How to efficiently fill U-tube manometers - I don't want a red mouth, HELP!!!

[MMack]

Finally fired up my bench Saturday night! Took me way too long to fill the U-tube manometer, and when it all settled down I was reading 1/2" high, 1/4" measured. I did get pretty good at adjusting it to 1-2" low, and 1/2" high, but hitting zero is a royal PITA! What am I missing?

I noticed in Tony's pictures of his new bench, a single tube with a reservoir. Anyone else do this? Electronics are out of the question, so I have to make this work.

Thanks!

Mike

[Tony]

A fluid reservoir is excellent to have for several reasons. But it makes filling really simple. I just use a small syringe to add or remove fluid. Another way to adjust zero would be to fit an adjustable vertical slide to the reservoir or manometer scale.

I have a second reservoir fitted at the top. If I blow all the fluid up out of the manometer due to overpressure, it just floods into the top reservoir, and then slowly drains back by gravity all by itself. It saves an awful lot of work after a little overpressure accident.

I suggest you make the surface area of the fluid inside the reservoir, perhaps at least 1,000 times the fluid surface area inside the manometer tube, which is not difficult. Then if the fluid goes up 1 metre (39") in the manometer tube, it only drops 1mm in the reservoir.

You can then use an ordinary steel ruler as a vertical test pressure manometer scale, knowing that the scale error will only be 0.1% (or less) on the scale, which can be ignored.

Note, it is the exposed surface area of the fluid in the reservoir that matters, not the volume, so with a cylindrical reservoir like mine, it should be exactly half full at zero to give the greatest exposed fluid surface area possible.



The whole thing looks rather askew in the picture, compared to the brickwork, because the manometer is leaning in significantly towards the wall at the top, and perspective has distorted some of the angles. Should have photographed it with a plain background and without the flash.

[2seater]

If you only flow one way, the reservoir and single tube work very well. If you flow both ways, you can switch the connection from the top (suck) to a fitting above the fluid level for (blow). Otherwise a conventional U-tube should have a movable scale to allow zeroing. You can move the tubes up and down, depending on how the tubes are fastened, actually only one needs to be movable, but that is more difficult than a movable scale.

[Tony]

I still prefer a single tube plus reservoir to a U tube, because the scale is much easier to read. But I do own a fairly large mercury U tube manometer that I use as a pressure calibration reference for checking pressure gauges against. I just use a loose steel rule to measure directly between the two mercury levels. It works but is not very convenient.

With a single tube, 28" pressure goes 28" up the scale. I could not be simpler. And a store bought ruler will probably look better than a scale you have to make yourself.

With a U tube 28" travels only 14" up one side, and 14" down the other. So resolution is really only half as good. That is if you are only measuring on one side of the U from zero, which is the most usual way to do it. Zeroing a U tube can be quite an entertaining process.

As 2seater says, one of these single tube manometers can measure either positive or negative pressure, and for a single direction flow bench, the connections to the bench and manometer need to be swapped around, which only takes about five seconds to do.

These single tube manometers are so simple to construct, why not fit two to the bench ? one for the blow side test pressure, and one for suck side test pressure. Then nothing need be changed around.

I did not think to do this, but in retrospect now wish that I had made a pair of identical matching test pressure manometers instead of only one.

[MMack]

Thanks for the input and ideas! I like the simplicity of the single tube, and your ideas about the scale are right on. I don't know how much direction changing I will be doing, if any so this may not be an issue.

Right now the bench is still in the main garage. I have to pull the race car out of the shop to move the bench in, and the caliper seals have started leaking and I have no brakes. And I didn't reorder caliper kits the last time I rebuilt the calipers! So as soon as the kits come in and I have brakes I will move the bench and make my wife happy!

Moving the bench matters as the top of the bench clears the garage door by 1/2", and I need the top reservoirs to sit on top of the bench. Once I get these last few domino's in it will be awesome!!?!!!

Mike

[115-1172523331]

I'm in the middle of building my manometers right now and was putting an adjustment under the reservoir for zeroing. Is there anything wrong with doing this? My reservoir is 7/8ID and the tube is 1/8. Thanks, slracer

[Tony]

A mechanical zero adjustment will be fine, I had originally planned to do that myself, but quickly realized that a small syringe to add/remove fluid would probably be just as fast and convenient to use, and a lot simpler to construct.

If your manometer tube is 1/8" bore and the reservoir is 7/8" diameter, then the ratio of fluid surface areas is only 49:1 That means that if the manometer rises 28 inches, the reservoir will fall by 28/49 = 0.57" So 28 inches rise on the manometer scale will require 28.57 inches of actual pressure.

It would probably be better if your reservoir had a much larger exposed fluid surface area than only 49 times. That is, if you plan to use a standard inch ruler as a pressure scale.

My manometer tube uses 1/4" od acrylic tube, and the bore size measures about 0.118", (3mm?) so the internal tube area works out to be something fairly close to 0.01 square inches.

The reservoir uses some thin walled two inch copper pipe four inches long. When half full, the surface area will 8 square inches. Ratio of the two surface areas should in theory end up somewhere around 800:1

[2seater]

There is one other advantage to the single tube and reservoir in that, I find no matter what fluid I use, it takes a while to settle down, not from movement but from some amount sticking to the sides. Not really visible drops, but a film that will distort the measurement somewhat. I use Teflon tubing for the manometers, and it is slippery, but there is always some amount of "slowness". The downside I see with the single tube is the lower ranges will be low on the scale. I know this sounds silly, or obvious, but I have a 90" U-tube, pretty much the height of the wall, so a single tube with the same range will have the frequently used range in a pretty inconvenient place. There is no perfect solution, but of course an electronic measuring device gets rid of all of these concerns. I agree the reservoir needs to be several hundred times the area, or you need to build in some conversion factor. I would shoot for at least 500:1

[larrycavan]

Oh how I remember the days of water guages on my bench. What a pain they were.

If you get totally fed up with playing around with those things, spend the money and go digital. You'll never, ever, in a million years, miss refilling and zeroing water guages.

[Griffin]

Hey Larry - I finally got to fire up FPExcel that I bought form you way back. Its great stuff! Thanks for the hard work.

[larrycavan]

[115-1172523331]

I am planning on building and using an Excel spreadsheet so the conversion for reservoir drop will be included. I am also thinking about using a metric ruler as that provides 1000 divisions in the 39+ inches so each mark is about .040 and reading between the marks gives .020 resolution. The 28 inch depression (27.43 on the scale) is then at the 696 mm mark. Is all of this correct? Building a flow bench is to allow me to work on my toys, not a business for me. If manometers are slower than digital, I have the time (more than the money!) Thanks for the inputs, sorry to steal the thread! -- Doug

[Tony]

I agree. There is a fairly large difference between running a busy commercial engine tuning business, and the back yard amateur hobbyist, that may only have one engine to play with at weekends.

So there is a whole spectrum of needs from the very low cost and basic, to the unlimited budget, fully automated, wonder bench.

[MMack]

Yes, I fall on the no budget/let's make this pig faster, trade my time for what I need.

But now that I have this cool gizzy, I can learn what I need to do to make the carbs work better, see which head is the one to put on the next hot dog engine since my class doesn't allow porting. Evaluate different valve seat combo's...

And I can make my little yard cart do a doughnut!

Mike

[larrycavan]