Garland 200 service manual General Calibration, Instrumentation, Weighing of Thermocouple

GENERAL CALIBRATION

Instrumentation

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°F, depending on the altitude.

Generally, a mercury thermometer can drop and excess of 25° F when the doors are opened to check oven temperature. This condition makes it extremely difficult to get a true temperature reading.

Almost all convection ovens 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 correctly. This is why we recommend that the thermocouple tip or the bulb of the thermometer be "weighed" Weighing (adding mass) to the test instrument compensates for oven temperature changes 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 temperature points in an oven is possible with either type of test device with out weighing, but is subject to possible inaccuracies. This is most true at a 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. "Weighing" provides the compensation for both time and temperature that is necessary. We produced test curved showing actual results in the low temperature area showing the difference in results when using an unweighted and weighted thermocouple or thermometer. Tests were made with the same control, same oven and dial set at 170° mark (not moved).

Chart No.1

260

240

220

200

180

160

140

120

100

Unweighted Thermocouple dial setting 170 F

Chart No.2

260

240

220

200

180

160

140

120

100

Weighted Thermocouple Dial Setting 170F

From the above, it can be seen that an error of 15°F is possible. Chart #1, unweighted thermocouple indicates an erroneous average oven temperature of 185°F, Chart #2 weighted thermocouple indicates the "average" or true temperature to be 170°F.

Weighing of Thermocouple

A thermocouple can be weighted by clamping the tip between two one inch square 1/16" inch thick aluminum. The thermocouple can also be weighted (just as successfully) by using a letter sized sheet of aluminum foil. Fold the foil five times doubling the thickness each fold. After the fifth fold, place the thermocouple tip in the center of the aluminum piece and fold once more. Finally fold in the sides so that the foil clings to the thermocouple tip. A mercury thermometer can be weighted in much the same way by wrapping several layers of aluminum foil around the bulb end thus creating the necessary mass. This procedure is a must if you open the door to check the temperature.

Page 40 Part # MCOSM06 Rev 1 (11/03/08)