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Thermometer Comparison
My wife uses a Taylor digital oven thermometer (model 1470) to measure the
interior temperature of roasts and the like. The first one got a bit too close
to a hot stove burner and the case melted, so I replaced it with a new one
earlier this year.
Recently, she remarked that the new thermometer was reading differently than
the old one, so today I compared the old and new oven thermometers along with
two glass "laboratory" thermometers, two analog dial thermometers and my Fluke
189 Digital Voltmeter and 80PK-25 Type K thermocouple combination.
If you don't have one, by the way, I recommend the Taylor 1470. As my
measurement data shows, both samples I tested agree well with my Type K
thermocouple.
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Taylor 1470 Digital Oven Thermometer
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My test protocol is to immerse a variety of thermometers in water at varying
temperatures, and record the data at intervals of about 30 degrees F over the
approximate range of 30°F to 212°F. To prevent temperature stratification, I
continuously stirred the water. As a desired test temperature was approached, I
reduced the applied heat until the temperature more or less stabilized and then
read the thermometers. I considered the water temperature "stabilized" if it did
not change more than 0.2°F during the time I read all the thermometers.
I compared the following instruments:
- Two glass tube "laboratory" thermometers, with scales
from -10°C to +110°C. I bought these at a cheap price and the accuracy
matches the cheap price.
- Two Taylor 1470 Digital Oven thermometers, one
about two years old with a partly melted enclosure and one about six months
old.
- A 2" diameter Taylor analog dial thermometer my wife
uses in the kitchen.
- A 1" diameter Weston analog dial thermometer I use in
the shop.
- Fluke 189 Digital Multimeter with an 80PK-25 Type K
thermocouple probe, and Fluke Type K thermocouple adapter I use in the shop.
The only instrument of these seven with a stated accuracy
that I can put my hands on is the Fluke Type K thermocouple, which is rated at
±1.1°C over the test range.
The figure below shows how the six thermometers compare
with my Fluke thermocouple. I've added lines to show the thermocouple rated
accuracy, which corresponds to ±2°F.
The two extreme test temperatures I used are melting ice
water and boiling water. The thermocouple was less than 1°F different from
theoretical temperatures of both extremes. I know that boiling water should be
around 211.7°F for my elevation and today's barometric pressure and the
thermocouple read 211.3°F.
At the lower end, the ice water mixture read 31.0 °F, a
bit lower than the normal 32°F. However, the ice in the mixture is fresh from
the freezer and is around 0°F. I did not have the patience to do the normal
ice/salt and water bath calibration, and in cooking, the normal temperature
range the oven thermometer is used with is in the 140 - 180°F range.
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Conclusions from the data are:
- The two glass tube thermometers don't agree with each
other, or with the thermocouple. I recall paying about $1 each for these, so I
suppose their accuracy is proportional to the price.
- The old and new Taylor oven thermometers track rather
closely with each other. The minimum temperature these indicate is 32°F and
below that the display says 'min" so I don't have a data point for the new
thermometer at 32°F. At all other temperatures, the two agree with each other
within a degree or so. Both read on the low side of the thermocouple data,
however.
- The large (2" diameter) Taylor dial thermometer is
quite close to the thermocouple data over almost the entire temperature range.
- The small Weston dial thermometer is also quite close
to the thermocouple data.
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In no case are the two Taylor oven thermometers more than
3.5°F different than the thermocouple readings. The question that follows is
"how much accuracy is required in an oven thermometer"?
Some guidance can be found in looking at a typical internal
temperature range for differing degrees of "doneness" for a beef roast:
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Condition |
Temperature °F |
| Rare |
140 |
| Medium |
160 |
| Well |
180 |
The interval is 20°F which suggests that an accuracy of
about 10 to 20% of the interval is reasonable, i.e., an error of between
2 and 4°F should be satisfactory. If we assume the thermocouple is the true
temperature, then both oven thermometers easily meet the 4°F error standard, and
almost meet the 2°F objective.
If the true temperature is somewhere around the average of
the ensemble, then the two Taylor oven thermometers are even better. |
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