Pitot Bench Operation

[roepke44]

ok, so few percent.

Since nobody has answered the other questions yet, i'll keep the tangent running.

what's the smallest increment of flow rate that you feel confident that an orifice bench can measure in apposed to a pitot static? say you measured something one day, and you came back to it another day after making a really small change and measured a 0.1 cfm difference? would anyone say that that is a real change or would you say that there is no change in flow? how about 0.01 cfm? or is it more like 2 cfm? I'm just trying to see what you guys would say is the difference in precision in the two types. my gut tells me it has a lot to do with the pressure sensor being used by the two benches, but that's another topic.

Bruce, you mentioned that the pitot bench required too many flow elements to cover a range of CFM. what do you mean by flow elements? are you talking about piping? are you saying that at low flow, the velocity was too low to measure in a normal pipe, and you had to make it smaller to get the velocity up to be able to measure it, and then at high flow, the small pipe is too much of a restriction?

-Josh

[Brucepts]

the only method for truely knowing the error in a system is to measure the system with a method that is at least an order of magniude more accurate than the original system.

I don't want anyone to get the impression that i am against orifice plates. i think they are great. for me, i just think that i could build a small flowbench with a pitot static tube, some pipe, some wood and some tubing. and my bench would be just fine. what i'm asking is how is the velocity measurement normally done to account for flow none-uniformity.

-Josh

For the sake of discussion; how would you calibrate your pitot bench?

Orifice plates can be calculated to a known standard of .62 Cd, testing various plates at various depressions one can compare results. As Rick would say regressive testing.

One assumes that any flowbench being commercially made is also accurate to some degree, do they tell anyone what % it could be off? How many commercial flowbench designs are pitot based?

How accurate do you need to be? How accurate do you think you could get a flowbench made out of some pipe, wood and tubing? Might surprise you what can be achieved or what is required? I have spent many hours testing flowbenches over the past few years way more time than the average end user would ever think of doing on their flowbench.

Repeatability is what you need to be after not +/- 1/2 CFM, get repeatability then work on accuracy if you even think it is required after getting repeatability

[Brucepts]

Correct, flow elements (pitot with pipe) would need to be changed based on airspeed.

Who is looking for less than .5 cfm changes? You would have to have your bench in a laboratory to see those changes even with digital gauges, yes you probably can see them but do you need to see them?

The PTS digital manometer can see changes at .XX decimal places, those numbers change so fast you can't see them change depending on how you set the sample rate and averaging. Would I worry about cfms at .XX probably not wouldn't even worry about them at .X

Personally, I just chuckle when I see someone post numbers on forums or magazines with XXX.1, XXX.4, XXX.6 etc cfms as more than likely they are averaged numbers so are they really correct?

[roepke44]

For the sake of discussion; how would you calibrate your pitot bench?

i think i would try to find another way of measuring air velocity or mass/volume flow rate and use it to check my numbers. I know someone with an LFE that i was going to borrow and see what happens. the LFE is accurate to +/-0.2% of the reading (not full scale) all the way from 20 up to 600 CFM.

Orifice plates can be calculated to a known standard of .62 Cd, testing various plates at various depressions one can compare results. As Rick would say regressive testing.

I am unaware of this standard. i thought Cd depended on the Reynolds number for the flow. if you have a source, or link, about this, i would love to read it. i have seen plots of Re vs. Cd for different values of d/D (orifice diameter to duct diameter) before and they are vary anywhere from 0.6 on up to 1.2 for certain situations. So this 0.62 is new to me. But i am not an expert in mass flow measurement, i just know what i've been taught.

i know of only one commercial flow bench company, and for the money of one of those, they better tell me what the precision is.

I think what you call repeatability is what i call precision. here's a link. http://en.wikipedia.org/wiki/Accuracy_and_precision Basically, precision is having a good tight cluster and accuracy is hitting the middle of target or the average of the shots taken is hitting the middle of the target.

I agree, that probably beyond the +/- .5 cfm range, is not worth the effort. I laugh at those people also who carry too many digits of precision. I know you probably don't need any more than this amount of precision, but i was just wondering what you guys were getting. on a 50 cfm bench, the one i'm going to be buiding,this is +/-1%. So I'm happy with that.

I was just trying to get some numbers as far as how much better the orifice benches are than the pitot. when someone tells me that one bench is better than another, ultimately, the precision to me is what would dictate this. I can deal with using a bench that is hard to use, if the extra precision is worth it.

I really appreciate the info about the need to change pipe and pitot based on airspeed for the pito bench. i will watch out for that. I appreciate the info Bruce, Tony, and John. I know my little bench will b a challene, but i'm here to learn.

[Tony]

I think what you call repeatability is what i call precision.

Never take your eye off the rabbit you are hunting.....

Something can be precise and repeatable, such as a very slightly tapered mandrel for measuring hole diameter.

But the mandrel itself could be home made, and of uncertain absolute dimensional size.
If you are more interested in knowing if a bunch of holes are the same, it could be an invaluable tool.

As to measuring to micron absolute accuracy it would be useless.

We don't want to know if the flow is 300.024CFM at 20 degrees Celsius and sea level standard pressure during a full moon.

What we want to know is if flow has increased or decreased by 1% from what it was previously, after we carry out the secret demon trick modification.

That is where REPEATABILITY and resolution is king, and absolute calibration accuracy to a National Standard comes a very distant second.

[Brucepts]

Beta factor is the minor diameter/major diameter. Lets look at a common design where the settling chamber is 24” square and you hope to flow 600CFM at a 16” Delta P. Your internal orifice will be close to 3.375” at a .62CD. Now since C in Bernoulli’s formula is equal to the CD / square root of 1 – the (beta factor)^4 you will see that this statement only will affect in this cases CD by .0001 Which in this case equates to 1 tenth of one CFM at 600 CFM all speaking to the internal orifice plate.

Now this only gets better as the orifice plate diameter gets smaller or D1 the virtual pipe gets bigger. In the event of a calibration plate on top of the bench it reaches toward infinity which makes the affect more like .00000000000000000000000000000000000000000000000001 or whatever! NILL

With this knowledge in hand and some simple regression testing between the internal and external calibration plate you can zero in on the actual CD of the internal and external orifice plates.

This quote is pulled from a 13 page discussion from speedtalk.com a few weeks ago discussing flowbenches. I edited the quote to pull out some of the non-relevant discussion. It goes into beta ratio and how it effects the plates Cd.

Here is a Wiki on orifice plates mentioning .62Cd: http://en.wikipedia.org/wiki/Orifice_plate

Rick (or anyone else) can expand upon this next time he stops by here and sees this post.

[roepke44]

So, i went and found the post on speedtalk that you talk about bruce. It makes sense. I see that a realy small beta factor is important.

I am still a little confused about the 0.62 CD value. i don't see where this is a standard. It just seems like a common value for CD. That's fine, when get around to building an orifice bench, i will probably understand it better. My college moto "Learn By Doing".

Also, i guess no one uses regular pitot-static probes for their pitot benches. i guess everyone uses averaging pitot tubes. i am doing some research on these, and the initial papers i've read are not flattering. the first paper i read indicated that the problem with an averaging pitot is in the data reduction. fails to be able to predict the mass flow correctly because of the square root relationship of pressure to velocity since itaverages pressrue and not velcity. this is accounted for by the correction factor that accompanies the probe, but i don't like correction factors. this paper might be biased because it is from a private company (FlowKinetics), so i'm looking for more sources.

btw, with an orifice plate bench, what percentage of the max flow for a given orifice would someone switch to a smaller orifice because the readings were too low to precisely measure? I'm trying to figure out what the range of flow rates are for a pitot vs. orifice bench, without switching orifices or flow elements.

-Josh

PS. you guys really know your stuff. It's nice to be able to interact wth people of a high caliber in their field.

[Tony]

With an orifice plate bench, what percentage of the max flow for a given orifice would someone switch to a smaller orifice because the readings were too low to precisely measure? I'm trying to figure out what the range of flow rates are for a pitot vs. orifice bench, without switching orifices or flow elements.

-Josh

It depends on what you use to measure the differential pressure across the orifice.

With a traditional sloping water manometer, a flow range of 2:1 is quite practical.
That is a 4:1 pressure range, or between the manometer scale points of 25% and 100% of manometer scale length.
Below 25% the square law manometer scale becomes increasingly compressed and difficult to read to the desired accuracy.
My own bench uses a rotating turret equipped with eight orifice plates in 2:1 flow range steps.
That would be far more difficult to arrange with a pitot flow measurement system.

With an electronic differential manometer, provided it has sufficient stability and resolution, it is possible to read very accurately down to astonishingly low pressures.
The Forum digital manometer, and associated software, is now commonly being used by many here with only a single orifice plate.
Provided a suitably sized orifice plate is selected for the job at hand, there is no real need to change ranges during a typical flow versus valve lift test run.

[1960FL]

Josh;

A couple of things, First of welcome aboard, you are obviously technical and have done your homework. Since this thread was duplicated and I read your other post then saw it was a dup I have been a little behind in this. I would not cast aspersions on any of the three major types of bench design each has its place (Pros & Cons) in a technicians mind and budget and use I/E an orifice bench would be difficult to measure live flow from a motor all be it, it could be done.

Pito, The pros are it is simple to build, a shop vac and some PVC etc, Cons are that it uses averaging, weather conditions, altitude etc. are very important. Yes a Pito Static would be great if you could ensure laminar flow in the tube, but then you are talking precision tube and air straightenors etc. Then you still need to deal with direction change.

LFE, The Pros they are pre made, NIST traceable and can be programmed to be accurate across the scale. .72% of reading. .72/100 = .0072 (0 to600cfmLFE) so .0072*300 = 2.16 CFM at mid scale ??? http://www.meriam.com/productcart/pc/vi ... Category=6 Cons, COST, flow direction change and last of all they are very sensitive to Weather and altitude input.

Orifice, The pros, They are ratiometric (no requirement for Weather input), very repeatable, easy to change flow direction, and can be built with simple hand tools. Cons, The size of the box and understanding the principle of operation to effectively be able to build and understand the orifice plates. (But you buy these from Bruce HAHA shameless plug)

Discussion, Orifice plates, 1 unless you are using .010 Aluminum for your plate or testing at some high depression flex should not be an issue. CD .62 is an engineering base calculation used in discussions for a sharp edge 45° orifice plates to approximate diameter/flow calculations I will look for the PDF I stole off the net that gives said data. It is also the starting point used in calibration until you dial in your bench. Yes we do use CD to calibrate the bench as it is more sensitive than adjusting the effective diameter of the plate, but you could do either.

Defining the CD of an orifice plate is sort of an outrageous statement if you do not have a standard or multiple plates machined in the same manor at the same time. Just think back to the guy that figures out CD and how to evaluate it. Comparative analysis! I call it regressive testing (Stole this from Steff) . It works like this if I have two plates machined at the same time and manor and measured by the same person (ME) , say two 100cfm plates. I can perform tests against each other one acting as the calibration plate and the other as the internal orifice. I test perform my calculations am I close? adjust the CD test again? Spot on! Next I swap the plates test again Adjust ? Spot on? I do this a couple of times until the numbers and error on Both plates becomes stable. I have now found the CD for this particular plate pair and I have created a standard for my bench.

Last and I will then listen 

Flow testing is a comparative analysis so thus the most important thing in my pea brain is to remove as many variables as possible in the process or you are creating room for error. So does it matter if your bench in comparison to NASA is off by 2CFM at 300 or is it more important that you are always off 2CFM at 300?

As for the discussion on speed talk, flow numbers on one porters head to another’s means nothing if you do not have all the data (calibration, Weather, Altitude etc.) besides one guys .050 lift is another’s .048 and another’s .052. From what I have read over there I can tell you I would never Fly in one of those airplanes!.

Rick