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.

[ROL]

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.

Well, if my CSI–sense serves, at least we will know accurately your time of death!

I love mine as well, but never thought about how to calibrate my other thermometers with it. Yeah, for most B/W procedures, I'm good to within 1ºC or so – I love B/W.

[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.