Measurement of temperature normally requires close, direct contact between the material and the sensor in such a manner that the sensor responds quickly to the temperature of the material.
OPERATING PRINCIPLES
The bi-metallic element consists of two strips of different metals, with different thermal expansion coefficients, inseparably joined together. This strip is spirally or helically wound with one end fixed to the body of the thermometer, and the other attached to the axis of a pointer. Measurement of temperature is made with the bi-metallic system inside the temperature sensor. Temperature variation influences the bi-metallic strip such as to rotate the pointer; this rotation is indicated on a dial.
INSTALLATION
The thermometer should be mounted at any convenient location where it will be subjected to the average temperature variations to be indicated.
Avoid bending the stem as this will cause misalignment of the internal parts, resulting in undue frictional errors.
To tighten the thermometer to the apparatus, use a wrench applied to the hexagon head of the threaded connection located just outside of the case.
Locate the stem so that at least two inches will be subjected to the average temperature to be measured.
Exposing the stem to a temperature in excess of the highest dial reading should be avoided.
The thermometer is normally provided with a threaded connection. To tighten the thermometer to the apparatus or into the well, use an open-end wrench applied to the hexagon head of the threaded connection. Turn until reasonably tight, then tighten still further in the same manner as a pipe elbow or similar pipe fitting until the scale is in the desired position for reading. DO NOT TIGHTEN BY TURNING THE THERMOMETER CASE.
Install the dry type thermometer so that the maximum case temperature is kept below 200°F at all times.
Install the liquid filled type thermometer so that the maximum case temperature is kept below 150°F at all times.
When a thermometer is equipped with a well, the well should be installed onto the apparatus first. The stem of the thermometer should then be coated with a heat conducting medium (a mixture of glycerin and graphite or vaseline or any other heavy lubricant may be used), after which the thermometer stem is inserted, and tightened into the well.
CAUTION: Thermowells should be used on all pressurized applications, to protect the thermometer from corrosion or physical damage, and to facilitate removal of the thermometer without disturbing the process.
TESTING AND CALIBRATION
Bimetal Dial Thermometers are carefully calibrated at the factory and under most operating conditions will retain their accuracy indefinitely. However, as in the case of all instruments, it is well to make periodic checks for accuracy against known standards.
ASME B40.3 STANDARD ACCURACIES*
Thermometer accuracy is graded as shown in the table below. Adjustment of the case of a thermometer, with an adjustable angle connection, may affect its accuracy. This effect should not exceed 0.5% of span.
*ASME B40.3 may be ordered from American Society of Mechanical Engineers, Three Park Avenue, New York, NY 10016.
Example #1: Range 0/250°F Grade A
Span = 250-0 = 250°F
Accuracy at 20% of span (50°F) = ±1% = ±2.5°F
Accuracy at 50% of span (125°F) = ±1% = ±2.5°F
Accuracy at 100% of span (250°F) = ±1% = ±2.5°F
Example #2: –40/160°F Grade E
Span = 160-(–40) = 200°F
Accuracy at 20% of span (0°F) = ±3.4% = ±6.8°F
Accuracy at 50% of span (60°F) = ±1% = ±2.0°F
Accuracy at 100% of span (160°F) = ±5% – ±10.0°F
Example #3: Range 50/300°F Grade AA
Span = 300-(–50) = 250°F
Accuracy at 0% of span (50°F) = ±1% = ±2.5°F
Accuracy at 50% of span (175°F) = ±0.5% = ±1.25°F
Accuracy at 70% of span (225°F) = ±0.7% = ±1.75°F.
ADJUSTMENT
If it is necessary to make an adjustment to the thermometer proceed as follows:
On thermometers fitted with an “External Adjustment”
– Use a small wrench, small screwdriver or a coin to turn the slotted hexagon head in the back of the case until the pointer indicates the proper temperature on the dial.
MAINTENANCE OF DIAL THERMOMETERS
Aside from occasional testing, little or no maintenance is required.
Be sure that the gasketed glass cover is on the case at all times, as moisture and dirt inside the case will eventually cause the thermometer to lose its accuracy.
If the thermometer is used for measuring the temperature of a material that may harden and build up an insulating layer on the stem, the thermometer should be removed from the apparatus occasionally, and the stem cleaned. Observe this precaution to ensure the sensitivity of the instrument.
(Courtesy of the Ashcroft and its related manuals)
