Continuing my discussion of how to check your heat process on a budget, I’ll pick up with how to check the control circuits of electric heaters as well as how to manipulate test signals and analyze the results. I also will be covering controllers, recorders and indicators. Again, let me remind you: None of your tests must endanger the plant or its product.
Electric Heaters. If there are no installed ammeters, put your clamp-on ammeter on the heater conductor. Check that current goes up and down as temperature goes down and up through setpoint. Without a sensor simulator, you cannot easily swing the temperature, so take the setpoint up and down through the indicated temperature and watch the output change.
If your final control device is an SCR with a 4 to 20 mA input, you can leave the controller out of the loop, inject a measured milliamp signal and watch the heater ammeter respond. It will pulse with increasing percent of on-time (time-proportioning control) or increase progressively as you increase the milliamp signal (phase-angle control).
Testing Controllers, Recorders and Indicators with 4 to 20 mA Inputs. Use your run-up box to inject a measured milliamp input. Check that the indication on the instrument shows the corresponding engineering units. Some current loops that you break into may have too high resistance to make the full 20 mA. In this case, add one or more extra 9 V batteries in series to your run-up box.
Connect this millivolt signal to a temperature controller in place of the thermocouple. If it is safe to do so, you can leave the temperature controller connected to the process and watch it modulate the heat as you vary the millivolts up and down through setpoint. Use your clamp-on ammeter to watch heater current.
To check the controller calibration, connect the millivolt signal to a temperature indicator intended for the same type of thermocouple. Better still, use a hand-held model that has several thermocouple selections. Otherwise, use a known good controller or thermocouple temperature indicator having the same input sensor and range. Don’t forget to use thermocouple extension cable. Compare both indications as you run the signal through its range. Disconnect any installed controller from the process at this stage.
With this simple run-up box, the minimum setting should show room temperature, not 32oF (0oC) on both instruments. You can verify this with a thermometer placed near the thermocouple input terminals.
If you can’t find a dependable spare controller or indicator, in its place, use a multimeter having 10 microvolt resolution. In your thermocouple table, look up the millivolts corresponding to the current room temperature. Subtract this from the millivolts corresponding to the temperature you want to inject.
Set the injected millivolts to this difference using the indication on your multimeter.
Checking Devices with DC Voltage Inputs. The most common are 0 to 5 V and 0 to 10 V. These devices usually come with input impedance in the megohm (Mohm) range. The circuit in figure 1 will tolerate impedances down to some 0.1 Mohm.
Use the +V Out terminal in figure 1 and monitor it with your multimeter. If you need more than 9 V, you can add more batteries.
Next month, I will continue with more on temperature sensors and shaft speeds.