How to Make 4-20 mA Current Loop Measurements

by Roger Lockhart of DATAQ Instruments

originally published 9/30/2013, reprinted with permission


It seems that at least one 4-20 milliamp (mA) measurement is required by our typical customer, and the way to do it is a constant source of confusion for many.  So I thought I’d zero in on the various 4-20 mA current loop configurations and elaborate on the specifics you need to know to make a successful measurement.  The following is ordered from the most to least common configuration, and I hope to cover all those that I encountered in customer applications.  If yours isn’t included, please contact DATAQ technical support.

dataq.com

4-20 mA Current Loop Basics

Sensors or other devices with a 4-20 mA current loop output are extremely common in industrial measurement and control applications. They are easy to deploy, have wide power supply requirements, generate a low noise output, and can be transmitted without loss over great distances. We encounter them all the time in both process control and basic measurement data logger and data acquisition applications.

The idea behind 4-20 mA current loop operation is that the sensor draws current from its power source in direct proportion to the mechanical property it measures. Take the example of a 100 psi sensor with a current loop output. With 0 psi applied, the sensor draws 4 mA from its power source. With 100 psi applied the sensor draws 20 mA. At 50 psi the sensor draws 12 mA and so on. The relationship of mechanical property measurement to current output is almost always linear, allowing the resulting current loop data to be scaled with a simple mx+b formula to reveal more useful measurements scaled into engineering units.

How you actually measure the 4-20 mA current loop signal is a function of the sensor’s architecture and the capabilities of the instrument you’ll use for the measurement.

Terminology

So that my discussion translates well across the various kinds of 4-20 mA current loop configurations, I’ve opted to standardize the terminology I use to describe each. Here’s an overview:

“E” (DC excitation)

Most configurations that follow will show a DC voltage excitation source that I denote as “E”. Many who use current loop sensors for the first time are surprised to learn that they need to supply this excitation source. Nonetheless, unless the sensor is self-powered (i.e. AC line powered) an external dc source is required. The good news is that this can sometimes be supplied by the instrument, and the range of acceptable supplied voltage values is usually very wide, typically 10 to 24 VDC.

“R” (shunt resistor)

Here’s a bit of trivia for you:

No instruments measure current directly.

They all do it indirectly by measuring the voltage dropped across a resistor of known value, and then they use Ohm’s Law to calculate actual current. The resistor is referred to as a “shunt”, is absolutely required to make a current measurement, and is either supplied externally to, or built into the measuring instrument. For clarity, I assume that it’s supplied externally.

“i” (current loop value ranging from 4 to 20 mA)

This is the 4 to 20 mA current signal generated by the sensor. Note that some sensors may draw 0 to 20 mA and even other values, but the vast majority of them use the 4 to 20 mA convention.

“v” (shunt voltage that’s proportional to current)

This is the voltage drop across the shunt that is actually measured by the instrument. Since our industry has standardized on a shunt value of 250 Ohms, “v” will range between 1 and 5 volts for a 4-20 mA current loop signal.

Note that shunt resistor value is arbitrary as long as it’s a known (fixed) value. You also need to ensure that it doesn’t burden the loop, so lower values are better than higher.

Yes, I mean lower.

Remember that we’re working with current, not voltage, so the rules are inverted. Just as infinitely-high resistor loads work well for a voltage source, you can take the load all the way to zero Ohms for a current source without consequence.

 Self-Powered Sensors (Most Common)

I promised to order these configurations from most to least common, and the self-powered sensor just noses out the first runner up. Self-powered sensors are those that, well, power themselves. The sensor may have an integral ac power supply, thereby negating the need for an external DC power source.

Or it may not be a sensor at all. It could be an output from a Programmable Logic Controller (PLC) or other source that is internally powered.

2-wire Sensors (Low-side Shunt)

Okay, this can get confusing for first-time  4-20 mA current loop users.

Yes, it is possible to both power the sensor and measure the current it draws over the same two wires. In the 2-wire examples shown here, only two wires connect the sensor to its power supply, and the sensor draws current from it in direct proportion to the mechanical property that it measures. As current changes, the voltage developed across resistor “R” will change, thus providing a signal that’s suitable to connect to a measuring instrument like a data logger or data acquisition system.

In most situations, care should be taken to place the resistor in the low-side of the loop as shown here, as opposed to the high-side. Doing so will allow non-isolated instruments to make the measurement. In the next section, I’ll deal with a high-side shunt placement and discuss these cautions in more detail.

2-wire Sensors (High-side Shunt)

This configuration is almost exactly like the low-side, 2-wire approach, but it places the shunt resistor in the high-side of the loop. Note that while the voltage across the resistor is proportional to the current drawn by the sensor (just like the low-side approach), there is also a common mode voltage (CMV) present on either side to ground. On one side to ground the CMV is equal to the supply voltage. On the other side to ground it’s equal to the supply voltage, less the voltage dropped by the resistor (v).

The presence of the CMVs places conditions on the instrument that you use to measure v. Specially, the instrument needs to have an isolated front end so it can float to the level of the CMV and still successfully make the measurement. Try this with a non-isolated, single-ended instrument and you will short-circuit the sensor to ground. A non-isolated differential instrument will either saturate or provide erroneous results.

3-wire Sensors

Three-wire sensors with a process current output have a separate wire for ground, signal (4-20 mA), and the power supply. This configuration is the easiest for current loop beginners to grasp, one input for power and a second for the current loop with a common ground. The primary advantage of a 3-wire sensor over its 2-wire counterpart is its ability to drive higher resistive loads. Resistors drop voltage for any given current in direct proportion to their resistance value. Holding current constant, higher resistances drop more voltage. Turning back to the 2-wire sensor and holding current constant, as the shunt resistance increases the voltage drop across it also increases. You might reach a point where the voltage dropped by the shunt lowers the voltage drop across the sensor below the minimum required for it to operate properly.

We had a customer whose 2-wire current loop measurements functioned beautifully until loop current reached about 18 mA, at which point everything went haywire. Upon close examination, we determined that the supply voltage she used was too low by at least 0.56 volts. She needed 2 mA more measurement to reach full scale, which translates to 0.56 V with her 250-Ohm resistor. The solution was to use a higher voltage power supply to ensure that the voltage drop across the sensor stayed above the minimum level. She could have also used a 3-wire sensor, which ensures that the voltage applied to the sensor is independent of shunt resistor voltage drop.

Watch Your Grounds (or use an isolated instrument)

Contrary to what many believe (and have been erroneously taught in school), grounds are almost never the same in industrial settings, exactly where most 4-20 mA current loop sensors are used.

Two or more grounds that are the same means that they are at the same potential. If so, a measurement between the grounds of the various field sensors and the instrument using a digital volt meter (DVM) on both its DC and AC settings will show zero volts, or very close to it.

In reality, you’ll measure at least several volts, and I’ve seen as much as 75 Volts. When grounds that are not at the same potential are tied together (which you need to do to make a measurement), current flows through them, creating several possible measurement outcomes for non-isolated instruments:

  1. The measurement is noisy.
  2. The measurement is inaccurate.
  3. You irreparably damage the instrument.
  4. You saturate the instrument (it’s not damaged, but you can’t make a successful measurement, either.)

To remedy these problems requires the following:

  1. Use an isolated instrument for your 4-20 mA current loop measurements. This single decision allows you to ignore all other grounding issues in exchange for successful measurements in any situation. If you don’t have an isolated instrument, read on…
  2. Ensure that the loop power source is isolated. This means that its output ground (the one connected to the sensor) is not tied to its input ground (the one that connects to AC line power.) An isolated power source means that the output ground can be tied to another ground (like a non-isolated instrument) without consequence.
  3. If using self-powered sensors, ensure that the low-side of the loop is isolated from its power source.
  4. If using sensors that require an external dc power source, ensure that the shunt resistor is placed in the low side of the loop (see “2-wire Sensors (Low-side Shunt)” above.)
  5. If you lack control over the power sources and determine that they are not isolated, then your only option is to power ALL devices (power supplies, self-powered sensors, the instrument, and its connected PC) from exactly the same power outlet. Don’t make the mistake of using outlets that are close to each other. If you run out of receptacles on a single outlet, then use a power strip.

Again, it’s worth repeating that all of the cautions associated with proper grounding disappear if an isolated instrument is used to make the measurement.

Sensors with 4-20 mA outputs are encountered in all disciplines and in many configurations.

Contact DATAQ with any questions that arise in your unique situation.

dataq.com


This article has been reprinted with permission from DATAQ Instruments, a manufacturer of quality data acquisition and data logger products used by many professionals and amateur rocketry hobbyists.  

The RRS is thankful to DATAQ for their assistance.

Also, you can watch DATAQ YouTube instructional videos on this and other subjects.

For information on DATAQ products, go to their website: 

www.dataq.com


September 2021 Virtual Meeting


by Keith Yoerg (RRS Secretary)


The latest meeting of the Reaction Research Society took place Friday, September 10th and had 16 attendees, including several members of the University of Michigan Aeronautical Science Association (MASA) team which conducted a long campaign of testing at our MTA site last month. After brief hellos & a short discussion on updates to this website, we got the meeting rolling with a presentation from the MASA team debriefing us on their August tests.

Screenshot of discussion during the monthly meeting

PRESENTATION ON MASA CAMPAIGN AT THE MTA

4 members of the MASA team who were present for the long campaign of testing at the MTA site were at the meeting and provided a presentation detailing how the nearly 2 weeks of work went for the team. A detailed write-up from RRS member Dave Nordling who was on site assisting for some of that time is available here.

MASA logos – Tangerine Space Machine is named after a craft brewed beer local to the Michigan area

While the end goal of conducting a hot fire test was not accomplished, there was a ton of great work and learning opportunities for both the MASA team and the RRS. The first challenge for the MASA team was in driving their equipment all the way from Michigan to Southern California. Unfortunately, they had some equipment suffer damage during the trip when their smaller “Ground Support Equipment” (GSE) trailer broke through the plywood floor of their larger travel trailer. This required them to stop in Texas and complete repairs to the travel trailer before continuing on to the MTA site.

Once at the site, the MASA team set up a 2-shift schedule (9am – 6pm and 5pm – 12am) effectively working 18-hour days to conduct the activities required for their tests. Many of these tasks took a lot longer than the team had anticipated, in particular with the supply and delivery of the gas & cryogenics needed. The MASA team was originally in contact with Airgas, but their communication was with the office closer to their home in the Midwest and not the Southern California branch. Ultimately sorting through those issues proved to be too difficult to secure a delivery while the team was at the MTA, but fortunately they were able to work with Praxair to secure a supply of the gas and cryo.

MASA presenting a great photo of nighttime work at the MTA to the members at the September meeting

Once the gas and cryogenics were at the site, the team was able to complete pressure tests of the fuel side of the system, and were ultimately able to perform 2 coldflow tests through the entire system near their target pressures. These tests revealed many more areas for design improvements that the team hopes to implement, including reducing fittings, changing the location of vents and drains, and possibly even replacing the LOx and fuel tanks.

Improvements the MASA team hopes to implement

This campaign goes down in the books as the longest to ever take place in RRS history, and proved to be challenging for both the MASA team and the RRS. Several society members graciously volunteered their time to help make this testing effort possible, and the experience revealed many ways that the society could improve our procedures to better support extensive tests like this. Namely: limiting the duration of a test period to no longer than 1 consecutive week, requesting that some members of University staff be on-site when these long campaigns take place, and requiring a longer notice time before approving this sort of test were all brought up by RRS president Osvaldo.

PERMANENT BATHROOM

Progress is continuing on the permanent RRS Bathroom structure. Work on cutting holes for doors and windows has been completed on the 20-foot shipping container and delivery is expected imminently to the new work site at Wilbur’s hangar. The next stages of construction including adding plumbing, fixtures, and the doors and windows, and Osvaldo has already acquired some of these items to install

View from inside the container with the doors and vent windows installed

SEPTEMBER MTA EVENT & WORK PARTY

The USC Rocket Propulsion Lab (USCRPL) plan to be out at the MTA site from Friday, September 24th – Sunday, September 26th to conduct a static fire of an 8″ diameter solid rocket motor. The first few days will consist of prep work and the firing is planned on that Sunday. Several members including Bill Inman and John Krell indicated in the meeting that they have stepped up to the Yoerg Challenge and built model rocket kits to fly at the MTA. This will give us a great excuse to test out the new PVC wire rail launchers as well as the newly purchased Cobra wireless firing system.

In addition, the society has decided to use Sunday, September 26th as the date for our annual “Work Party” to perform maintenance and cleaning tasks at the MTA site. The expected tasks we would like to complete are:

  • Weld the plate on the vertical test stand
  • Removal of dry vegetation
  • Move drum of RP1 (from MASA testing) into one of the lockable containers
  • Fix the 2 broken latches on the Dosa Building roll door
  • Prep/measurements in area for new container bathroom

YOUTH ROCKETRY CLASS

RRS Vice President Frank updated the membership on the upcoming youth rocketry class in Boyle Heights with details on the schedule and overall plan. As opposed to 2 alternating classes, we will now be working with a single class of up to 30 students. The classes will run every Friday starting on September 24th, with a launch planned at the MTA site on November 13th. The plan is for each student to build their own Estes Baby Bertha kit and fly it twice on launch day. RRS Secretary Keith is currently working on 3D printing fin alignment jigs for the students, which will help in both teaching the students about that technology as well as properly installing straight & aligned fins on their rocket kits.

Several of the 3D printed fin alignment jigs

MISCELLANEOUS DISCUSSION

The end of the meeting consisted mostly of miscellaneous discussions around the various projects that RRS members are currently working on. Some of the highlights included John Krell’s new, very small (20x80mm) electronics board capable of collecting 7 channels of data onboard a flight, with over half of those channels at 500 Hz! Bill Inman updated members on the progress with his Solar Cat steam rocket, and there was a brief discussion of ham radio operations with Tom Hendricks sharing some of his wealth of knowledge in that subject. Overall it was a fun meeting with a lot of good discussion & participation from the membership.

NEXT MONTHLY MEETING

The next RRS monthly meeting will be held virtually on Friday, October 8th at 7:30 pm pacific time. Current members will receive an invite via e-mail the week of the meeting. Non-members (or members who have not received recent invites) can request an invitation by sending an email to:

secretary@rrs.org

Please check your spam folders and add secretary@rrs.org to your email whitelist to make sure you receive the invitation.

MTA Launch Event, 2021-08-28


by Dave Nordling, Reaction Research Society


The RRS Mojave Test Area (MTA) was used by the student group, Michigan Aeronautical Science Association (MASA) of the University of Michigan at Ann Arbor.  Given the remoteness of the RRS MTA and the great distance that the Michigan team was willing to travel, MASA had planned an extended test campaign to use the site for their cold flow and ultimately hot-fire their RP-1/LOX 2,550 lbf liquid rocket engine.  Originally planned for a week, the team arrived on Monday, August 16th, and continued to use the MTA site through August 28th.  In the end, Michigan faculty called the end of the MASA test series as the new semester was starting and many materials needed to be returned before MASA left southern California.

The MASA logo on the back door of the mobile trailler.
Initial checks of the mobile propellant supply trailer. Over the road travel loosened a lot of plumbing joints.

MASA is a new student group to the society and had very ambitious goals in what they wanted to accomplish in the planned test series.  Typically, the RRS will work with new universities and new clients over a period of many months before agreeing to a first test series at the MTA on a weekend campaign.  Proper planning is an essential requirement for success and the RRS must become thoroughly familiar and comfortable with all planned use of the MTA site.  Like with all attendees to the MTA, indemnification waivers were required from all attendees including spectators.  MASA limited their staff to only essential personnel and ran a day and night shift to both safeguard their equipment through the night and provide continuous support to prepare for the next day’s events.  The team was able to find rented housing accommodations in the local areas of California City and Ridgecrest.

Nate Campbell verified valve functions in conjunction with the mobile control trailler.
The MASA control room was well equipped and was able to safely and remotely conduct test operations.
More leak checks under the rising moonlight and electric lamps at the RRS MTA.

MASA has had a couple years of experience with their propellant flow systems in laboratory tests at the university and was willing to hold several meetings with RRS members sharing their full test procedures and schematics, and answer questions posed by RRS pyrotechnic operators in advance of their arrival.  The MASA fluid systems had many appropriate safety features and used high quality valves and parts.  MASA has developed a control system that uses motorized needle valves in place of a pressure reducing regulator for independent propellant tank pressure controls.  MASA had conducted many tests of this system and held several tests to confirm proper operation in the early steps of their MTA campaign.

The MASA liquid rocket engine, RP-1 and LOX, rated for 4300 lbf thrust but the hot-fire goals of this campaign was 2550 lbf.

MASA’s system designs also had some problems with the nitrogen compressor (booster) system being unable to operate due to a regulator failure. The team was able to bypass the unit, but it limited the top pressure of the blowdown tests to the bottle pressure (2000 psi). A few changes were necessary for vent line routing to improve operational safety. Remote pressurization operations were safely executed but proceeded very slowly and thus a great degree of boiloff in the LN2 limited run time.

The composite overwrapped pressure vessel (COPV) originally was intended for compressed natural gas service in ground vehicles. These vessels have a good safety record in demanding applications and are often used in de-rated aerospace applications.
The MASA team made several changes to their vent line routing for improved safety.

The event was successful in some respects that it gave the students a practical understanding of how to conduct test operations under desert conditions.  It also revealed some of the shortcomings in their plumbing design (leaks) which they were able to fix well enough to get to cold flow with cryogenic LN2 and water on the last day of testing (when this report is dated).  The cold flow tests provided useful data in their control algorithm which will be useful to the next series of tests.  MASA also gained experience in safe cryogenic tanking and operations with these hazardous fluids.

MASA team proceeded into LN2 tanking of their oxidizer propellant tank for the cold flow test through their engine and plumbing.

Logistics was a big challenge for the MASA team due to errors in their communication with local suppliers.  Nitrogen and helium gas bottles were significantly delayed and cryogenic liquid nitrogen cylinders also were very late to arrive at the MTA.  Some of these problems can be easily mitigated for the next test campaign now that relationships have been better established. While MASA was disappointed with some of the outcomes from the test series, they are interested in returning to the RRS MTA in the latter part of this calendar year.  This follow-on test series will be discussed at length in the coming months.

A gang of nitrogen bottles sit chained togeither in a pressurant feed manifold.

The society was similarly challenged in supporting this MASA campaign.   The society is grateful to everyone who assisted at the MTA (Osvaldo Tarditti, Waldo Stakes, Bill Inman and myself) or those who gave their comments and concerns (Larry Hoffing, Jim Gross).  Several members spent multiple days at the MTA both during the week and on weekends.   The RRS provided the necessary oversight during the hazardous portions of the testing campaign which was particularly difficult to schedule during weekdays.  The MASA team was very open and disciplined in their interactions with the society.  The RRS was also glad for the University of Michigan’s support and communications throughout this event.

It was a challenging event which was made possible by the contributions of many RRS members over many days.  Frequent communication between MASA faculty and the RRS was a firm requirement on all days of this tenacious campaign and the MASA team provided daily briefings on their progress.

The MASA team showed tremendous dedication and perseverance sex shop over this extended campaign in the summer heat of the Mojave deset.
Cold flow testing complete with pressurized LN2 through the oxidizer path and water through the fuel path.

This testing campaign and current RRS policies will be discussed at the next monthly meeting, 9/10/2021.  Pursuant to our mission statement, the society is glad to support projects of this kind to universities capable of conducting safe experiments at our unique testing site and to those who are willing and able to provide the society with sufficient advance notice to review their reports, schematics and inspect their hardware.  This campaign is firm proof that we will need more licensed pyro-ops and more members available to support any similarly extended test series in the future if they are accepted by the council.  By building and enforcing a consistent and fair policy for all new and prior clients, the RRS can better operate to the benefit of everyone.

All requests to use the RRS MTA must be made to the RRS president and reviewed by the executive council.  For any questions about this test series or any future test series, please contact the RRS president.

president@rrs.org




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