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How to Make 4-20 mA Current Loop Measurements

2021-10-02 02:39:34 dnordling

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


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CSFM committee meeting on rocketry, 2020-02-21

2020-02-23 19:27:09 dnordling

by Dave Nordling, RRS.ORG


The California State Fire Marshal’s (CSFM) office held a sub-committee meeting to discuss potential changes to the definitions in the state laws concerning amateur rocketry. This meeting on Friday, 2/21/2020, at the CSFM offices in Monrovia was the second of two meetings held between CSFM and amateur rocketry representatives to informally discuss and review ideas for improvements. The laws had several ambiguities and areas for improvement which would better reflect the needs of our growing amateur rocketry community and provide for clarity and safety at all points.

The RRS was glad to host Ramiro Rodriguez, the state fire marshal of the local Hollywood office last year both at our February 2019 meeting and later as a speaker at the 2019 RRS symposium in April. The RRS and representatives from the Friends of Amateur Rocketry (FAR) and the Rocketry Organization of California (ROC) had met several times over the last calendar year leading to a consensus opinion on a few areas that would be presented to CSFM for consideration. Many of the ideas were with regards to fees, transportation issues and the different licensing classes of pyrotechnic operators for rocketry.

The RRS, FAR and ROC were glad to have the opportunity to speak frankly and give the state reasonable ideas that would preserve the freedoms in our hobby while keeping only responsible measures for assuring public safety as is required by CSFM.

CSFM will report their findings back to the home office in Sacramento and begin to discuss the next steps to amend the legislation governing amateur rocketry. We hope to hear more in the coming months as several of our ideas were accepted.

For any questions, contact the RRS secretary.


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50 Years After One Small Step for a Man

2019-07-21 07:27:03 dnordling

By Dave Nordling, Secretary, Reaction Research Society


It was a half century ago today that mankind landed on the Moon. This event has had an impact on both generations present to witness this landmark event and the generations born afterward, such as myself. The Apollo 11 moon landing was a daring extension of an aggressive program that was progressively built from the dawn of the space age with abundant resources, acceptance of risk and political will never seen before (and never since). The herculean task set by the late President Kennedy in 1961 of landing a man on the moon and safely returning him to Earth by the end of the decade (1970) was fulfilled on July 24, 1969.

A grainy image of the American flag planted on the moon.

It was only eight years before that time when manned spaceflight began with the humble beginnings of riding a derivative of an intercontinental ballistic missile (ICBM) into low earth orbit scraping the bounds of the upper atmosphere. The journey was fulfilled with the enormous 6,540,000 lbm tower of three stages of the Saturn V vehicle filled with kerosene, liquid hydrogen and liquid oxygen that pushed three brave men into a new sphere of influence of the Earth’s closest celestial body just three days away. New systems and new rocket motors were built from scratch and flown in less than a decade. The massive Saturn V rocket could throw an unprecedented 107,100 lbm to trans-lunar injection (TLI) orbit. No other past or operational launch vehicle has surpassed this ability to this very day.

The Saturn V leaves its pad with a thrust of over 7,500,000 lbf.

Looking back, landing a man on the lunar surface appears simple and almost certain. But to those watching from their black and white televisions across the country and to the men and women behind the launch consoles, all of the Apollo missions were truly audacious with the looming deadline, a Cold War rival busy at work to maintain their leadership in space and an ever-present risk for tragedy at every step. Lives were lost, sacrifices were made and the goal remained steadfast. Excellence was demanded from hundreds of thousands of technical professionals, suppliers, shop workers, clerks and everyday people and was delivered such that two astronauts could walk on a foreign world opening the door to our species visiting a place beyond our blue Earth.

Skipping along the lunar surface getting work done. Beyond the human experience and reflection, this was an expedition filled with experiments to extend human knowledge.

At this 50th anniversary, it is interesting to reflect on what has happened since. After six more Apollo flights with five resulting in 10 more Americans walking, even driving over the lunar surface, the program came to an end under the Nixon Administration’s budget cuts. No other nation, including our own, has returned. It is probably due to this fact alone that more and more people begin to doubt whether the moon landing was ever real.

Also, it is the opinion of this author that because the Soviet Union’s then-secret moon program failed to place a cosmonaut into lunar orbit with their massive N-1 rocket, let alone a successful landing on the lunar surface, that our country saw fit to halt the progress of Apollo and turn our back on the Moon for five decades. I can only imagine how history would be different if the any of the four Soviet launches of the N-1 from February 1969 to November of 1972 had been a success.

The first Soviet N-1 rocket sits on its pad at Baikonaur in September 1968.

https://en.wikipedia.org/wiki/N1_%28rocket%29

The first man on the moon, Neil Armstrong, has passed away just a little less than seven years ago. Buzz Aldrin and Michael Collins remain as living historical witnesses, but in time, they too will pass on. NASA has a huge discontinuity in their chronology of exploration after the Apollo and Skylab program’s success. A long period of quiet then the Shuttle followed by eight years of paying the Russians for rides to the International Space Station (ISS) from Russia is all that remains. Our unmanned program has continued with ever more impressive returns as we learned about the moon, Mars and places throughout in the solar system, but our manned space program remains at a stand-still.

The legacy of Apollo has been more of historical legend and pride than any tangible progress eclipsing this feat of human achievement. The Space Shuttle program and its nearly four decades of life brought us the historical achievement of the first American woman in space, the first African-American in space, the launch of the Hubble Space Telescope, the first visit to a Russian space station, Mir, the first Russian cosmonaut to fly on an American space vessel, and of course the multi-year construction of the ISS celebrating its third decade of operation even after the Shuttle’s retirement. There are many people who feel that the Shuttle program failed its basic promise of routine access to space and certainly to fulfill the loftier goals of men reaching beyond low Earth orbit.

Since the days of Apollo, there have been new discoveries about the Moon. Thanks to the Lunar Reconnaissance Orbiter (LRO) launched in 2009, the Apollo launch sites have been seen in higher detail.

https://www.space.com/14874-apollo-11-landing-site

The Apollo 11 landing site and the crew’s discarded equipment as seen from lunar orbit courtesy of the Lunar Reconnaissance Orbiter.

The Indian ISRO Chandrayaan-1 lunar orbiter, the Japanese Kaguya lunar orbiter and the American LRO have each found evidence of lunar lava tubes and “moon caves” in several places along the lunar surface which offers a tantalizing possibility of a ready-made shelter for future manned exploration.

An excellent new point of interest on the Moon’s surface, lunar lava tubes found by orbiting spacecraft.

https://en.wikipedia.org/wiki/Lunar_lava_tube

Further evidence of ancient lunar lava tubes as seen from orbit.

The discovery of water ice in the permanent shadow in craters at the Moon’s poles starting from the Soviet Luna 24 probe to the ISRO Chandrayaan-1 orbiter provided strong evidence of an important resource awaiting future lunar explorers. .

https://en.wikipedia.org/wiki/Lunar_water

Distribution of water ice at the Moon’s South and North poles

Most recently, on January 3 of this year, the Chinese with the Chang’e 4 have soft-landed a rover (Yutu-2) on the far side of the Moon, a first for any nation.

https://en.wikipedia.org/wiki/Chang%27e_4

Lunar tracks by the Chinese Yutu-2 rover in the soil at the von Karman crater in the South-Pole Aitken Basin region on the far side of the moon.

With the end of the Space Shuttle program in 2011, planned since the Columbia disaster of 2003, the Constellation program, later renamed the Space Launch System (SLS) was built and extended from legacy technologies with years of flight experience.
At this moment in time, NASA has redoubled its commitment to returning people to the surface of the moon in just five years from now, 2024. It is possible this goal can be realized, but there are abundant reasons to be skeptical.

Technology is no longer the perceived barrier to finding our way back to the Moon. The ability of any government or administration to muster the cohesive, sustained political will and necessary funding to build and fly the SLS program to put men back on the moon is the question that remains unanswered. More so, will we have the fortitude to recover from failures should they occur and surmount them to make a permanent colony as was envisioned for after Apollo? To date, my generation has waited in vain on the many promises from NASA to deliver something of the magnitude of Apollo.

There is no shortage of passionate, intelligent people in this world. Many share the vision of mankind becoming an interplanetary species. Our art and culture have been permanently changed from seeing the whole of our world as a small blue marble against the enormous blackness of space. The true legacy of Apollo is the inspiration that was given to this nation’s people and any nation seeking to find pride in their abilities to put their citizens in space. Regardless of what may come in the next few years with NASA, the dream is alive with the people of the Earth to be explorers. To move beyond dreams is what will extend mankind to the Moon and beyond.


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