Continuing my discussion of how to check your heat process on a budget, I’ll pick up with temperature sensors and shaft speeds. Again, let me remind you: None of your tests must endanger the plant or its product.
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| Figure 1. You can improvise an RTD simulator using this circuit. |
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Platinum Resistance Detector (RTD) Instruments. The most commonly used RTD has a resistance of 100 ohm at 32
oC and 138.51 ohm at 212
oF (100
oC). Tables of temperature vs. resistance are readily available for the whole useable temperature range, so you can connect a known resistor to your instrument in place of the RTD and check the instrument’s calibration.
You normally would use a calibrator -- also known as an RTD simulator. This is a precision variable resistor circuit with a digital indication expressed in degrees. You can improvise one from the circuit in figure 1. This design covers approximately -60 to 400
oF (-50 to 200
oC). Use your multimeter to set the resistance to represent various temperatures, then connect it to your controller in place of the RTD. To extend the range to 932
oF (500
oC), change P1 to 200 ohm. Remember that multimeters can introduce an uncertainty of some 0.4 ohm (1.8
oF [1
oC]) on top of the tolerance of the sensor in your process.
Two-Wire Connection. Some low-cost instruments have only two input terminals, so they add the connecting wire resistance in their measurement of temperature. This amounts to about 4.5
oF (2.5
oC) error per ohm of go and return wire. This may be acceptable and can be minimized by using short runs and thick cable.
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If you can’t find a dependable spare controller or indicator, in its place, use a multimeter having 10 microvolt resolution such as this one.
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Three-Wire Connection. Most instruments use the three-wire connection, where the third wire allows the instrument to compensate for line-resistance error. Here, one white lead connects to one end of the RTD and two red leads go to the other end. They are connected to three terminals on their instrument.
Colors vary, but all you need to know is that one end has two leads of the same color and the other end has one of a different color. The same code should apply in the plant wiring.
If you are calibrating an instrument with your simulator direct on the input terminals, you don’t need correction for line resistance, so you connect the two red-wire terminals together.
Signal Converter Calibration. Here, you have to inject a range of known signals at the input and at the same time measure the output -- for example, a Type J thermocouple in, and 4 to 20 mA signal out. So, you need two precise instruments.
Shaft Speed. If you can locate any rotating steel part, bring a magnetic pickup or proximity probe up close and read the signal as revolutions per second on the Hertz function of your multimeter. A few turns of wire on a small magnet works. A rotating strip of foil can do the same job activating a photo cell.
Other checks using your multimeter include AC and DC power supply voltage, heater resistance and continuity using the beeper. PH
