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A thermocouple type test instrument is preferred for measuring oven temperatures accurately. Mercury thermometers
are acceptable providing they can be proven accurate.
Regardless of the type instrument used, it is most desirable to double-check it just before making an oven temperature
check. This can be done simply and quickly by placing the thermocouple tip (or immersing the entire mercury
thermometer) in boiling water. Note: Mercury oven thermometers should be the "total immersion type." The
resulting reading should be within several degrees of 212°, -depending on the altitude.
Generally, a mercury thermometer can drop in excess of 25° when the doors are opened to check oven tem perature.
This condition makes it extremely difficult to get a true temperature reading.
Almost all ranges today have oven burners which cycle "on and off." The high and low points of the resulting
temperature "swing" (differential) in the oven must be measured-and averaged-to determine the true operating
temperature. A thermocouple type test instrument is best suited to measure these temperature changes quickly and
accurately, and without opening the oven door.
With an accurate, thermocouple test instrument or mercury thermometer, it is still difficult to measure these changing
oven temperatures and then, average them correctly. This is why we recommend that the thermocouple tip or the bulb
of the thermometer be "weighted." Weighting (adding mass) to the test instrument compensates for oven temperature
changes by making the test instrument less sensitive to these constant changes in temperature. Note: How this
weighting can be done is discussed later.
Measuring these changing high and low temperatures points in an oven is possible with either type of test device
without weighting but is subject to possible inaccuracies. This is most true at low temperature setting because, in this
area, the function of "time" becomes a factor. The simple averaging of temperatures, then, may not produce the true
operating temperature. "Weighting" provides the compensation for both time and temperature that is necessary.
We produced (next page) test curves showing actual results in the low temperature area showing the difference in
results when using an unweighted and a weighted thermocouple or thermometer. Tests were made with the same
control, same oven and dial at 170° mark (not move d).
From the above, it can be seen that an error of 15° is possible. Chart #1, unweighted thermocouple indicates an
erroneous average oven temperature of 185°. Chart #2, weighted thermocouple indicates the "average" or true
temperature to be 170°.
WEIGHTING
A thermocouple can be weighted by clamping the tip between two one-inch squares of 1/16" thick aluminum. The
thermocouple can also be weighted (just as successfully) by using a letter-size sheet of aluminum foil. Fold the foil
Chart No.2
Weighted Thermocouple Dial Setting 170F
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Chart No.1
Unweighted Thermocouple dial setting 170 F
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