Friday, December 19, 2008

Accuracy vs Precision

Fiddling with new toys - got a $5 oven thermometer and a $40 meat thermometer.  Set the oven to 350 - nice digital display, so it *should* be accurate and precise.

Testing that theory.  The $5 oven thermometer reads precisely 10 degrees low - the oven claims it's at 350 degrees; the thermometer reads 340.  The $40 meat thermometer shows the initial warmup overshot to 375 degrees, then cooled to 365, even after opening the oven several times to check the $5 thermometer.

After about a 10 minute 'rest' -- long enough to type up this blog post - the $40 meat thermometer has dropped to 351 degrees.  I think the oven agrees - I just heard the relays kick on to re-energize the heating elements.  The $5 thermometer still shows low - 342 or thereabout.

After a 20 minute 'rest' and a full heat cycle the oven thermometer now shows 160, and the meat thermometer shows 165 degrees.

So, which is right?  I don't know.  They are all precise - same measurements every time.  But until I can find a definitive answer, I'm out of luck for accuracy.

Update: Thought of a new test. We know how hot boiling water is - 212 degrees at sea level. Well, I'm 1,000 feet above that, but I pulled a pan of hot water (less dissolved solids) and set it to a rolling boil. The meat thermometer registered 212 degrees. So it is accurate and precise.  The oven thermometer isn't submersible, so no way to test it - but comparing against the meat thermometer it appears to read within 10% or so, just slowly.

The good news is that the oven display is right on the money, at least good enough for home use.
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