error sources. The Raytec is a Pyrometer and therefore subject to variations in emissivity of the surface you measure, in addition to possible electronic drift. The Fieldpiece seems to be thermocouple-based, and thermocouples not only have large absolute errors, but also tend to drift (their advantage is a fast reaction time). Both types are not the best choice for precise darkroom measurements. A regular precise thermometer like the one Neil mentions is good, or for electronic units, thermometers based on resistance like a Pt 100 or Pt1000 type are the most accurate. Out of the three you have , I would give the benefit of the doubt to the color thermometer - it might have some absolute error, but its unlikely to drift over time, and an absolute error such as in the scale position, can be checked with ice water.

[ic-racer]

Try ice water, that will short them out.

[Bruce Barlow]

You'll have to pry the Zone VI Compensating Developing Timer out of my cold, dead hands. Using it, and only it, I can calibrate all my times and be consistent. Works for me.

Otherwise, a cheap thermometer for measuring wash temperature and such is way close enough for photography.

[BetterSense]

Use the glass one as your standard and calibrate the others to it.

[Jim Noel]

I have 5 dial thermometers which are regularly adjusted to match my Kodak Precision mercury filled one from the 1940's. I measure the boiling point, freezing point in a container of ice water left sitting for 15 minutes,and at 100 deg F.

[Harold_4074]

Unless you break it, any glass thermometer with an engraved scale is going to retain its calibration forever. If the Paterson was made for photography, it was probably calibrated for best accuracy in the 68-75 F range, and should be entirely adequate for even color processing. If it is a general-purpose thermometer, the accuracy may not be there, but if you know someone with a Kodak Process Thermometer (there are color versions for 75F processing and B//W at 68F) you can locate your target temperature on the scale of any other suitably precise and stable thermometer. Ideally, you will have one "reference" thermometer and a "working" thermometer which can be lost or damaged without too much grief.

Bear in mind that being able to measure the temperature is not the same thing as being able to control it...