measuring air duct static pressure with a digital manometer

[J-thetechwizard]

Hey Dudes,
I can't afford to buy a good tester to measure air flow or static pressure however, I have an off brand digital manometer I bought a few years back to adjust gas valves. it measures inches of water column.

My question I pose is this: is it possible to modify this digtial manometer with a homemade pitot tube of sorts to use to measure static pressures in ductwork.

I'm thinking water column is water column. I was going to get an aluminum tube of some pre-designated i.d. and plug the vinyl hose to it and stick it into the ductwork.

Do you think it will work within reason until I can buy a proper, fancier model???

[jpsmith1cm]

Before I bought a proper set of static tips with my Testo 510, I just used some rubber hose and a bit of copper tubing to take a reading on my magnehelic. It did the job.

That said, proper static tips aren't that expensive and, had I known how much easier they were to use, I wouldn't have waited nearly as long to buy them.


http://www.trutechtools.com/product.asp?itemid=629&gclid=COD8kN_XqbMCFREx4AodeiIAng

[J-thetechwizard]

So what you're saying is if I buy this tip and connect it to my cheapo digi manometer, I'll be on my way to measuring static pressures??

[jpsmith1cm]

Actually, I'm saying that if you buy this tip you will be measuring static pressures.

Well, you're gonna have to splurge and buy a little tubing, too... ;)

[ga-hvac-tech]

Using a single head digital manometer (or a magnehelic) with a pitot tube will give you the basics of measurement... there is a little math to do also:

First: If you are comparing to the furnace or AH blower charts; measure ONLY through the furnace or AH, NOTHING more. Otherwise, you cannot compare to the fan charts, as they are based on through the furnace or AH only. Note AH's generally are rated with the coil, since it is part of the AH. (Someone correct me if I am wrong on this).
OTOH; TOTAL ESP (external static pressure) is from before the air filter to after the coil... this is what the fan actually has to fight against.

Having said both, here is an example:
Say you read -.025 before the air filter, and you read +.035 after the coil... you have .06 EXP (forget the minus and plus, just add them).

Now if you have a Testo 510 (what I use) with two pitot tube probes... remember to check you have the tubes on the proper side of the meter. Otherwise you will get in-accurate readings.

Having the ability to measure ESP is a HUGE help when you have a system that does not respond the way it should (especially in AC mode).

[Phase Loss]

there is a little math to do also:

TOTAL ESP (external static pressure) is from before the air filter to after the coil... this is what the fan actually has to fight against.

MATH?!?!?!? :gah: EEFFFFF that!

Isn't TESP measured after the filter and before the coil?
That way it includes all resistance in the return side?

[ga-hvac-tech]

MATH?!?!?!? :gah: EEFFFFF that!

Isn't TESP measured after the filter and before the coil?
That way it includes all resistance in the return side?

Well... it seems to depend which way one learned TESP... I have heard it both ways.

As noted: If one is referencing the fan charts for the furnace or AH, then measure across the furnace or AH...

However if I was diagnosing a system I suspected of having too much SP, I would do it before AF to after coil (furnace/coil install). This way I would have what the blower was fighting against, including the ductwork.

I think we had this discussion somewhere before, I have forgotten the name of the thread though...

[jpsmith1cm]

Well... it seems to depend which way one learned TESP... I have heard it both ways.

As noted: If one is referencing the fan charts for the furnace or AH, then measure across the furnace or AH...

However if I was diagnosing a system I suspected of having too much SP, I would do it before AF to after coil (furnace/coil install). This way I would have what the blower was fighting against, including the ductwork.

I think we had this discussion somewhere before, I have forgotten the name of the thread though...

I believe that math AND reading are required.

Some charts assume certain filters installed and others do not, so ya gotta read the wee tiny print.

[ga-hvac-tech]

I believe that math AND reading are required.

Some charts assume certain filters installed and others do not, so ya gotta read the wee tiny print.

C'on now JP... we are men... we do not follow directions until there is no alternative... LOL

[jpsmith1cm]

C'on now JP... we are men... we do not follow directions until there is no alternative... LOL

I must be all bass ackwards.

Working on an unfamiliar unit today, I went out on the roof, reset it, grabbed the manuals, sat down and read them to lead sequence of operations and other basic stuff. Rolled back out on the roof, found the broken connector and went home...

[SBKold]

Hi guys I remember that thread.....ga calls before filter ..well at the blower door on a furnace.

The way the air flows this would be called after the filter..

[ga-hvac-tech]

I must be all bass ackwards.

Working on an unfamiliar unit today, I went out on the roof, reset it, grabbed the manuals, sat down and read them to lead sequence of operations and other basic stuff. Rolled back out on the roof, found the broken connector and went home...

You know I was blowing off steam... :) You are to serious today JP... <grin>

[gravity]

hey rundawg..

It's this one!

LOL

[rundawg]

Well... it seems to depend which way one learned TESP... I have heard it both ways.
I think we had this discussion somewhere before, I have forgotten the name of the thread though...

I sent you an email of a presentation I put together awhile back on TESP testing.

I would post it here, but I don't post technical info or manuals, in this "OPEN" forum.

If anyone else wants it, send me an email (address is in my profile).

Footnote:

Using a pitot tube in a residential application not only requires you to do math in public, but will require alot of straight duct, before and after the pitot tube, to get an accurate reading.

Even using the smaller static probes can give inaccurate readings because of turbulent airflow.

[cuervo]

I sent you an email of a presentation I put together awhile back on TESP testing.

I would post it here, but I don't post technical info or manuals, in this "OPEN" forum.

If anyone else wants it, send me an email (address is in my profile).

Footnote:

Using a pitot tube in a residential application not only requires you to do math in public, but will require alot of straight duct, before and after the pitot tube, to get an accurate reading.

Even using the smaller static probes can give inaccurate readings because of turbulent airflow.

Hey Rundawg, would you be able to send me this info also? Thanks!

[air1]

Has anyone tried to build their own flow hood and use a digital manometer with fpm display to measure airflow? I was thinking of a testo 510 and a 1'x1' square by 6" long piece of sheet metal duct with a pressure sensing grid. The grid would be 1/4" tubing with holes drilled to measure the velocity pressure and a second grid underneath the upper grid to measure the static pressure. Then attaching a homemade hood to the measuring assembly. You could even make the hood from cardboard to funnel the supply air.

[mark beiser]

Has anyone tried to build their own flow hood and use a digital manometer with fpm display to measure airflow? I was thinking of a testo 510 and a 1'x1' square by 6" long piece of sheet metal duct with a pressure sensing grid. The grid would be 1/4" tubing with holes drilled to measure the velocity pressure and a second grid underneath the upper grid to measure the static pressure. Then attaching a homemade hood to the measuring assembly. You could even make the hood from cardboard to funnel the supply air.

Old trick, works best with a vane anemometer.
Set the dimensions of the outlet of the box, and do an average velocity reading, the anemometer calculates the CFM.

On residential grills it can be quite accurate if you use a rectangular box, long enough for the turbulence to mostly settle, and are not choking the airflow down with the box.

If you look into how shockingly inaccurate the results from expensive flow hoods can be, you may come to a similar conclusion as other people have.
The anemometer traversing the outlet of a box method is at least as accurate as a $2000+ flow hood, probably more so if done properly.

[air1]

Old trick, works best with a vane anemometer.
Set the dimensions of the outlet of the box, and do an average velocity reading, the anemometer calculates the CFM.

On residential grills it can be quite accurate if you use a rectangular box, long enough for the turbulence to mostly settle, and are not choking the airflow down with the box.

If you look into how shockingly inaccurate the results from expensive flow hoods can be, you may come to a similar conclusion as other people have.
The anemometer traversing the outlet of a box method is at least as accurate as a $2000+ flow hood, probably more so if done properly.
So an anemometer would be better than a average of the velocity pressure?

[Hvac216]

Hey run dawg would you be able to send that to me too? Thanks.

[mark beiser]

So an anemometer would be better than a average of the velocity pressure?

Absolutely, because a traverse with an anemometer is a direct measurement of the average air velocity, so there is only one step to calculating the CFM.
With a vane anemometer, no corrections for air density are needed either.

You can do it with a pitot tube and manometer, but then you have to convert velocity pressure to velocity, with associated air density corrections, for each measurement point, average them, then calculate CFM.
The measurements are also MUCH more subject to error due to turbulence than using a vane anemometer.

[air1]

Absolutely, because a traverse with an anemometer is a direct measurement of the average air velocity, so there is only one step to calculating the CFM.
With a vane anemometer, no corrections for air density are needed either.

You can do it with a pitot tube and manometer, but then you have to convert velocity pressure to velocity, with associated air density corrections, for each measurement point, average them, then calculate CFM.
The measurements are also MUCH more subject to error due to turbulence than using a vane anemometer.

I was thinking of using a "velogrid" arrangement. A velogrid is effectively a pitot tube that mesures velocity pressures at multiple locations simultaneously. The result is an average velocity pressure of the area being measured. Connecting the velogrid to a Testo 510 would convert the velocity pressure to fpm and if the area is 1 sq. ft. the fpm would equal cfm. The testo 510 has air density compensation capabilities although I'm not sure how one would go about setting it.
I don't mean to be argumentative. I'm just trying to understand the why a anemometer is better. Personally, I've never trusted vane anemometers. Seems to me that a vane anemometer would be affected by friction caused by wear over time.

[rundawg]

Hey Rundawg, would you be able to send me this info also? Thanks!

I would need an email to do that.

Put an address in your profile, or send me an email (address is in my profile), and I will get it off to you.

[rundawg]

Hey Rundawg, would you be able to send me this info also? Thanks!

double post - sorry

[rundawg]

Hey run dawg would you be able to send that to me too? Thanks.

Check your email.

[cuervo]

I would need an email to do that.

Put an address in your profile, or send me an email (address is in my profile), and I will get it off to you.

Thanks for the info!

Here's a question............
If there's no way to pop a hole in the back of a residential furnace (walls or other restrictions) and evaporator coil is right above the furnace, is the limit hole (if centered) an option to take the TESP or is it still too much turbulence from the blower to get an accurate reading? I've never tried this myself.
Leaving the only choice to popping a hole into the evaporator coil casing.

[jrbenny]

I only drill a hole in the plenum above the coil. I use holes already in the furnace.

NCI has found that the average difference in pressure between the limit hole and bottom of the coil to be 0.04 IWC.

I measure return pressure in the thermostat wire hole or the fault code peep hole. Slide the filter out for filter pressure drop.

Easy breezy.

[rundawg]

Here's a question............
If there's no way to pop a hole in the back of a residential furnace (walls or other restrictions) and evaporator coil is right above the furnace, is the limit hole (if centered) an option to take the TESP or is it still too much turbulence from the blower to get an accurate reading? I've never tried this myself.


Yes, I have used the limit hole on more than one occasion. Testing at that location, although not ideal(due to possible airflow turbulence), is better than not testing at all.

[stomper56]

I recently stopped drilling and plugging holes in favor of using some cap tube in place of my static probes and just using the "factory installed holes" lol
Iv tested the variances between the two and as long as you hold the cap tube 90* to airflow the measurements are correct.
You can just remove a screw and place the cap tube in to take a measurement, easy peasy and no hole to plug.
Much quicker and easier to use once you learn to trust the readings after doing alot of testing and becoming familiar with it.
I use this for measuring SP on exhaust fans too since you cant drill type 1 hoods that carry grease through them.

[ckartson]

Rundawg,I atempted to email you for your information on the topic but your email is shut off on this site. Am I doing some thing wrong?

[ckartson]

Sory I found your email forget the last post

[powderedtoastman]

As long as it can measure tenths of inches of WC, you're good to go. I use the fieldpiece manometer head with a dwyer 303 tip and a 1/4" bit. It has a switch to change the scale for anything above 1"(gas valves), and below 1"(static)