difference between pitot and orifice

[whtrthanu]

i would like to know what the differences and benefits are ..i have a 110 right now I believe its the orifice type..........

[bruce]

Well since I have now built one of each I'll give my perspective on this subject:

Pitot is eaiser to build, no need for a cabinet just some PVC pipe. Reverse flow testing can be a pain since you have to flip the flowtube or buy a bi-directional one.

I do like the orifice style now that I have built one, reverse flow is a lot easier if designed to be so. I'm going to make a self-plug here but using my plans to build a bench has made it so easy! 3-4 months ago one customer had no idea (well maybe not "no-idea") about flowbenches and now he has one sitting in his shop capable of flowing some good cfm for around $500!

There is so much info on both designs on this forum on pro's and con's of each style you can spend days reading.

I'm sure I'll hear about being a turncoat and heading for the orifice style side after being a diehard pitot style guy for so long.

[whtrthanu]

as far as repeatablity, are they the same? difficulty wise, is the orifice style alot harder to build?
Is there also a specific cabinet size that the bench has to be in order to flow a certain amount of air?
When I build it, I want to make sure that its big enough for several years to come......I seen that some of the builds are pulling 500 cfm+ @28"

[86rocco1]

A Pitot tube bench can be built to have fewer internal restrictions to air flow and so can make more efficient use of the blower motors. On the down side, as the weather changes from day to day, the readings will vary but fortunately, the reading changes are predicatible and can compensated for but means there's a bit more math involved in operating a Pitot bench.

In a large bench such as the one you have planned, it's very useful to have several range, in that regard, that's a lot easier with an orifice type bench.

[Tony]

86roccol has summed it up pretty well.

Orifice bench is arguably simpler to build, and easier to reverse the flow, and it is entirely practical to use water manometers to begin with to keep the cost down. The only real disadvantage is the required built in pressure drop across the measurement orifice. This requires a slightly more powerful blower.

Pitot is a bit more high tech. It really needs electronics to fully exploit the pitot advantages. But I doubt if the final results are going to be any more accurate.

Either type is capable of excellent results, but how well your own bench ends up depends upon the time and effort you put into building and understanding it.

My own advice for a first time novice, especially on a budget, would be to use the plans Bruce can supply for his new orifice bench. There is a fairly long learning curve with all this, and plans for a known successful design are the very best way to begin.

[whtrthanu]

as far as the orifice bench goes, do you have to warm the bench up and calibrate like I do on my 110?
I always have that correction factor to figure on, will I still need to use the c/f for the day, and can I still hook up the flo com and sensors to that style bench.........

[86rocco1]

The reason you need to warm up the sf110 is because the blower motors are located in between the tested piece and the measurement orifices causing the air to warm up and change density between the time it flows through the tested piece and the time it's flow is measure as a result, you need to let the temperature stabilize so that the appropriate correction can be determined.

This sort of correction is not necessary with Bruce's style bench or the larger SF benches because the airflow pattern through the bench is different so that there's no change in density between the time the air enters the test piece and is measure.

I'm not exactly sure how the flowcom is configured but the sf110 measures airflow in the same manner as all orifice type benches so I think it's quite likely you'll be able to use it. There are several other people on these forums that are much more familiar with SF products than I, perhaps one them will be able to give a more definitive response.

[Tony]

That is the beauty of an orifice bench it is completely ratiometric. It is just two flow restrictions in series, with exactly the same mass of air going through both.

If you have an exact known flow calibration figure for your measurement orifice plate, and you measure pressure across what you are testing, and pressure across the measurement orifice, flow through the test piece can be worked out without knowing anything at all about the actual air. It is just the ratio of two restrictions, one known, the other what you are measuring.

It could not be simpler, and it eliminates a lot of unknowns. You will get the same flow reading in scorching summer, or freezing winter, without applying any corrections to the readings.

All this assumes no temperature rise in the air between what you are testing and the measurement orifice. It has to be the same air at the same temperature. Some benches locate the blower between what is being measured and the measurement orifice. That is bad design and complicates things greatly.

Some commercial orifice benches are made to be very neat and compact and suffer from this defect. There is no reason why a home built bench has to be built very small. So do it properly and you can ignore the blower motors heating up and changing the flow readings.

With a pitot bench, you need to measure ambient conditions and make corrections. That is why electronics are really required to make a pitot bench practical and easy to use. Nothing really wrong with that, it just requires a bit more cash to get going than building some home made water manometers.

There is another thing. If you can measure say twelve inches of water level change in a water manometer, that is what it is. It simply cannot be wrong!

But your fancy electronic gizmo might read 12.002 inches of water. But how do you know the thing is not faulty and the real pressure 11.5 inches or 13.7 inches or something else ?

*edit Ha-ha 86rocco you beat me to the draw on the blower placement.

[thomasvaught-1]

Quote:

"I'm sure I'll hear about being a turncoat and heading for the orifice style side after being a diehard pitot style guy for so long."

Some guys are slow learners!!! Just Kidding Bruce.

Is the 12 motor deal similar to what we discussed?

Tom V.

[bruce]

Tom, I blame it on fact I never read the orifice area of the forum for many years

The 12 blower design has larger chambers, when we talked on the phone we were looking at 16 blowers with your setup? You were going to send me a sketch of what you proposed.

[whtrthanu]

Ok i got the info i needed about the orifice bench...Now, if I dont run my "electronic gizmos", can I still have some type of data logging? Im sure that there are other flo com type devices out there, but what do I really need or want to capture my air speed and cfm...I dont mind doing some calcualtions, but it would be nice to have the pc do it for me

[Tony]

Sure you can have automatic electronics result logging, no problem at all.

I just wanted to get away from the general idea that something with lots of computer power and a big bright color screen is the only way to get any sort of good results.

A very simple orifice bench with low cost water manometers is not some sort of crappy inferior thing to be avoided. It may not LOOK as impressive, but it is an excellent way to build a first class accurate flow bench on a budget.

Data logging can be added to it later, but it is certainly not required to have a perfectly usable and practical flow bench. After you have adjusted and checked the valve lift, corrected the test pressure, and taken a flow reading, writing the numbers down with a pencil and paper is no big deal.

My bench does not have electronic monitoring, and I am a retired electronics engineer that worked for many years as a supervisor in a nationally accredited government run gas flow and standards laboratory.

I get along fine here in my home garage without yards of paper spewing out of a printer with thousands of numbers arranged in multiple columns.

[whtrthanu]

Im still not clear on what type of device i can use to get the air speed, other than calculating it........Like i said i dont have a problem hand writing my results, I thought that the flo com type device will collect more data.....

[larrycavan]

[whtrthanu]

Thanks Larry thats exactly what i was looking for.............
Now i understand the pitot tubes.....
I have been using my 110 for about 4 years now and i have just about ran out of home made ideas to see how the air travels acroos the floor ....
Now I need those new bench plans................