Loop Calibrators

Process instrumentation needs to be checked and calibrated periodically. Any deviations are to be noted and adjusted to ensure that they are operating within acceptable specifications. Loop calibrator devices are designed for the specific purpose of troubleshooting 4-20 mA current loop circuit. These electronic devices are generally capable of not only measuring current but also providing current to devices in the unpowered loop. They can also simulate 4-20 mA transmitters. Often this device comes in a portable hand-held package which is quite easy to use and test circuits in the field. It has a few controls such as a rotary knob for current adjustment and a few toggle switches for setting its various modes.

 

What is a 4-20 mA loop?

Current loops are quite common in industrial measurement and control applications due to their ease, simplicity, and low cost. They have a wide range of power supply requirements, low noise output, and can transmit a signal over long distances without any significant loss of signal.

In a plant in which a large number of sensors are reading a variety of process parameters and sending this information from the field to the controller or control room, 4-20 mA loop is quite common. Various types of thermocouples and RTDs provide temperature readings while other sensors provide pressure, flow level, etc. Most of these parameters are transmitted as analog signals in the form of 4-20 mA current signals. The information about the process variable is transmitted via varying amounts of this current in the loop. The 4-20 current loop is by far the most dominant standard in the process industry for signal transmission. The 4-20 mA current loop is designed in such a way so that the current drawn by the sensor is directly proportional to the process parameter it is measuring. In this loop, the 4 mA signal is considered as zero value and 20 mA as the full-scale reading. Thus it is very easy to detect the failure of the sensor embedded in the circuit loop.

The 4-20 mA current loop is a quite popular way to send process data to controllers and displays because of its many useful properties which are hard to match by other analog technologies. It is a very good method because the current does not vary as it travels for even long distances from the transmitter to the receiver. It is also much simpler to design and is economical to install. It is not subject to frequent errors. However, the voltage may drop in the circuit, which should be monitored closely. If the number of process parameters is large then it impacts the cost and complexity of the system.

The 4-20 mA loop circuits are commonly found in process control, basic parameter measurement data logger, and other data acquisition applications.

Troubleshooting a 4-20 mA loop using signal current simulation

A very useful function of the calibrator is its current simulation function for checking and troubleshooting 4-20 mA process control circuits. In the current simulation mode the calibrator device duplicates the function of a process instrument when it is connected in place of the transmitter. The current simulation function is often sometimes referred to as transmitter simulation on the loop troubleshooting tool. In the current simulation mode, the loop calibrator acts as a current sink which means that it regulates a current signal value while drawing power from the 24V power supply of the circuit.

 

How to use the current simulation?

If for example a loop is operating incorrectly and a pressure transmitter is suspected of being defective then disconnect the wires from the transmitter and connect it to the test leads of the loop calibrator. Now select the current simulation mode and set the loop calibrator to a current test signal values.

When a current of 4 mA is generated by the loop calibrator, the controller or display that is connected in the loop should indicate the 0% value of the transmitter. And when the current is set to generate 20 mA, the indicator or display should indicate the 100% value of the parameter from the transmitter. This will cover the entire range of the parameter from 0% to 100% of its range.

 

Interpreting the current testing results

If there is a difference between the actual value and the expected value of the parameter then it would indicate that the A/D converter of the controller input card requires adjustment. If there is an open circuit or no value is indicated then it is possible that the input card is defective or perhaps the input fuse in the card has blown.  If there is no voltage then it is possible that the 24V loop power supply is defective. Check the fuse on the input card with a digital multimeter to see if it is blown or not. To check if the 24V power supply is working properly measure the voltage at the transmitter wires. The full 24V should be detected and if not then the power supply has a problem, it is either faulty or there is a break in the circuit.

Using current simulation to test a process control loop is a very powerful method that ensures that the 24V loop power supply, the signal wires, and the input of the control system are all operating correctly. If a loop operates correctly during this test, but still shows errors, then the problem may lie in the transmitter or the process sensor.

A good loop calibrator makes the task of loop calibration easy. With hundreds of loops to check in an automated plant, the process of calibration requires diligence, precision, and planning. It is important to plan a logical strategy for loop testing that will result in an improved control performance without compromising the quality, reliability, or safety of plant operations.

 

e2b calibration provides reliable and cost-effective calibration services for all your 4-20 mA measurement loops.  We can test and repair your loop calibrator. Our verifiable and traceable services are unmatched in the industry.  We provide NIST traceable calibration and issue calibration certificates and calibration labels to meet your requirements. We are ISO-17025: 2017 compliant and registered with ANAB. We are also ANSI/NCSL Z540-1-1994 certified. We have the NIST Traceable Wide scope of ISO/IEC 17025 accreditation. Contact e2b calibration for more information about our on-site calibration services.

 

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