At Alliance Calibration, we deal with all kinds of calibrators, from dimensional calibration to torque calibration. In fields like engineering where precision is a must, calibration is an essential part of the process. One complex tool that we work with is the loop calibrator. The loop calibrator is a hand-held electronic test instrument with a very specific purpose — to work with 4-20 mA current loops. If you need to measure electrical currents, send currents to unpowered devices in the current loop, or find something to work as a loop-powered 4-20 mA transmitter, a loop calibrator is your tool. Since the function of a loop calibrator is not very straight-forward to those unfamiliar with the territory, we need to take a few steps back to explain this instrument. Image Source: https://www.sensorsone.com/4-to-20-ma-current-loop-output-signal/ What is a current loop? Since the purpose of a loop calibrator is to troubleshoot 4-20 mA current loop circuits, then a quick study in current loops is essential for understanding this tool. Current loops are an analogue signal that can be used to monitor or control a device remotely. Such devices might include transmitters, transducers, sensors, meters, or indicators. An analogue signal such as this one is currently the most common form of input used in applications that require process control. Let’s take another step back and look at process control, what it is, and how it relates to loop calibrators. As its name might suggest, process control is a means of controlling and monitoring a process in order to maintain a standard of quality and performance. There are numerous types of processes that may need controlled in any given number of settings. For instance, it may be used to control physical parameters like pressure or temperature. One straightforward example of process control is the thermostat you use to manage the temperature in your own home or office. If you set your thermostat to 68 degrees, the thermostat monitors the temperature and then turns on the heating source or air conditioning when needed. When the room temperature reaches 68 degrees, the thermostat will turn the heat or air conditioning off. Current loops are used in process control to carry signals from process measuring instruments to devices like PID controllers and programmable logic controllers. The industry standard for current loops is the 4-20 mA loop. That’s 4-20 milliampere, where an ampere is the unit for measuring electrical currents. Why 4-20 mA? Current loops are almost always set at the 4-20 mA standard, but that wasn’t always the way that process control worked. Before electronic circuitry came into the picture, process control was done mechanically. Without access to the option of current loops for sensor signaling, processes had to be controlled with pneumatic control signals. Controllers were adjusted via the changing pressure of compressed air, and the standard was measured by pound-force per square inch rather than ampere and electrical currents. At that time, the standard was 3-15 psi. This range started at three rather than zero for several reasons. First of all, measuring signals that were less than 3psi wasn’t an option because the signals were unrecognizable unless one was willing to spend much more money for a system that could effectively detect signals under 3 psi. It was also easier to notice a failure in the system, which would register at 0 psi. If the range had started at 0 psi, then low signals would be hard to differentiate from a problem with the system. In the 1950s, things began to change. Electronics became more affordable, as well as more reliable. Process control started to favor electronic controllers over the mechanical, pneumatic options. When current input became the more popular process control signal, the standard shifted to a 4-20 mA range. The 4-20 mA range was chosen to reflect the older psi range. It began at 4 mA rather than zero for similar reasons. It’s too expensive to build systems that can detect a 0 mA signal, and it’s too difficult to differentiate between a 0 mA signal and a system failure. The 4-20mA standard has continued to work well in current loop circuits. How to use a loop calibrator Like all categories of meticulous measurement, process control requires routine calibration to help it stay accurate. The loop calibrator itself is a hand-held device with multiple mode settings and functions. There are several instances in which you would use a loop calibrator for your work. Here are some of the more common instances when a loop calibrator may come in handy. Testing loop powered isolators A loop powered isolator is a switch that is designed to cut off ground loops in a control system while allowing the control signal current to travel to another part of the system. Loop powered isolators get their power from the input portion of the isolator at 5.5 to 13.5 V. The output of the loop isolator will mirror the current on the input side. A loop calibrator will test and troubleshoot the functionality of a loop powered isolator. Testing two-wire transmitters Like a loop powered isolator, a two-wire transmitter can isolate the ground loops from other control signal currents, but also offer signal conditioning for different types of inputs. A two-wire transmitter gets its power supply from the output side of a transmitter, and it controls the current of the power supply in proportion to the input. A loop calibrator can also be used to test the behavior of a two-wire transmitter. Testing valve positioners Electronic valve positioners need to have routine calibration to ensure that they’re functioning properly. A loop calibrator can be used to follow the valve positioner manufacturer’s specific instructions on calibration. Calibrating voltage input signal conditioners An input signal conditioner takes one type of electrical signal and turns it into another. It may, for instance, take voltage and turn it into a standard process output of 4-20 mA. Loop calibrators are perfect for calibrating the many different types of input signal conditioners, whether they are dealing with standard and non-standard voltage. Wondering what we calibrate? Check out our Visual Index.
