HVAC Field Tools: Measuring Air Pressure and Flow

This post was inspired by a question a student asked the other day in class.  I meant to answer during a break and then ran out of time.  So, I thought I would answer it here in case it was of use to others. 

The question came up after I had been talking about making flow assessments in the make up air handling system that is the subject of the current, on-going string of blog posts on retrocommissioning a make up air handling unit that showed signs of  unnecessary simultaneous heating and cooling.  Specifically, Richard asked a very good question, that being what I carried with me in the field to do that sort of work;  i.e. what should he consider adding to his toolbox if he wanted to do the same sort of thing.

There are a number of ways that I go about measuring flow when I am out in the field.   Four of the simplest are qualitative (give me an observable indication but nothing I can measure directly) versus quantitative (provide a measurable parameter).  The others require some basic instruments (a.k.a toys).

The links below should take you to the headings indicated if you want to just jump to a specific topic of interest.

Qualitative Air Flow Measurements

Qualitative data is data that can be sensed and observed but not necessarily measured.  In formation that you gather with your eyes and ears and other senses is generally qualitative.

Using Your Senses

My grandmother often referred to simply”using the good sense God gave you”.  While old fashioned, that is pretty good advice when you are out doing field work (and in my experience, out living life).   Our senses tell us a lot;  we just need to pay attention, and maybe to put some limiting parameters on them to start the move from qualitative data to quantitative data.

One very handy qualitative measurement you can take with regard to air flow  is to simply see if you can feel it and if you can feel it, see if you can tell which direction it is coming from.  For most people, a gentle breeze that you feel as something just starting to brush your cheek is about 50 Feet Per Minute (fpm).   To put this in perspective in terms of instrumentation, the rotating vane anemometer I will discuss later is probably not accurate below about 80 fpm.  But an instrument like a Shortridge Air Data Multimeter with a Velgrid probe is accurate down to about 25 fpm.

Air flow is generally driven by a pressure difference.  The gentle sea breeze you feel at the beach is driven by the sun heating the beach, which causes the air their to rise, creating a low pressure area, which causes a breeze to flow in from the ocean to the beach.  The rising air flow over the coast can actually create a line of clouds that follow the coastline.

Putting Some Numbers to What You Feel

Similarly, the breeze you might feel as you open the door to enter a building might be created by a pressure difference between the inside and outside of the building.   Here is where your other senses start to come into play.

  • If the door was surprisingly easy to open, its quite possible that you were getting some assistance from a positive pressure difference between the inside and outside of the building. 
  • If the door was tough to open, then the building might have a negative pressure relationship to the exterior, essentially sucking the door closed.

Pressure applied over an area creates a force and little pressures over a large area can create a surprising amount of force.  In fact, if the Owner of the building you are working in is complaining that they have problems with doors either blowing open or being difficult to open, that tells you that something is likely pressurizing the building above about 0.10 – 0.15 in.w.c. (semi-quantitative data). 

You can reach that conclusion because that pressure applied over a typical door will generate a force that exceeds the door opening force requirements established by the Americans with Disabilities Act (ADA). The requirements of ADA along with how it is interpreted by the local jurisdiction determine the force that can be applied by door closers. 

If the pressure in the building creates a force on the door that exceeds ADA requirements, the doors can blow open, which causes a number of problems including issues with comfort control and security.  Going the other way, if you fight a tug of war with the door and then feel like you get sucked into the building once its open, the building likely has negative pressure problems.

So, while you don’t know how much of an airflow imbalance exists, by simply using your senses and some logic, you know that there is an issue that needs to be addressed and quantified.  In fact, the indicators may focus you on the make up air systems, exhaust systems, economizers, and relief fans.

Adding Simple Tools to the Mix

Sometime, I can feel flow but other clues that I might use to tell me the direction of flow are not there.  This happens a lot when I am working at hotels and we are looking at how well the guest room exhaust systems are  working.   Fortunately most hotel rooms are equipped with instrumentation for this purpose.

Kleenex on Grill

Facial tissues (a.k.a. “Kleenex”) are pretty good detectors of low flow rates and pressure differences.  I’ve held them next to grills (as illustrated above) or windows or doors to see what happens.  Narrowing the gap by closing the window or door can make an undetectable flow more obvious since it will increase the velocity of what ever air flow is exiting or entering the space.

Lacking the sophisticated instrumentation found in a hotel guest room, I apply a different tool to the situation; the flame of a lighter I carry with me just for the purpose.

Flame at lab door 02

In the picture above, I was working in a lab and trying to understand the pressure relationships.   The relationships were subtle and if you opened the door, it was difficult to tell what was going on.  If you left the door closed and used the “lick your finger” trick (evaporative cooling makes the side of your finger facing the breeze cool) the result was inconclusive.   If I tried using my handkerchief, it was too heavy to detect anything.  But the flame from my lighter quickly showed me that air was flowing out of the lab when I held it up to the door.

I also carry sage leaves with me and a smudge stick.  For one thing, I think the scent of burning sage is very pleasant and relaxing.  But in terms of building science, the stream of smoke from a smoldering sage leaf, smudge stick, or incense stick can tell me about air flow patterns I can’t feel.

Sage 02

If you look closely in the picture above, you can see two streamers of smoke coming off the save leaf and going out and up through the window.  (They sort of blend in with the folded up walker that I was getting ready to donate;  I should have moved it in hind sight)   The smoke streamers told me there was a bit of flow out of the room, even thought I couldn’t feel it.

Ron Simens, one of our senior field engineers has a very cool technique he uses to detect drafts that involves an camera tripod and Teflon tape.

Ron's Draft Test 01  Ron's Draft Test 03

The Teflon tape is dangling from the little blue spools.  In the picture on the right, there are no detectable drafts.   In the picture on the right, there is:

  • A 38 fpm draft at 43″ (the top  tape;  most people probably would not feel it),
  • A 134 fpm draft at 24″ (the middle tape;  most people probably would feel it), and
  • A 29 fpm draft at 4″ (another one that most people would not feel, although we humans seem to be sensitive at our ankles).

If Ron observes the tape moving, he mounts a calibrated test instrument capable of measuring low velocities and takes some data, which is where the numbers come from.   Way cool.  Hopefully he doesn’t kill me for giving away his technique.

I should point out in the context of the question that inspired this post, the instrument that Ron would use to get a number is not one that most of us would have in our field tool box.   But we might have a roll of Teflon tape.   And if we were really lucky, we might have Ron’s cell phone number in our “favorites” so we could call and ask him what he thought after we deployed our tape (no, I will not give his number to you).

Qualitative is Semi-Quantitative

Ron’s measurements turning qualitative data to quantitative data aside, its still possible to put some dimensions on a quantitative observation.  For instance:

  • If a can feel a breeze/draft, its reasonable to assume its at least 50 fpm and some calculations on that basis may be revealing as long as you don’t take yourself to seriously;  basically, you are playing a “what if” game when you do this.
  • While I may not be able to put a number on it, the sage smoke streaming out my window says that air was moving from inside my house to outside my house, at least at that location. So, there definitely a positive pressure relationship at that point;  how much, I’m not sure.  But I also know that there was alight breeze because the wind chime outside the window was just starting to resonate and we had the door open on the front of the house and the NWS said the wind was out of the NW which is the directly the front door points.   So my guess is Mother Nature was providing a little natural ventilation cycle and moving air through the house from the front door to the window.
  • My guess is that having done it a lot, Ron can probably look at his Teflon tapes and tell you about how fast the air is moving based on the angle of deflection with out placing instrument in the flow.  So, a rough assessment of the implications based on his observations (which are informed by years of experience) may provide some insight as long as you remember that the numbers were and “estimate, not an exactimate” to quote Pat Murphy.

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Rule of Thumb Based Air Flow Measurement

Rules of thumb can also be used to put some numbers to something as long as you remember what the constraints on the number are (one is that it was based on a rule of  thumb and another is that it was based on a field observation).  One that I use a lot is based on the fact that for most HVAC processes, the face velocity through a filter bank or coil will not be above about 500 feet per minute (that’s where fpm comes from).

  • Velocities over 500 fpm through a coil will cause a number of issues to come up, especially for cooling coils where the pressure drops can become untenable and you can start blowing water off the coil.
  • For filters, you can actually blow the media out of the frames and most manufacturers will not rate their filters above this velocity.

At the other end of the spectrum, ASHRAE and others, like Lee Eng Lock, have demonstrated that in terms of operating costs, you can justify increasing the size of an air handling unit to provide face velocities of 350 fpm (including the value of the real estate that takes), mostly because of the reduction in fan power associated with the lower flow rates.

So, if I look at a filter bank like this one …

AHU1 filters

,,, I can count the filters and come up with a face area based on the dimensions of the filters, which are usually printed on the filter someplace.

Then, if I multiply that face area (square feet) by 500 feet per minute, I end up with cubic feet per minute.  In other words, I just “discovered”  how much air the unit might move at the upper end of the design limits for the components it contains (the filters and coils).

If I multiply the face area by 350 feet per minute, I also end up with cubic feet per minute, but this time, it’s the low end of where an energy conscious designer might have selected the unit.

So, the system flow rate, at design, is probably someplace in between those numbers.

Is it exactly one of those numbers? 

No;  but its probably someplace in between so calculations based on those numbers might bracket things.

Is it always between one of those numbers?

No;  for instance, a Variable Air Volume system (VAV system) might (hopefully, will) operate at flow rates significantly less than design a lot of the time.  So, the rules of thumb are handier for constant volume systems vs.VAV systems.  

But for VAV systems, the rules of thumb give you an idea of what the peak of the load profile might be.  And things like Table 1 in the 2007 an previous versions of the ASHRAE Applications Handbook might give you some insight into the load profile (a good reason to keep older versions of the handbook around).  Trend data from the facility might also give you some insight.

Meaning that you could do calculations based on percentages of the peak flow based on load profile data that you might acquire from reputable sources like ASHRAE or the building itself.   Just remember that you’re, at best, bracketing the answer, vs. coming up with an exact answer.

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Putting a Number to It;  Quantitative Airflow Measurements

That brings us back to Richard’s original question;  what tools do I carry around with me in my tool box to measure air flow and pressure.  Aside from my lighter, sage, and smudge sticks, here is what I have.

My Trusty Four-In-One Tool

The Four-In-One tool is one of the handiest tools I have. If you follow this blog, you have already heard me mention it before.   Here it is measuring the flow I created by blowing on it.

Measure Pressure and Flow 05

In addition to having a rotating vane anemometer to measure air flow, it allows me to:

  • Measure temperature via an internal sensor or external probe,
  • Measure relative humidity via an internal sensor,
  • Measure lighting levels via an internal sensor, and
  • Measure velocity via a rotating vane anemometer.

Its not particularly “great” at any of those things, but, it’s a lot better than I can do with out it. 

For instance, since the temperature probe is a thermocouple, I only assume that that the temperature is right plus or minus a degree or two.  But, since I usually am concerned with temperature differences, as long as I use the same probe for the measurements I am going to compare, the error cancels out. 

And testing against a fairly common parameter (ice water) says its not too bad in terms of absolute accuracy, at least at 32°F.

Ice Point Calibration

Not bad for a thermocouple (it sat in the bucket of  ice water overnight in my  hotel room one day when I got curious about how close it might be to an absolute).

Even if I don’t take my instrument case (it’s the Pelican case you see in the background of the Four-In-One picture at the beginning of this section), I will always put this tool in my suitcase, along with:

Incidentally, the links take you to web pages that show the various items.

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    Total Pressure, Static Pressure, and Velocity Pressure

    Most of you probably started to experiment with total, static, and velocity pressure when you were fairly young, you just didn’t realize it.  Specifically, when you stick your hand out the car window and make it fly, in general terms, it is the force associated with  the moving air that makes your hand move.

    For those who are not familiar with the terms:

    • Static pressure is what you feel every day due to the weight of the atmosphere pushing down on you.
    • Total pressure is what you feel when the wind blows against you; i.e. there is an added force created by the motion of the air running into you and being deflected around you that is in addition to the static pressure.
    • Velocity pressure is a measure of the force produced by the wind.

    As a frame of reference, a quarter inch of air pressure – 0.25 in.w.c. – corresponds to the velocity pressure associated with 2,000 fpm which is a fairly high velocity in an HVAC duct, especially a small one.

    The rotating vane anemometer in my four-in-one tool uses the force created by velocity pressure to turn a propeller that creates a signal that is an indication of the velocity.  But another very common way to measure velocity pressure is to use a pitot tube, along with a manometer or some other form of differential pressure measuring instrument. 

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    Pitot Tubes and Manometers

    This string of slides from the VAV systems class I do will give you a feel for how a pitot tube works.







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    Tools for Velocity and Pressure Measurements

    The Lightweight Approach

    If I take my Pelican instrument case with me (90% of the time I do because Murphy’s law has demonstrated via functional test that if I don’t, I will need something from it), then I also will have a Magnehelic and some short pitot tubes.

    Measure Pressure and Flow 01

    The Magnehelic takes the place of the manometer in the slides.  Since it is calibrated to take readings with the instrument face in the vertical plane, I have mine mounted on a bracket that lets me sit it on something to orient it that way.  The bracket has magnets on the back of it that let me mount it to something like a duct while I take a reading.

    Measure Pressure and Flow 02  Measure Pressure and Flow 03

    The “Mag”, as they say out in the field, measures pressure based on the movement created by the deflection of a diaphragm, which is transmitted to the needle via a magnetic coupling.  The magnetic couplings eliminates some of the issues associated with a purely mechanical connection, allowing the sensitivity to very low pressures.

    Usually, I’m only trying to measure a low differential pressure relationship or get a feel for the flow in a duct, vs. trying to do a full blown, high accuracy duct traverse1.  The shorter pitot tubes allow me to get centerline velocities in ducts up to about 36” across or in diameter.   The thin pitot tube lets me slip it under the crack at the bottom of the door and use the static ports to read the pressure across the door.  I used that a lot in my clean room days when I was the HVAC and Fire Protection system owner at Komatsu’s Hillsboro fab .  I also have a longer (60”) pitot tube I can ship ahead to a site if I think I will be dealing with larger ducts.

    Bringing Out the Big Guns

    If I anticipate the need to measure fairly high pressures and/or velocities, I will take my 10” inclined manometer.

    Inclined manometer 13

    The actual manometer is the plastic block with the red fluid filled capillaries in it. The other items are:

    • The gray metal carrying case that you put it all in to transport it.
    • A 24” pitot tube (the silver L shaped thing lying across the case),
    • Two static probes (the brass tubes below the pitot tube),
    • Some spare gauge oil (the little red bottle),
    • A slide rule device for converting velocity pressure to velocity (the white rectangle below the static probes),
    • Tube holders (the little spring-shaped things below the slide rule;  you drill a hole in the duct and thread them in and they then hold the static probes steady for you), and
    • Flexible rubber tubes for hooking the probes to the manometer.

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    Inclined Manometer Details

    The inclined manometer is just a fancy version of the U-tube manometer illustrated in the slides with a few features that make it easier to use. 


    The capillaries filled with the red fluid in the first picture and highlighted in green and yellow are the actual gauge.   The green line is the “inclined” part of the “inclined manometer”.  More on that in a minute.

    The little white fittings at the top are manual valves that you turn counter clockwise to close and seal the oil in.  turning them clockwise opens the connection.  The tubes to the pitot tube slip on over the fittings that are pointing towards the back in the picture.

    The expanded section between the capillary tube and the white fittings provides a little reservoir to minimize problems with blowing the oil out of the gauge when you insert the pitot tube into the duct.  That doesn’t mean you can’t do that;  most folks who have used one of these have done that at least one time.  That’s usually when you learn something about specific gravity;  more on that in a minute.

    To get an accurate reading, you need to make sure the gauge is level, which is the purpose of the built in bubble level and leveling adjustment.   Leveling is trickier when using the magnets to mount the gauge to a duct since you have to tap on the corners to shift them around against the magnetic force clamping the gauge to the duct, which is pretty strong.

    Once you have the gauge level, the black knob on the left end of the fluid reservoir is used to zero it.   Turning  it changes the volume of the fluid reservoir, forcing gauge oil into or out of the actual manometer capillary tube.

    Reading Small and Large Pressures on the Same Gauge

    To be a useful field tool, the pressure gauges like this one need to be able to measure the relatively low pressure readings associated with velocity pressures (frequently, much less than 0.50 in.w.c.;  remember 0.25 in.w.c. is associated with 2,000 fpm, a relatively high velocity, especially in a smaller duct) along with the higher static pressure readings one might want to measure at various points in a system (filter and coil pressure drops, discharge pressures, etc.).  Part of the trick behind the ability of this gauge to do that lies in the inclined portion of the scale.  

    Specifically, by sloping the first inch of the manometer over several inches horizontally, the scale is expanded significantly for that portion of the range.  


    For instance, if 1 inch of elevation change were to be spread out over 10 inches of horizontal distance, then the fluid level change associated with 0.10 inch of pressure change would be spread out over an inch of the scale, making higher resolution readings possible.

    Playing Games with Specific Gravity

    Another part of the “trick’ making low pressure readings possible is the gauge oil itself.   For one thing, its specific gravity is 0.826 (water is 1), meaning its lighter than water.   So, the pressure that it would take to elevate water 1 inch will elevate the gauge oil about 1.21 inches, which expands the scale a bit.  That’s a tricky thing to take a picture of due to the effects of parallax (the phenomenon that makes lines tend to converge in the distance).   But in the picture below, I took the enlargements by lining up the camera to eliminate parallax at the 2 inch point (left) and the 3 inch point (right) on the gauge scale.


    If you compare the readings on the ruler, you can see that 1 inch on the gauge scale corresponds to about 1-1/4 inches on the ruler; i.e. the scale has been expanded bit.

    That means if you lost your gauge oil, you could not fill up the manometer with water and expect to get a good reading,  at least not with out correcting for the difference in specific gravity.

    The Impact of Surface Tension

    The gauge oil also has another characteristic that helps with taking readings;  its surface tension.   Surface tension is what makes a liquid tend to bead up when a drop of it is placed on something.  Surface tension can vary with the type of fluid and also can vary for any specific fluid as a function of temperature.


    In the pictures below, I had put (approximately) equal sized drops of water, 70% isopropyl alcohol, and gauge oil on the smooth surface of Kathy’s travel make-up mirror. 



    If you compare the height and diameter of the various drops, you will notice that the alcohol spread out more than both the water and the gauge oil and that the gauge oil spread out a bit more than the water.  Note that the nature of the surface in addition to the nature of the fluid will impact how much the fluid tends to cling or spread out.

    The 70% isopropyl alcohol solution (I’m assuming the % is by volume) has a specific gravity very similar to the gauge oil (.0877 vs. .0826), so it using it in the gauge instead of water would also tend to spread out the scale.  But, since it would be less “clingy” inside the capillary tube of the gauge, the result would be more like the upper image than the lower image in the following illustration.


    So, you sill have to eye-ball it a bit, but there is less margin for error.

    The curved surface created by the surface tension of the gauge oil is called the meniscus.  That brings me to the purpose of the reflective scale.  The second image of the meniscus tube created by the reflection will appear shifted from the actual location except the one point directly in line with your eye due to the effects of parallax.  You can see that in this close-up image.


    By moving your head and eye around relative to the gauge when you are reading it, you can line up the meniscus in the reflected image with the actual image, meaning the scale and the meniscus in the gauge oil are on the same line of sight and thus, you are reading the true value of the indication.

    Incidentally, this issue comes up with any instrument where there is some space between the indicator and the scale it is read against.  Pressure gauges and Magnehelics are a common examples of this and that is why high end gauges will  have  a mirrored scale feature.

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    The Downside of the Inclined Manometer

    That brings me to one of the disadvantages of the inclined manometer;  accurate, repeatable results are very much dependent upon the technique you use to read the meter, in addition to the technique you use to position your sensors.  The air data multi-meters eliminate this human factor from the equation (but not the human factors associated with positioning the probes).

    And the electronic meters bring other advantages too.  Consider what you need to do to set up and use the inclined manometer, which is how I learned to do this sort of thing.

    1. You have to find a place to set the gauge up, level it and then zero it.  That can actually take a bit longer than you might think since, when the gauge is horizontal, the oil films out over the reservoirs at the end of the capillary tubes and needs to drain back to the capillary tube. 
    2. If the gauge has been in the the back of your Jeep in the winter or in the cargo hold of an airliner, the fluid will be cold and more viscous and will take longer  to drain back into the capillary tube.  If y0u don’t take time to allow this to happen, you will discover that your zero has shifted after the gauge has warmed up
    3. If you aren’t careful, when you insert your probes into the duct, you can subject the gauge enough pressure that you will blow some or all of the oil out of it, especially in the case of a gauge that has  a full scale value of an inch or  less. Aside from the mess that creates, you then need replacement oil of the correct  specific gravity (not sure about your local Ace Hardware, but mine does not have a gauge  oil aisle). 
    4. If you have been down this path before, you probably have  some extra gauge oil (hopefully, with you, not back at the hotel, or worse yet, the office).   But, it’s  probably still cold so it will take a while to get everything filled and re-zeroed.   Plus, until you have done it once, you will likely over-fill the gauge, meaning you have  to get a little bit of oil, but not a lot of oil  back out of it.
    5. After doing all of that, you will likely need to re-level and re-zero the gauge.
    6. You probably are not going to be able to read the gauge and hold the pitot tube, especially if you are going to be climbing a ladder to do this and especially if you are familiar with the parallax issue.  So, to take  readings, you need a team of two  people.
    7. For a large duct, you can easily need to take 20-30 readings.  So you will need a form to write all of that  down on.
    8. Having made all of the adjustments and making sure the assistant that is going to read the gauge knows how to take consistent, reliable readings and knows how to document them on the form appropriately, you head up the ladder with the pitot tube and start taking readings.
    9. After taking a number  of readings, your “reader” wants to make sure they are doing it right, so you climb back  down to take a look.  After simulating a reading, verifying that your assistant has in fact taken parallax into account in their observation and also documented the number properly on the form you commend them on their technique and head back up the ladder.
    10. Unfortunately, one of the rubber tubes connecting the pitot tube to the gauge catches on your boot as you climb the ladder, and the tube pulls off the gauge, but not before  knocking it over and spilling the gauge oil all over the place, including all over your form.
    11. This means you need to return to step 1 and also make a trip to the store to get your assistant a pencil since you just discovered that the ink in the pen they were using is not water or gauge  oil resistant.

    I’ll leave it to you to guess how I know that stuff like that can happen, but a clue is that I have acted in both the assistant role and the lead role.

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    Modern Electronics to the Rescue

    Modern air data multimeters like the Shortridge I have mentioned numerous times in this post overcome a lot of the shortcomings I listed above.  For one thing, one person can do the sensor positioning task and the reading task since the data can be stored in memory and  the click of a button.  And, they do not need to be  leveled every time you move them, meaning you can be taking readings pretty quickly upon reaching your destination. 

    And, with the proper attachments, the meters can read flows that we can’t even feel, velocities as low as 25 feet per minute, the kind of stuff I detect with my sage leaves and lighter but can’t quantify.  Here is one of the students at the Pacific Energy Center using a Shortridge Multimeter with a Velgrid adapter to measure the minimum outdoor air flow into the main air handling system. 

    Using a Velgrid and Shortridge 01

    The velocities across the intake were below 40 fpm, so we could not really feel the flow, but we could measure it with the air data multimeter equipped with the appropriate sensing probe

    The one thing you don’t want to do is knock it to the floor.  Do that with the inclined manometer, and you will probably be cleaning up a mess and looking for some oil.  But do that with an electronic multimeter and you are going to be looking for FedEx to ship it back for repairs and recalibration.  Of course, that is what the neck strap if for, so you should be sure to use it, just like the student in the picture above.

    So in a perfect world, and if money were no object, I would say “buy a Shortridge (or equal;  there are other companies that make similar items.  I just happened to be familiar with the Shortridge product).  In fact, when folks ask me what they should get after they have their basic tool kit assembled, I often say the the same thing.  

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    Why Even Bother with the Old Fashioned Stuff in the Age of Electronics

    Some of you may wonder why, in this age of sophisticated electronics, I don’t just carry something like a Shortridge Air Data Multimeter.  Our company has a bunch of them, including the cluster of folks I work with in Portland.  But there a number of reasons I have have and turn to some of the other instruments I mentioned.

    1. The list price for a basic  Shortridge  kit currently is probably in the range of $3,000 – $4,000.  The list price for the Dwyer 10 inch inclined manometer kit in the picture currently is $1,040 (I bought mine used for about half of that).  The list price for my Magnehelic gauge was about $130, including the carrying case, bracket and magnets.  The list price for my four-in-one is about $175.  Point being that I can do a lot with instruments that total  about $305 (the Mag and the four-in-one) and, with proper technique, I can  approach what  the Shortridge can do with my inclined manometer for significantly less upfront cost (and also significantly less risk in terms of the loss if something were to be damaged or lost in the field or while traveling).
    2. The four-in-one tool provides a lot of capability for the cost, size, and weight.  If I complement that with my Mag, I really can do a lot with out having a lot of bulk and weight to carry around, something that becomes important if you have to get on airplanes a lot.
    3. The inclined manometer is based on a fundamental principle and there is not a lot that can go wrong with it.  That’s not to say that you don’t have  to use proper  technique, as described above. But there are no batteries and its basically a liquid in a tube that the air pushes around, so pretty simple in terms of how it works and how you calibrate it.
    4. Because it is based on a fundamental principle, if you take a reading with an inclined mamometer, you can actually “see” how a higher pressure “pushes” against a lower pressure to reveal the pressure difference or how static pressure “pushes back” on total pressure to reveal velocity pressure.  When I am teaching classes, this is a valuable training aid.
    5. Keeping the Shortridge calibrated requires special equipment;  typically we send ours in for calibration about once a year.  In  contrast, keeping the inclined manometer calibrated only requires some replacement fluid and consistent technique in terms of leveling, zeroing, and reading it.
    6. They didn’t have air data multi-meters when I learned to do this.  So, Mags, inclined manometers, and rotating vane anemometers are the type of instruments I was trained with, meaning there is a certain level of comfort associated with using them.
    7. Because of the preceding, and probably, because I am getting older, there is a certain amount of sentiment attached with using the more basic instruments.   I just enjoy taking a reading them.

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    So that’s a long answer to Richard’s original question.  But hopefully, it will give you some insight into the pros and cons of some of the ways to go about measuring air flow, including some fairly low cost options to get you started out there in the field.


    David Sellers
    Senior Engineer – Facility Dynamics Engineering

    Click here for a recent index to previous posts


    1. To find out more about pitot tube traverses, there is a discussion on the topic along with related references in the third chapter of the NBCIP Return Fan Capacity Control Guideline that I have  linked to under 01 – Commissioning Resources on the right side of the blog home page.
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