What manometers ? - Newbie, questions, sorry !

[Phil]

Hi
This is my first post here. I've been reading this forum for some time and there's certainly a wealth of information on here, most of it over my head at the moment ...
I've decided to make an orifice type bench, as this seems the simplest. I will probably make a steel frame and fasten the panels of the bench to this, as this seems to be a good way of making a strong unit that should seal well and be easier to maintain.
I've collected a few of the parts needed to begin construction of the bench. I have 6 1200w vacuum motors, switches, some steel box section & some sheets of 3/4" ply.
I've searched but i'm still unsure on a few basic things.

{1} What size manometers do i need ?
I see most people measure at 28" of test pressure, so i guess i need the vetrical manometer to be at least this length. But what about the inclined manometer ? what size does this need to be ? Are there certain criteria that dictate this ?

{2} Regarding the orifice plate, what size do the holes need to be ? What decides this ?

{3} Besides this site is there anymore basic reading i can do to learn a bit more of the basics ?

I only need the bench to flow around 300cfm, as it's for relativley small capacity 4cylinder engines.

Sorry if this seems a bit dumb, but i'm just trying to get all the pieces together in my head so i can start to get on with the projest.
Thanks a lot for any help you can give.
Phil.

[larrycavan]

Here's my advice before you go any further. If you're going to buy high end water manometers from someone, unless you're getting them dirt cheap, don't buy them. Go digital for that.

It's important to decide that first because it will affect your orifice sizes and the number of orifices you would actually need.

John's FP1 is far superior to water guage manometers when it comes to accurate reading across a wide range. Something referred to as turn down ratio. In other words for a given size orifice, a water guage tends to be most accurate over a given range.

That being said, you can see how a better measuring system will allow you to go with less ranges.

And as for cost comparison. For new professional manometers purchased from a reputable manufacturer or reseller, it's about the same.

On top of that, you won't have to be dinking around refilling over pressured guages which is always a pain..

Larry C



Edited By larrycavan on 1159912033

[Phil]

Hi
Thanks for your reply. I've only just found my way back to this site after a long time away. Time to start the project again.
So if i go digital, what is the FP1 you talk about ? I've seen talk of it in other posts & have done a quick search.
I've seen plenty of digital units on ebay, are any of these any good ?
Thanks
Phil.

[Tony]

The FP1 recommended by Larry is certainly an excellent way to go.

But water manometers will still work and are a truly low cost solution. For a first attempt at building a flow bench, water manometers will get you started on the learning curve. An FP1 can always be bought later as a future upgrade. If your budget is limited, spend it all on the actual bench to begin with. Much better to have a large capable bench with water manometers, than a small capacity bench with electronics.

The answer to your question, is that an orifice bench is built so that the design pressure drop across the measurement orifice matches the design pressure measurement range of the sloping manometer.

The higher this design pressure is made the better, but having a very high pressure drop across the measurement orifice creates a lot of extra flow restriction for your air blower source to overcome. So it is all a bit of a compromise.

Suppose you want to test at 28", and you have already determined by some tests that your air blower can easily develop 40" at the required maximum airflow. Then deciding that something around 12" might be a realistic design pressure for the flow measurement part of the bench.

But four inches or thirty inches design pressure would work too. Suppose you settled on twelve inches as being a good all around compromise pressure for the blower combination you have.

You then calculate the required orifice sizes so that each orifice just reaches twelve inches pressure drop at the 100% flow figure for that particular range.

And you build your sloping manometer so the high end is twelve inches above the low end (at the 100% flow mark). The length can be made anything convenient.

Length is not important, except if it is made very short the angle will be steeper, and the flow divisions will be closer together. There will be a practical limit where something over three or four feet long might be rather difficult to mount on the bench.

Everyone has their own ideas on what is optimum. But the starting point should always be to measure the capacity of your air blower first, and build the flow measurement system around whatever surplus pressure is available beyond the test pressure.

[Thomas Vaught]

Many people have their water manometers made with a "well" at the bottom and a vertical tube. On some inclined manometers you could have the well mounted low or high.

In either case, the point was made that sometimes you inadvertently
push water or manometer fluid out of your manometer.

I solved that problem (and a slight oscillation issue) by installing a large dampening reservoir/ "catch can" in the line where fluid would naturally move. No more oscillations and if the fluid blows out of the top of the manometer in is trapped in the "catch can". The fluid would completely have to fill the large "catch can" before it could continue further. The "catch can" is about the size of a quart fruit jar but made of plastic. Two normal nipples are at the top of the can. (Cut the manometer line and just put the can in the circuit). On the bottom of the "catch can" is a valve with a hose that will allow me to easily drain the fluid back into the well of the manometer.

The RED .826 sg fluid is not cheap but the BlUE "Super Flow" style 1.91 sg fluid will make you cry if you have to replace much of it.

I think the electronics have their place in the flow bench world but I also think, like Tony, that manometers can do a fine job too if you put a little extra thought into the process. JMO

Tom V.

[larrycavan]

[Tony]

I agree 100% with Larry.

Homebuilt manometers are fun, ultra low cost, work well, and a great learning exercise. But the cost of commercial manometers just prices them right out of contention. Your hard earned cash would be much better spent on electronics, and the FP1 system definitely appears to be the best way to go.

[Phil]

Thanks for the replys chaps, some great info there.
I think i'll go with making my own manometers to begin with. Like some of you said this way i can get the bench up and running on a low budget & half the fun is making things yourself, hey ?
What size ID clear tubing is best/suitable to use ?
Also how do i know how much my vaccum source will flow ? I've just got a load of 1200w motors from Hoovers.

Thanks a lot for your time.
Phil.

[Tony]

Best bet is to fit all your motors to some sort of crude temporary blower box, and do some testing. See how much pressure and flow your motors can produce across a range of test orifice loads. Each different sized orifice will give you a spot flow and pressure reading. With about six points from six different sized orifices, you can then plot a graph of flow versus pressure.

Pressure will obviously fall off as flow increases, and then you can make some logical decisions about the overall capacity and ability of the vacuum motors you have.

Extreme accuracy is not important, and neither are a few air leaks, but it will give you a pretty good idea of what you will have in the way of bench capacity, before committing yourself to spending a lot of time and money on the actual bench.

As for manometer tube, either clear acrylic, or laboratory glass tubing will both work fine. The standard size seems to be quarter inch o.d.

I am using clear acrylic 6.35mm o.d. with a 3.0mm bore. It is less expensive, easier to cut, less likely to shatter than glass, and can be easily formed with a hot air gun.

[Griffin]

I can't speak to water manometers, but I will say this: we just built our first DIY bench and we use the FP1 and its has been a godsend. Aside from the utter ease of flowing heads when you have the FP3 to regulate your depression for you, the system calculates SCFM for you, it is VERY easily calibrated, requires literally no complicated math if you buy some professional calibration plates, and its invaluable for hunting fown leaks, foibles, and inconsistencies that you will Probably wind up building into your bench without knowing it.

John at Flow Performance has also been a godsend as he has lots of experience, is easily accessable, and consistently has good well thought out answers to many questions I have asked, a lot of which didn't even relate to his product.

Anyways I think the FP1 system is well worth the money, and I think the FP3 is as well, but ultimately its up to you.