# Thread: Questions for traversing with a pitot tube

1. ## Questions for traversing with a pitot tube

I would only be using one for service work, So I don't need to be ultra precise.

Do I need to take temperature into account, and if so, how?

For rectangular duct, do you do one traverse in the center on each side of the duct, or the 5 or so equally spaced holes on one side?

Is it important to know what the barometric pressure is?

What is a good rule of thumb for how far you need to be from any elbows to get an accurate reading?

I'm most likely going to by the 24" pitot tube, is there anyway to traverse a duct larger than 24" to obtain a somewhat accurate reading?

Thanks guys

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Temperature is not a factor on HVAC. That will be your smallest potential for error.
For what you want use the equal area method. For a 24" duct you take 16 readings. The holes are drilled 6" apart starting at 3" from either side.
Barometric pressure isn't important because you will be measuring velocity pressure.
The 24" will do bigger ducts but you have to drill holes on both sides of the duct opposite each other. You lose a little bit of accuracy but not enough to worry about.
Acceptable traverse can normally be obtained the equivalent of 5 duct diameters downstream of elbows etc. The readings will tell you if the traverse is good.

3. Originally Posted by WAYNE3298
Temperature is not a factor on HVAC. That will be your smallest potential for error.
For what you want use the equal area method. For a 24" duct you take 16 readings. The holes are drilled 6" apart starting at 3" from either side.
Barometric pressure isn't important because you will be measuring velocity pressure.
The 24" will do bigger ducts but you have to drill holes on both sides of the duct opposite each other. You lose a little bit of accuracy but not enough to worry about.
Acceptable traverse can normally be obtained the equivalent of 5 duct diameters downstream of elbows etc. The readings will tell you if the traverse is good.
Thanks a bunch for the reply Wayne, what do you think about devices like the testo 510 that have a timed traverse function?

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This usually gets me in trouble. Time weighted average accuracy largely depends on the continuity of mass. They are less accurate in low pressure systems than in the higher pressure VAV systems. The time weighted meter is more accurate when used for duct traverse than the same meter is when used on the digital flow hood. When used for duct traverse the results is slightly high but acceptable. Time weighted digital flow hoods are usually a little high and in some cases unacceptable. I would not use them in life safety situations such as railroad crossing control building equipment conditioning.
Time weighted can get you in trouble taking static pressure readings also but normally doesn't unless you have to prove your readings.
I'm not familiar with Testo equipment but time weighted all works the same. The problem with it is that it is time weighted average and doesn't start and stop in the same place during the cycle every time because the cycle is not regular.
Having said that it is pretty much state of the art and should have the manufacturer's stated accuracy you need.

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I almost forgot that for pitot readings negative readings are added as zero and at least 75% of the readings have to be 10% or more of the maximum reading. That isn't a very good traverse but is the minimum acceptable.

6. What do you mean negative readings? As in a negative static in the return for instance?

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Reverse airflow or turbulence can give negative velocity pressure readings. When that happens the airflow is assumed to be zero. On digital meters the read-out is "NEGATIVE PITOT". That bites into your accuracy somewhat but was determined by studies to be the best approach because negative pitot readings are common unless you have a really good place to run the pitot.

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Originally Posted by WAYNE3298
Temperature is not a factor on HVAC. That will be your smallest potential for error.
For what you want use the equal area method. For a 24" duct you take 16 readings. The holes are drilled 6" apart starting at 3" from either side.
Barometric pressure isn't important because you will be measuring velocity pressure.
The 24" will do bigger ducts but you have to drill holes on both sides of the duct opposite each other. You lose a little bit of accuracy but not enough to worry about.
Acceptable traverse can normally be obtained the equivalent of 5 duct diameters downstream of elbows etc. The readings will tell you if the traverse is good.
so as understand it, and i may be wrong. three things affect air density, therefore airflow measurements.

temperature.

moisture contents (RH)

barometric pressure absolute (altitude above sea level)

two questions. are the other factors? and in what order of having most impact on accuracy are they?

TIA.......

9. Originally Posted by WAYNE3298
Reverse airflow or turbulence can give negative velocity pressure readings. When that happens the airflow is assumed to be zero. On digital meters the read-out is "NEGATIVE PITOT". That bites into your accuracy somewhat but was determined by studies to be the best approach because negative pitot readings are common unless you have a really good place to run the pitot.
Okay last question then I'm done!

I've read that using a pitot is better for higher velocities and a hot wire is typically better at low velocities.

I do commercial/residential so I might be encountering lower velocities more often. Would a hot wire better suit me? I'm kinda torn between them

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You are correct. In a duct really high temperatures have the greatest impact. Normal HVAC temperatures don't get high or low enough to be a problem. You are measuring the pressure created by the air flowing without reference to atmosphere other than the original density. My meters measure barometric pressure and duct air temperatures. For balance work you are really after cubic feet per minute in most cases not mass flow. The air balance specs call for CFM as determined by the engineer which should take into account mass.
I did have one job where I did have to do mass flow in a chemical process. The air was so hot I had to wear gloves and manually calculate the effect of temperature because it was too hot for my meter's thermometer.
Relative humidity varies widely between heating and cooling but I have never heard of compensating for it.
Accuracy is normally impacted more the duct velocity pressure profile and method of taking readings than anything else

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The pitot is universally recognized as the industry standard baseline for accuracy. I don't like the hot wire but that is my own personal opinion. I know two guys that swear by the hot wire and they are good balancers. You have my opinion and that's honestly all it is worth.

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humidity has a negligible effect on air density.

its a bit of a bigger factor in compressed air systems, but still most design manuals do not call for it.

its temperature and in a compressed air system- the Psia as well.

ive done the humidity calcs exactly one time... i dont recall it being that hard. you need the partial pressure charts for dry air at STP, saturated air at STP, then adjust them for your site conditions, then run them through this calc that ASHRAE lays out in their psychrometrics document.

now... humidity might come into play if you were right at the threshold of acceptable margins of say a 40hp blower verses a 42 or a 5 stage or 6 stage compressor... in that case do the calcs to get the best possible estimation and to cover your ass.

but most component requirements inevitably fall in between inch increment diameters or incremental HP's... and the effect on humidity simply would not change any design decisions what so ever.

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I developed a formula from existing formulas for calculating cooling BTUH that corrected for density etc. and found that the corrections made almost no difference until around 130,000 CFM. At that airflow you have enough accumulated errors it just doesn't matter.
There are some on here that have the ASHRAE spread sheet that is great for calculating about anything you want related to psychrometrics.

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