4-20 ma Current Loop Experiments – Position Sensitivity

The next few posts will supplement a string of posts I did a while back on 4-20 ma current loops, including a couple of posts that show you how to build a DC power supply panel so you can use current loops with a Hobo data logger.

In this string of posts, I will look at a number of different transmitters while they are connected to a data logger and do some experiments with:

  • How position sensitivity can impact some sensors, which is the subject of the current post.
  • The temperature response of a thermal sensor when it is stimulated by a hair dryer.  We will look at the sensor response with the sensor in a thermo well and with the sensor outside of a thermo well.
  • The impact of lead resistance when it is introduced into the wiring on the Resistance Temperature Detector (RTD) side of temperature transmitter vs. introducing the resistance into the current loop.

As has been my practice of late for long posts, the following links can be used to jump to the content of interest.

Our Sensor of Interest

As the title implies, in this post, we will look at how mounting position can impact some times of sensors.  Specifically, we will take a look at how mounting position impacts the output from a Dwyer Magnasense differential pressure transmitter.

Dwyer Magnasense 01

These transmitters are typically used to measure relatively low pressures in duct systems.  For instance, this is the sensor I used to monitor filter pressure drop in the field trial I posted about in the past where I was comparing the life cycle operating costs of two different filters which were applied in very similar systems with very similar operating profiles. 

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Logging Filter Pressure Drop – An Application of This Type of Transmitter

Incidentally, you you are wondering how that turned out, I am still working my way through the data.  But in general terms, the more expensive filters proved to deliver the performance we had anticipated.  Specifically, by the end of the trial period, the pressure drop through the more expensive, rigid bag filters, which had a depth loading characteristic and a lot of dust holding capacity, was approaching the clean pressure drop of the flexible bag filter with roll media prefilter that we were comparing them too.

Stated another way, after over two years of operation, the more expensive filters, which had projected the best life cycle cost when we modeled them, had a pressure drop of just under 0.20 in.w.c.


In contrast, the less expensive bag filters, with relatively clean prefilters had just about twice the pressure drop after a similar time in service and several prefilter changes along the way.


So, while I’m still “crunching data” to develop hard numbers on the results, it sure looks like the filters performed as we anticipated when we modeled them. More on that in a subsequent post.  The point in bringing it up here is to illustrate the type of application where you might use one of the sensors we are going to discuss.

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Variations on the Theme

There are a number of different models of Magnesense pressure transmitters and there are significant differences between the various models, as can be seen from the following picture, which contrasts a MS-121-LCD (left) with a MS-131-LCD (right).

Compare 121 with 131

If you are staring at the picture thinking that you don’t see any major differences (or any differences at all for that matter), well, that’s my point.   These two 4-20 ma differential pressure transmitters look identical from the outside.  But inside, there are significant differences that make the one on the left much more position sensitive than the one on the right.

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Taking a Look Inside

The MS-121 can be configured to have a range of 0.10 in.w.c., 0.25 in.w.c., or 0.50 in.w.c.  In contrast, the MS-131 has a range of 10.0 in.w.c ;  an order of magnitude more than the MS-121. 

Compare 121 with 131 03

Frequently, instruments that measure pressure do it by applying the pressure to a diaphragm and then using the force generated by the pressure over the cross-sectional area of the diaphragm to move some sort of mechanism that generates a signal. 

The round shape of the Magnesense transmitter implies that there might be a diaphram involved.  That made me curious about how much force might be available to actually move something and generate a signal, so I did a bit of math and here are the results.


For the MS-121 on its low scale, that is not a lot of force when you consider that if the device really is electro-mechanical, the force has to create a meaningful movement in a repeatable, reliable manner.

The differences between the two transmitters start to show up when you take a look inside, as illustrated in the pictures below (MS-121 – left;  MS-131;  right).

photo 03 photo 05

The MS-121

As you can see, despite looking identical from the outside, the transmitters are two totally different devices on the inside.   When I tried to lift the circuit board on the MS-121, I found that something was holding it back. 

I didn’t want to break it so I did not pull very hard.  But given what I know of the technology and a reference in the manufacturer’s literature to a diaphragm (more on that in a minute), my guess is that there is a physical connection between a round diaphragm in the back of the transmitter and a device on the circuit board. Meaning that the diaphragm physically moves something in one of the circuit board components to generate a signal.

The MS-131

In contrast, for the MS-131, I was able to lift the board and flip it over which revealed a little black component with tubes connecting it to the sensing ports on the instrument housing.

photo 06

If you do a Google search for the model number on the little black component, you will discover that it is a piezoresistive pressure sensor manufactured by Freescale Semiconductor, Inc. (the internet really is an amazing thing, isn’t it?). 

In other words, inside the little black box, there is a very thin silicon diaphragm across which the pressure to be measured is applied.  It turns out that  that the resistance of a semiconductor or metal will tend to vary with strain in a fairly linear manner, a phenomenon called the pezioresitive effect.  The little black box inside the Dwyer M-131 leverages this phenomenon to generate a signal that is proportional to the differential pressure applied to it.

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Diaphragms and Gravity

If the MS-121 really does use a diaphragm to move something to generate a signal, then given the tiny amount of force available at full scale to produce motion and a signal (0.41 ounces at 0.10 in.w.c.), it occurred to me that the weight of the diaphragm may have an impact on the output of the transmitter.  Meaning that if the transmitter was mounted with the diaphragm in the horizontal plane, then maybe its weight would contribute to the over-all motion of the sensing system more than if the transmitter was mounted with the diaphragm in the vertical plane.

MS-121 Signal vs. Position

So to test that theory out, I powered up the transmitter and rotated it from horizontal through vertical and back.  Here is what happened.

MS-121 Signal Varies with Position with No Pressure Input


Obviously, we have very high video production standards here at the Facility Dynamics Engineering NW Satellite Office.  If you watch the video, you will discover that even though there is no pressure signal connected to the device, I am able to generate a signal from about negative .089 in.w.c. to positive 0.0024 inches w.c. simply by changing the orientation of the sensor relative to horizontal. 

The output is 0.0 in.w.c (the actual differential pressure that is applied to the instrument during the test) when the device is positioned vertically.  So, it seems like you would need to mount this device in that orientation to get an accurate measurement.  I bet there might be a few people out there who have used this device and wish somebody had told them that.

The Instruction Sheet

It turns out, somebody did (try to) tell them (I say this as someone who has learned this lesson the hard way a number of times).  The screen shot below is the instruction sheet that ships with the MS-121 with an important detail highlighted.


If you look at the instructions for the MS-131, you discover that they still want it mounted in the vertical, but  there is no mention of the impact of gravity on the diaphragm, just the concern about moisture entering the device, which was also mentioned in the MS-121 instructions.


MS-131 Signal vs. Position

Given the type of signal transducer that is supplied in the MS-131 and the information in the instructions, my guess was that the MS-131 was less sensitive to mounting position in terms of the accuracy of the output relative to what I had observed with the MS-121.  So, to test out that theory, I hooked my MS-131 up to a power supply and repeated the same test I had performed on the MS-121.  Here is what I saw.

MS-131 Signal Does Not Vary Much with Position


If you watch the video, you will see that while there is a modest variation in signal with position for the MS-131 (I was able to make it change from about 0.04 to 0.09 as I rotated it from horizontal to vertical) it is an order of magnitude or more less significant than the change I could generate by rotating the MS-121.

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The Practical Implications

You may be thinking that this is all very interesting technically, but what are the practical implications.  It turns out that I just had a recent experience with this down at the Pacific Energy Center. 

Specifically, the control system at the PEC was recently upgraded.  Subsequent to the upgrade, we noticed that the relief fan for the AHU seemed to run quite a bit more than it had in the past. So, Ryan used the problem as a lab exercise for one of the Existing Building Commissioning Workshop lab sessions and guess what they discovered?

It turned out that the signal controlling the relief fan came from a newly installed Dwyer MS-121 and that the MS-121 had been mounted horizontally, not vertically.  As a result, it was showing that the building was pressurized above the set point that cycled the relief fans on most of the time, even if the AHU serving the building was off.  That, in turn, meant that the relief fans tended to run any time the AHU associated with them was in operation (there was a interlock that shut the fans down if the AHU was shut down).

Reduced Fan Energy

The benefits of correcting this situation were two-fold.  The obvious improvement is the elimination of the unnecessary relief fan operation.  How much that is worth is a function of a number of variables including how often the fans ran unnecessarily and the electric rates in effect, which for the PEC, vary both with time of day and time of year. 

But in broad terms, based on our observations before and after the problem showed up and before and after the fix, getting the sensor mounted properly and supplying good data probably saved at least $262 a year and may have saved as much as $786 annually.  So paying attention to the details, including the instructions, can have its benefits.

Improved Comfort and Efficiency at the Perimeter

Another benefit associated with getting good data from the sensor was that after fixing the problem, the building pressure relationship was positive vs. negative relative to the outdoors.   Keeping the building positive means that someone sitting next to a leaking window will be more comfortable since air will be forced out the cracks vs. drawn in through the cracks.

That has an energy impact associated with it because the air that is being exfiltrated was brought in by an integrated economizer cycle to cool the core of the building.  That means that in essence, recovered energy energy (heat from the core) is being used to offset a perimeter load.  In contrast, if cold air leaks into a crack at the perimeter, it will tend to drop the temperature in the zone where the leak is occurring and that zone will use more heat.

Estimating the savings associated with this improvement is surprisingly complex, so I didn’t even try and just took it as “icing on the cake” as they say.  But in a Portland high rise where we made a similar discovery and adjustment, we estimated that the reduction in perimeter load amounted to 10-20% of the overall savings.

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So there you have it, a practical of example of why the details can matter, including taking the time to read through the instructions. Hopefully in the next week or so, I will have another post or two up that will illustrate how some other details associated with applied temperature sensors and current loops can come into play and impact what you think is going on based on the indications vs. what is actually going on.  Until then, hope everyone is having a nice transition into spring.


David Sellers
Senior Engineer – Facility Dynamics Engineering

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This entry was posted in 4-20 ma Current Loops, Controls. Bookmark the permalink.

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