Continuing my discussion about the instrumentation installed and used on your process, in this column, I will deal with test instruments.
At this deeper level of investigation, there is danger. Chances are, you will be opening enclosures without shutting your process down. If you are touching live wires and terminals with test probes, make sure that you are qualified and competent to know where electric shock hazards lie. You will notice that the top test equipment manufacturers supply test leads that are covered at the instrument terminals and have finger stops on the probes that prevent your fingers slipping onto the live probe tip. Alligator clip adapters have a rubber sleeve over the clip to prevent finger contact. Multimeters have fuses at the input to avoid destroying the instrument - or your eyeballs - when you probe a line voltage with a current or resistance range selected.
Check that oscilloscopes have floating input terminals; that is, neither terminal is connected to a metal case or ground. The modern liquid crystal display (LCD) scopes with plastic cases are good examples of safe design.
Digital MultimetersThis is your best choice for accuracy, resolution, high impedance and versatility. High-end models can read RMS values, capture highest and lowest values and record time over the sampling period. Some have a bargraph or simulated pointer display that can make sense of a jittery signal. When the leads are lying unconnected or connected to two different points that are isolated electrically from each other, you may see random voltage indications. This is due to capacitive coupling of nearby voltages through the leads into the meter's high impedance input and is no cause for concern.
Analog MultimetersIn this category, I refer to those without amplifiers and based on a moving-coil meter and an arrangement of resistors, rectifiers and rotary switches. Movement sensitivity can be as high as 50 KA to minimize the loading effect on voltage ranges, but they do not match digital meters in this respect. They have their advantages, being virtually immune from capacitive and noise interference. They are good for showing intermittent contact resistance - say, on adjustable potentiometers - and for observing jittery and suspect signals. They are not sensitive enough to measure low millivolt signals or those from a high impedance source.
Clamp-On AmmetersThe digital version generally is the most accurate and sensitive and comes in DC and AC-average or AC-RMS responding versions. Max/min capture and sample-and-hold features are available. The analog moving-coil version is cheap and simple but accuracy is rarely better than 5%, and only the AC, average-responding version generally is available. It will sample and hold by means of a thumb-operated pointer clamp. It is good for locations where you can reach and clamp on to the conductor but cannot see the indication.
CalibratorsThese can be anything from a small box with two output terminals and a rotary switch marked in steps of °F or °C to a full-service thermocouple/ resistance thermometer/voltage/ current source simulator with digital readout and continuous signal adjustment. All versions feed an accurate calibration signal into the temperature controller or indicator to simulate the installed sensor. They come with a small or large variety of sensor calibrations and are priced accordingly. A calibration usually involves injecting five to ten signal levels throughout the range, including 32°F (0°C). It is important to use connecting leads having wire that corresponds to the sensor.
Digital ThermometersThese devices look like digital multimeters but indicate in °F or °C and have an input socket that takes a thermocouple with a male connector. They are used for spot checks of process temperature. Some multimeters incorporate this function. Immersion probes work well but surface temperatures do not transfer readily; thin ribbon-type thermocouple probes have been designed to mitigate this problem.
You can do a handy spot check of a controller's calibration (at the working temperature only) by clipping the digital thermometer's input leads to the thermocouple input terminals of the controller without disconnecting the wiring. To avoid cold junction errors, use the correct extension cable with its wires soldered to the inside tip of the alligator clips that go to the controller.
Sensor and Wiring ChecksMost thermocouples have a resistance of less than 1 or 2 A, but when you check from the controller terminals, expect an additional 1 A per foot of double run of extension cable. This varies greatly with material and wire gauge. Most indicators and controllers will tolerate up to 1,000 A.
For RTDs, expect 100 A at 32°F (0°C) and about an extra 38 A per 212°F (100°C). To this, expect an additional resistance for double run of copper wire (look up the gauge and the ohms per foot).
Used properly, testing equipment will provide a snapshot of your process. Understanding where to use what will help you achieve accurate results and a reliable, repeatable process.