Loop calibrators are commonly used in process calibration and control. As the name suggests, process control monitors and controls a process to maintain a standard of quality and performance. The process can be a physical quantity, such as pressure or temperature. Loop testers can also be used for electrical calibration. In these cases, they check the performance of an instrument that measures electrical quantities such as voltage, current, capacitance, inductance, resistance, time and frequency.
A unit under test (UUT) is an electrical measuring device (such as a voltmeter or ammeter) that undergoes electrical calibration. Typical devices include loop testers, oscilloscopes, multimeters, frequency counters, data loggers, electrical meters, and insulation testers.
In this article, we will discuss loop testing, current loops, and the use of loop calibrators, especially in relation to process calibration.
What is loop testing?
Process calibration is important to ensure industrial safety, efficiency and productivity. Advanced instrumentation, such as digital instruments and multivariable instruments, requires sophisticated calibration tools that can communicate with industrial networks, including HART and Fieldbus. They must ensure high-precision field calibration.
Typically, these calibration instruments have software to store and analyze detailed and complex instrumentation data. The data is widely used to produce calibration certificates and reports.
Calibration instruments often rely on advanced techniques based on loop testing.
Conventional instrumentation testing relies on testing individual instruments, which is time-consuming and impractical in many situations. Today's advanced techniques rely more on loop or partial loop testing. The complete process circuit is calibrated as a unit and not as an individual instrument. It tests overall circuit tolerance to ensure efficient control, quality, safety and minimal downtime.
An example of a process loop is a temperature loop. In this circuit, a temperature device (an RTD or a thermocouple) is connected to a transmitter, which is joined to a local indicator and a PLC/DCS input board. In an end-to-end circuit test, the temperature element is separated from the process circuit and performance is evaluated against a simulated temperature.
Unlike newer end-to-end loop testing, conventional instrument testing relies on manufacturer specification sheets. Specifications may not quote actual accuracy or include repeatability, long-term stability, temperature effects, or other environmental factors.
However, end-to-end loop testing may be expensive or may not be ideal as a sensor or measurement instrument. In such cases, partial loop testing is useful. In the temperature process example, the partial test can be conducted by isolating the temperature device from the transmitter. It can simulate signals to the transmitter with the help of a loop calibrator. Remember, in full loop testing, the entire loop is calibrated rather than an individual instrument.
A loop calibrator is an instrument commonly used for loop testing. Loop calibrators are useful in instrumentation testing and for initial calibration of process loops. For example, a submersible level transmitter cannot be field tested in an electronics laboratory. But a loop calibrator can be used to simulate the level transmitter signals to calibrate the control circuit. Afterwards, the control circuit can be field tested with a real level transmitter at the test site.
The 4-20 mA current loops
Loop calibrators are generally designed to work with 4 to 20 mA current loops. They can measure currents, supply current to devices not powered in the current loop, and simulate signals like loop-powered transmitters.
4-20 mA current output is the typical electrical signal generated by sensors and instruments. The sensor or instrument within a series acts as a constant current source, where the current output depends on the physical quantity measured, such as temperature or pressure.
The current signal provides an output signal because it remains the same at all points in the series network. It also allows high-power analog output, which can be transferred over long distances.
The current signal can be converted into a measurable voltage at any point in the network by connecting a resistor in series. For example, 4-20 mA current can be converted to a voltage range between 1 ~ 5V by connecting a 250Ω resistor in series. When doing this, ensure that the 4-20 mA transmitter has sufficient voltage between the positive and negative lines so that the measurement is not understood at the bottom of the scale. The 4-20 mA current signal works easily with a two-wire connection and remains strong against radio frequency interference or other potential sources of error.
Why 4-20 mA?
Current loops are typically set to the default 4-20 mA. The default starts at 4 mA (not 0 mA) for historical and technical reasons.
Before electronics became the industry it is today, process instruments were calibrated mechanically. For mechanical calibration, 3~15 psi was the standard. Anything below 3psi was unrecognizable. Additionally, 0 psi was easy to identify as a system failure. When electronic controllers replaced mechanical calibrators, the 4 to 20 mA range was selected to reflect the same 3 to 15 psi range.
Likewise, with electronic controllers, 0mA is too low to be detected and is better suited to indicating a system fault.
What is a loop calibrator?
A loop calibrator is an electrical calibrator that:
- Troubleshoots 4 to 20 mA current loops
- Simulates signals as loop-powered transmitters
- Calibrates current loops
- Measures electric current
- Current sources for unpowered devices in current loops
- Run loop tests
Loop calibrators operate on a wide power supply, transmitting signals over long distances without loss or noise. These instruments can also measure electrical current, current source, and simulate loop-powered 4 to 20 mA transmitters.
Two or four wires?
Loop calibrators can be used with two-wire and four-wire transmitter configurations. In a two-wire transmitter configuration, the measurement signal uses only two connections for the power supply and the output signal.
A two-wire 4-20 mA loop transmitter.
In a four-wire transmitter configuration, the power supply and output signal are isolated using a Wheatstone bridge network. This setting is more accurate because it compensates for errors independently.
A four-wire 4-20 mA loop transmitter.
The 250Ω resistor connection
To convert a 4-20 mA current signal between 1 ~ 5 V, connect a 250Ω resistor. To clear up a common misconception about whether a series or parallel connection is used with the resistor, this may help:
- If the DCS/PLC input board accepts 1-5Vdc input, the 250Ω resistor must be connected in parallel to the 1-5Vdc input wires.
- If the measurement needs to be made from the current loop network to work, the 250Ω resistor must be connected in series with the 4-20 mA current loop.
Measuring current
To measure current with a loop calibrator:
- Disconnect the current loop from the negative terminal
- Connect the loop calibrator as a load between the loop powered transmitter and the local indicator controller
- Note: The loop-powered transmitter may be a temperature, pressure, level, float level sensor – or other process instrument
- The loop calibrator selector switch must be placed in the “read” position to measure electrical current
The electrical connections for measuring current with a loop calibrator.
Current source
It is easy to test a controller or indicator circuit with a loop calibrator, which can generate signals in the range of 4 to 20 mA. It is useful for calibration before any device is field tested with a sensor or process instrument.
To obtain current:
- The red wire of the calibrator must be connected to the positive terminal of the process variable input
- The black wire from the calibrator must be connected to the negative terminal of the process variable wire
- The loop calibrator selector switch must be placed in the “source” position
- The output current signal can be set using the adjustable knob
The electrical connections for current supply with a loop calibrator.
Simulating a loop powered transmitter
The loop calibrator can also be used to simulate a loop powered transmitter. See how it works:
- Calibrator red wire connects to process variable input power terminal
- The black wire from the calibrator connects to the positive terminal of the process variable input
- The loop calibrator selector switch must be placed in the “two wire” position
- A predetermined current level is set using the adjustable knob
The electrical connections to simulate a loop calibrator as a loop-powered transmitter.
