Current Loop units use a continuous voltage, usually 24 Vdc, with a varying current level that changes in direct proportion to the flow, weight, volume, etc. of what they are measuring. The most common type (90 to 95 %) is the 4-20 mA Current Loop, with a few 10-50 mA Current Loops in use. This current loops from a scale or meter out to a load cell, motion sensor, speed sensor, volume sensor, etc. and back again. As the weight, speed or volume increases, the current will also change in a direct correlation. The program at the primary end of the Current Loop is calibrated to read the exact current level and translate that into the correct value on the scale or meter.
A truck scale is a good example to explain how a Current Loop works. The load cells convert force into an electrical signal. The load cell sensor detects force (the weight of the unloaded, or loaded truck) and moves the strain gauge. This changes the resistance, which changes the current. This is read by the monitoring system in the scale house. The monitoring system reads extremely small changes in the current, thereby allowing for a wide range of weight measurement on the truck scale. There is no data signal (Mbps) or frequency (Mhz) on a common Current Loop. It is mostly an analog system reading the changes in current level.
- Measure water or other liquid flow rate past a given point
- Measure liquid or gas pressure
- Measure liquid or dry material level in a tank
- Measure weight on a scale or load cell
- Measure heat level
- Measure speed of a production line
- Measure the volume or pressure in a pipe transporting liquids or gas
Current Loops can be 2-wire, with or without a shield for terminal units. For conduit type units there can be 2-wire or 3-wire, plus ground. It is important to know the number of wires and the presence of a shield or ground in order to specify the correct current loop unit. You also must confirm the voltage is 24 Vdc, that it is a 4-20 mA current loop and not just a 24 Vdc relay circuit or a digital data circuit, and the type of enclosure required for the environment where it will be applied.
If you want to measure:
Volume Flow Speed Pressure Level Weight
Or any other variable where a sensor can detect a change within a known range, a Current Loop is most likely part of that process.
A Current Loop Surge Protective Device at one, or both ends of that circuit is often the difference between continuous, accurate, available measurements, and constant downtime and parts replacement.