Temp measurement and accuracy

[Vaughn]

Thanks for the info, Louie. I have always seen it as it relates to target shooting (which I do not do, so chances I am wrong with this, too).

Accuracy is hitting the bullseye, precision is a good grouping (shades of "Young Frankenstein", LOL!)

Vaughn

PS -- here at the university, we have a bunch of dial thermometers. I "calibrate" them all from the built-in sink thermometer. Accurate? I do not know, but all our dial ones read the same! (for awhile, anyway!)

[Heroique]

For perfect clarity, we need an example where the thermometer is repeatable, but neither accurate nor precise. ;^)

[Drew Wiley]

Depends what you are doing. Repeatability is always critical, accuracy sometimes is. No
different from a measuring graduate or weighing scale in this respect - for some formulas
you need to be spot on, for others ... But as far as Vaughn's analogy goes, that's why
they invented shotguns

[Leigh]

For perfect clarity, we need an example where the thermometer is repeatable, but neither accurate nor precise.

That's easy.

A good dial thermometer (very repeatable)
where the dial has been turned 90° from its proper position (quite inaccurate),
and only has temperature lines at 20°C increments (very imprecise).

- Leigh

[rdenney]

That's easy.

A good dial thermometer (very repeatable)
where the dial has been turned 90° from its proper position (quite inaccurate),
and is only calibrated in 10°C increments (very imprecise).

- Leigh

Better yet, a thermometer with no scale at all.

Rick "accuracy=0, precision=0, highly repeatable" Denney

[tgtaylor]

This is not 1812 but 2012 and the thermometer has been around for a long time now, is sophisticated, and inexpensive. I just placed my 4 therometers side by side for a comparison and the 2 Jobo's and Patterson Dial read identically - 65F; the small probing thermometer for cooking read 66F. While I'm not the betting type, I'd wager that the former 3 are within 1 degree of "true accuracy."

Thomas

[Leigh]

The following will serve to define the three parameters under discussion...

Precision:
The granularity of the measurement, i.e. how small an increment can be detected and displayed.
This is a function of the measurement technology used and of the method used to display the value.
E.g. if you stick your arm out the window, you can tell how warm the air is, but with very low precision.

Accuracy:
How close the reading is to the true value of the parameter.
Stated another way, it's the magnitude of the error, less error = more accurate.
For example, measuring air that's 20.0°, a reading of 20.1° is more accurate than a reading of 21.0°.

Repeatability:
When you take ten readings of the same thing, they should all be exactly the same. This is perfect repeatability.
The extent to which they differ is a measure of the repeatability. The greater the variation the worse the repeatability.
In target shooting this is the size of the group of holes in the target... smaller is better.

HTH

- Leigh

[Leigh]

Better yet, a thermometer with no scale at all.

Lacking a scale, the instrument could not properly be called a thermometer.

It could be a strain gauge.

- Leigh

[Leigh]

...I'd wager that the former 3 are within 1 degree of "true accuracy."

Disregarding some very odd and unlikely source of error, I would agree.

As a practical matter, any thermometer that is accurate within one degree is certainly satisfactory for photographic work.

- Leigh

[Drew Wiley]

Junk is junk, and there are a lot of junky thermometers out there. Does this matter? Maybe
not for garden-variety black and white work, but just try something like color separation
negatives, which have to match precisely between processing cycles. And I certainly wouldn't trust ANY dial thermometer or chepie mercury unit for that kind of thing.