vertical manometer

[Eagle Eye]

Hello flowbenchers. Can anyone tell me how to calculate the reservoir size or area for using a velocity probe in the port. This is for a diy vertical manometer that I already have. The id of the vertical tube is about .105 to .120. I will have to check to be sure. I think I can also use a U tube as the reservoir and manometer. Been a while since I've posted (old forum) so hello again to all the old members and to the lots of new members too. Jim

[Flash]

Are you taking about the math to convert Inch of water to fps or calculating port area by the air speeds you find in a port section, with a velocity probe?

[86rocco]

Manometer calculations are fairly simple. Ideally, you want to measure from the surface of the fluid in the reservoir to the surface of the column but because both levels change when pressure it applied, it's not really practical to do, it's easier if one end of the scale is fixed in which case we need to calculate how much of a correction we need to add or subtract from our measurement to get the correct value. Normally, we measure from the static level to the fluid level in the column, let's called that measurement "M", in that case, we need to add the amount the reservoir level has dropped below the static level.
That amount turns out to be M*(area of the tube/area of the reservoir)

The total pressure = M + M*(area of the tube/area of the reservoir)
= M * ( 1 +(area of the tube/area of the reservoir))

Notice that a simple U-tube manometer is just a special case of the manometer in the diagram in which the area of the reservoir equals the area of the tube.

In your case where you're using the manometer with a Pitot probe, you'd connect the static port of the Pitot probe to the top column and the other port to the reservoir. And to calculate velocity in FPS, open up the spreadsheet found in THIS THREAD, click on the tab at the bottom labeled "Pitot velocity calculations" then you'd take your calculated total pressure (in inches of water) plug that number into the spot labeled "Velocity pressure", and the spreadsheet will calculate you air velocity in feet per second.

[Eagle Eye]

Thanks for the reply from both. I'm wanting to check air speed in the port. I tried with a reservoir with a large surface area that you gave me before in the old forum. I believe it was around 2000 to 1 surface ratio. The fluid in the vertical tube didn't move so I decided that the reservoir area was too much to move down. Would I be wrong thinking that the surface area should be the same in both? In other words a U tube in essence? Thanks,Jim Sorry for the slow responce. The holidays!

[Brucepts]

In my opinion I would go with a U tube for velocity and get your tube IDs as small as possible, reason being the holes in the probes are small and the closer you match those holes the faster the probe reacts, least this is what I have found with my probes.

[Eagle Eye]

Thanks Bruce. My tube is about .120 inside dia. For the U tube can I use two rigid vertical tubes with a soft tube in a U shape connecting the two at the bottom? If so, should the soft tube have the same ID or very close? Have a very Merry Christmas for all who celebrate it. Jim

[86rocco]

The diameter of the flexible tube can be whatever is most convenient for you it won't make any difference to the reading but keep in mind that to make the manometer as responsive as possible, you want to keep the total volume of fluid as low as is practical.

[Eagle Eye]

Thanks Rocco

[ftch]

Following this thread. I am interested. Not as a pitot tube manometer. However as a straight vertical manometer how does tube diamter change a given reading in a vertical manamoter.

I have a couple of dwyer 16 inch vertical manometers and I want to build a simple 40 inch vertical manometer and just use water in it.

cheers
Michael Marriott
Australia

[jfholm]

Here's what I have found when I have built the manometers. The smaller the diameter sometimes causes your fluid to not entirely return so you end up with some residual water left in the tube. I then had to tap the tube to get it to go back in the reservoir. This was more of a problem with the inclined manometer.

At that time I was using .125" id tubing. I am now switching to .250" id tubing. Maybe somebody knows of something you can add so the water is slicker and won't stick.

John