How Should I Calibrate? Part 2
Last month, I covered the test equipment you should have in your calibrating toolbox. In this issue, I'll explain control signal and temperature sensor calibration.
Control Signals. Control signals are put out by controllers and expect to be obeyed by such final control devices as motorized valves, electropneumatic valves, motor drives and SCR heater controllers. How well they are obeyed is often pretty rough and nonlinear. Checking at this stage -- while not called calibration -- is important to control performance.
You can inject simulated control signals into these devices and see what the output does. Be wary about what their feedback signals (if any) represent. They likely will be linear mimics of the control signal rather than a measure of what you are trying to deliver (for example, fuel flow or electric power). You may be more assured by noting valve travel, dead zone, fan speed, heater current, etc., but these will not amount to a proportional delivery of heat. With electroheat, you can use an accurate clamp-on RMS ammeter to check your heater ammeters. Then, you can do a quick I2R calculation to determine heater power given that heater resistance (R) is reasonably constant.
Sensors. Sensors usually are replaced only when they burn out or break. As their calibration drifts over time, operators sometimes adapt instinctively by adjusting controller setpoints to maintain good results or product. The revised settings will not be reliable when the sensor is replaced.
The two wires inside a thermocouple may come together, sometimes intermittently, some way back from the hot junction, and the controller will receive a reduced signal. The controller will respond and turn up the temperature but will not show the increase. It takes an alert operator or an independent alarm device to detect this kind of overheating.
On plants that use more than one type of thermocouple, someone may put in a spare thermocouple or plug in a spare controller that is a mismatch. During maintenance, an interchange of type J and K is not uncommon, and the indications are not wildly different.
Check critical sensors once or twice a year at their working temperature. Tag them with the type and any known calibration errors.
Calibrator Choices. A liquid-bath calibrator is a common choice up to 480oF (250oC). A dry-block calibrator is useable up to about 1,100oF (600oC). A fluidized bed of alumina particles can be used up to some 2,000oF (1,100oC). Up to about 2,900oF (1,600oC), specially designed furnaces are used. The temperatures are controlled using a platinum RTD or a platinum/rhodium thermocouple.
Sensors with a heavy metal protection tube can pull heat out of the calibrator and may have difficulty reaching the exact temperature indicated by the controller. Insert the sensor deep and, if practicable, remove the tube. Make sure that all of the wire-wound sensing section of an RTD lies deep in the hot zone.
Infrared (IR) Thermometers. These can lose accuracy due to targeting misalignments, emissivity changes and dirty lens or sight path. Fix what you can with targeting and cleaning, then if you can get a thermocouple onto the target, read its true working temperature. Trim the emissivity adjustment on the infrared instrument to make its reading agree with that from the thermocouple.
General GuidelinesReplace instruments that show an unacceptable spread of dancing digits when being calibrated. When in service, you can expect some instability from fast-moving signals without attributing it to interference or defect.
For analog indications, check for dead band by taking the signal slowly up to and down to the check point. This will reveal bad pivots on meters and excessive friction on recorder or other servos. Remember that digitally positioned, non-servo chart recorders and paperless recorders have no slidewires or other position feedback devices and do not suffer from dead band.
Consider using a well-established instrumentation service company that has invested in trained technicians and accurate NIST traceable test equipment.