Physitemp Instruments TH8 - Manual (Page 5)

6.0 CHECKING CALIBRATION
6.1 The Thermalert TH-8 thermometer incorporates an automatic zeroing amplifier which
produces near-perfect stability of calibration. A calibration check is not necessary before each
use. Under normal circumstances, the instrument will provide years of service without requir-
ing recalibration.
6.2 Should it be suspected that the instrument is out of calibration, it may be checked with
the Physitemp type T calibrator (Model CT-3), or if this is unavailable, with an NIST calibrated
thermometer. Verifying instrument calibration with an NIST calibrated thermometer requires
extremely careful technique, an accurate reference standard thermometer, and a reasonably
draft-free location. The following apparatus and supplies are needed:
1 1 - quart thermos flask with 2-hole cork
1 Stirrer
2 NIST traceable mercury in glass thermometer, 0.05°
graduations, partial immersion, 25-50°C (78-122°F) with ice point scale.
2 Physitemp type T temperature probe
Ice made from distilled water
Distilled water
6.3 Preparation of a 0°C icebath:
a) Crush the ice and fill the thermos flask till full. Top off with distilled water. Leave for
30 minutes to allow temperature to stabilize, stirring periodically.
b) Immerse the standard mercury-in-glass thermometer in the ice bath to the immersion
mark on the stem. Stir the bath well and compare with thermometer reading at 0°C with
the correction table supplied with the thermometer. If the error is greater than 0.05 °C,
wait 10 minutes, stir the water bath and remeasure. If the error remains greater, test with
another standard thermometer.
A complete calibration check requires two different measurements, one for each of the tempera-
ture ranges of the TH-8.
6.4 CHECKING THE LOW END OF THE TEMPERATURE RANGE
(a) Attach one of the thermocouple sensors to the standard thermometer with an elastic
band. The tip of the sensor should be at the same height as the bulb of the mercury thermome-
ter. Connect the sensor to any input of the TH-8. Immerse the mercury-in-glass thermometer
and thermocouple sensor in the ice bath to the immersion mark, through the cork, and stir well.
Wait 10 minutes. Be sure sensors are not touching side of vessel.
(b) Switch the TH-8 on with the slide switch on the back panel in the degrees F position.
Make sure the “LO BAT” display indication does not appear. Stir the water bath well, and read
the mercury-in-glass thermometer to an accuracy of 0.05 degrees, making sure to minimize par-
allax errors in the reading. Apply any necessary corrections from the mercury-in-glass ther-
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