How to engineer 4-20 mA loops
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Current loops are a very common way to transmit data from a sensor to a remote location. Using a 4-20 mA signal opens the door to measuring just about anything, from flow to pressure to carbon dioxide, in measurement applications ranging from BAS in operational buildings to data loggers supporting commissioning or troubleshooting efforts.
What 4-20 mA current loops are and how they work
In general, current loops use a transmitter between the sensor measuring the process variable and the input device associated with the control system. Process variables could include temperatures, flows, humidity, and carbon dioxide. Today, if you want to measure it, there is probably a sensor with a 4-20 mA output that is available to do so. Typical input devices include the field panels and controllers associated with current technology, direct digital control systems, independent stand-alone controllers, and local indicators intended to display the process variable but not necessarily control it.
The transmitter generates a dc current that is directly proportional to the measured-process variable. For instance, a 4-20 mA transmitter rated for 0-100 F would generate 4 mA at 0 F, and 20 mA at 100 F.
The common standard is to provide a linear signal that varies from 4-20 mA, as the measured variable shifts over the field sensor’s range. Other standards exist, like 10-50 mA, but they are not common in the commercial HVAC market and usually limited to process control applications.
If you’ve had some experience and exposure to controls, you will likely notice the voltages are also common input standards.
Current loops can serve multiple devices by simply wiring the loop in series through the load resistors, and as long as the power supply has enough voltage to drive 20 mA through the sum of the resistances (including the wire), the process will work.
Data logger applications
“How do you hook up a 4-20 mA current loop to a data logger?” Questions like this often arise while an engineer is in the field on a project. In addition to building up power supply panels, teams must also realize that these panels are major components of the solution (see Figure 2).
The dc power supply panel is the gray box and the data logger is the tan square inside of the lower left-hand corner. The green rectangle toward the top center of the panel is the actual dc power supply, which is wired to the terminal blocks immediately below it on the low voltage side and to a standard 120 vac cord and plug on the high-voltage side. The square black boxes to the right are transmitters that I was about to install to monitor the 3-psi to 15-psi signal serving a couple of pneumatic control valves and dampers. The picture in Figure 2 was taken while I was checking to make sure everything was working properly before I installed the panel in the field. Figure 3 shows the panel installed in the field.
picture, but the pneumatic signal transmitter is the black box tie-wrapped to the catwalk railing in the upper left corner. In Figure 4, you can see the black tubing leading from the transmitter to the pneumatic line serving the actuator we are monitoring.
Transmitting information over distances
When you are sitting at a modern control system front end, it’s easy to forget that there are a lot of things between you and the process parameter that you are trying to measure, as illustrated in Figure 5.
For instance, on the input side of the system in Figure 5, the transmitter, wiring, controller, network device, and workstation all can introduce errors into the reading observed by the operator relative to what is going on at the sensor.
Current loops offer advantages in noise immunity and allowing information from a sensitive but low-gain-measuring element to be accurately transmitted over long distances. For instance, a 100-ohm platinum resistance temperature device (RTD) is an accurate way to measure temperature, but the resistance change associated with a temperature change is modest—fractions of an ohm per degree F.
RTD measurements typically are made by applying the RTD in a resistance-bridge and using the change in voltage as in indication of the change in temperature. Since the changes in resistance are small, the associated changes in voltage are small, typically on the order of millivolts (mV) per degree F.
Often in the field, the induced voltages from the conductors serving the machinery can exceed these voltage levels by several orders of magnitude. For example, when troubleshooting a chiller-interlock circuit, I kept picking up 10-15 vac in a circuit that was isolated from any power supply. Then I realized that my control conductors were running in a long cable tray in parallel with the large conductors carrying hundreds of amps to a different chiller in the plant. Consequently, the long run of parallel wires was acting like a transformer, inducing a voltage in the interlock circuit from the power feeders. Turning off the chiller made the induced voltage go away.
10-15 V of induced signal would completely obscure an mV-signal on cable carrying information from an RTD to a controller. Shielding helps address the issue by blocking and channeling away the undesired signal. For control systems, shielding typically is a metallic foil or metallic braided sheath that is wrapped around individual wires or pairs of wires with the entire assembly enclosed in an insulating jacket of some sort (see Figures 6 and 7).
Typically, control wiring for commercial office buildings is in the form of #18TSP, which stands for one pair (P) of 18-gauge wire (#18), which is twisted (T) and shielded with a foil jacket (S).
In Figure 6, from left to right, are Ethernet Category 5 cable (CAT5) #16 unshielded twisted pair, #18 three conductor twisted shielded, and #18 TSP. In Figure 7, from left to right, are the stripped jacket (blue), the three conductors, (black, red, and white), the fiberglass strand used to strip the jacket off the conductors, the foil shield, and the shield drain wire.
It’s important to realize that just because a cable is shielded does not always mean the shield will be effective, because installation practices can play a big part in the success or failure of a shielding system. For instance, if the shield wire is grounded (accidentally or intentionally) at both ends of the cable, then the shield becomes a current-carrying conductor between the two grounding points. A common misconception is to think of ground as being 0 V, but if you measure the ground voltage at different places relative to a common point in a building, there are actually minor voltage differences.
generates electromagnetic fields, which can couple to adjacent conductors and induce voltages and currents.
Even if shielding were perfectly implemented and eliminated the potential for noise in the measured signal, the wires that are carrying the mV signal from the RTD to the controller still can cause a problem. More specifically, wire has a resistance; even in good conductors such as copper. Consequently, a long run of wire adds to the resistance of the RTD, and without special compensation circuits, can be interpreted as part of the signal. The Control Design Guide on the Portland Energy Conservation Web site contains an example of how much this can impact the information from an RTD in a typical HVAC application.
The relationship between shielding and 4-20 mA current loops is that by their nature, 4-20 mA current loops are resistant to noise and it’s possible to use unshielded cable when installing them. While possible, the cost of shielding for a #18TSP is relatively modest compared to the cost of a typical wiring installation. So, you are better off using a shielded cable that can serve multiple types of inputs. This makes it less confusing to install the wiring in the first place and provides some flexibility in terms of changes that might be made down the line to the input devices and/or the control equipment.
Early in my career, I learned a valuable lesson about the significance of lead resistance and the impact it can have on the parameter one is trying to measure. After measuring and re-measuring, I discovered that the lead resistance in a 1,000-ohm-copper-RTD circuit, which amounted to 1.5 F, was greater than the temperature change the control system needed to detect to make the decision to start or stop a chiller (about 0.5 F).
Compounding the problem was the fact that resistance of the lead conductors between the control panel and the RTD varied with temperature. As a result, when the mechanical rooms were cool, the lead resistance error was different from when the mechanical rooms where hot. Again, the change was more than the change I was trying to measure because in the winter the mechanical rooms could be in the low 60s F, while temperatures in the summer could exceed 120 F.
|Table 1: Scaling resistor voltage drops for common resistors with a 4-20 mA current flowing through them.|
|Load resistance, ohms||250||500|
|Current loop current, amps||0.004||0.020||0.004||0.020|
|Load resistance voltage drop, volts||1.000||5.000||2.000||10.000|
Adding a 4-20 mA transmitter to the RTD circuit was one way to address some of these issues in my particular situation as well as in general. For one thing, since the signal is a current rather than a voltage, it is fairly immune to the impact of induced voltages. In addition, as long as the power supply is adequate, the wires can be run for literally miles serving multiple controllers and indicators without degrading the signal.
It’s important to remember that the transmitter itself, being another device between the process and the controller, will introduce its own errors on a number of fronts including hysteresis, linearity, temperature effects, and mounting effects. However, in most instances, the cost is worth the benefit.
In addition to the technical reasons, current loops are attractive because they are a common standard supported by many manufacturers. This is beneficial because a user or device that can pick up a signal from a 4-20 mA current loop can measure just about anything, from temperature and flow to pressure and toxic gas levels.
If this article has tweaked your interest in current loops, you may find my other posts on the topic to be of interest. The blogs start in April of 2009 and are summarized in the sidebar, which includes instructions for building a dc power supply panel so you can use 4-20 mA transmitters with field deployed data loggers.
Given how flexible the technology is, future blog posts will expand on this topic, including how to apply current loops to measure and troubleshoot the systems we all work with to control the built environment.
|Sellers is a member of Consulting-Specifying Engineer’s editorial advisory board. His background includes more than 30 years of experience with commissioning, design engineering, facilities engineering, mechanical and control system contracting, and project engineering in a wide array of facilities. This article was developed in collaboration with the Building Commissioning Assn|