Tractorsport Flowbench Forum Archive

Posted: **Mon Mar 30, 2009 3:16 pm**

Hello. I have read most of the postings on this but I am confused about some. I need to make a vertical man. to verify depression on my bench. I'm confused about the size of the tube. Won't different ID's in the tube change the height of the fluid? This is a fairly temporary device for me so what is the best fluid for me to use? Also regarding the flow calibration plates, I have made head fixtures and more from plexiglass. It is easy to machine and very rigid and fairly cheap to buy. I think you can find scraps if you look around. Thank's for any advice. Forgot to ask the significance of the surface area of the fluid verses the volume. Jim

Posted: **Mon Mar 30, 2009 5:26 pm**

The diameter and type of tube makes absolutely no difference to how far the fluid is forced up the tube by air pressure. Make the manometer tube any convenient diameter.

The best fluid to use for a quick test is plain water straight out of the tap.

To make it a bit easier to see, get some food dye from your local supermarket. One small drop of liquid food dye will turn a surprising amount of water a very deep vivid colour.

If you use a U tube vertical manometer, one leg goes up, the other goes down an equal distance. That can be rather inconvenient to measure.
If you use a vertical tube with a well, the fluid goes up the tube, and the level falls in the well. How far each moves depends on the relative surface area of each.

If the surface area in the well is a 1,000 times the surface area in the tube (not difficult to do) then the fluid might go up the tube say 21 inches, and the fluid in the well falls by .021 inches (1/1000). This very small level change in the well can be ignored, and you can just measure the vertical rise in the manometer tube directly with a steel ruler. If it goes up 21 inches, the real actual pressure will be 21.021 inches. Close enough !!

So use a fairly skinny manometer tube, and a well with a large exposed water surface area. The well does not need to contain a large volume, just have a lot of surface area. So something wide and shallow (like a saucer) is the way to approach this.

Posted: **Mon Mar 30, 2009 5:32 pm**

If you're making a U-tube style manometer, the size of the tube has no effect at all so long as both sides of the U are the same size. But, if you're making a vertical manometer with a reservoir or if the tubing size on both legs of a U-tube manometer are not the same size, you'll need to compensate for this. Go , download the spreadsheet you'll find there. You can use the inclined manometer calculations for a vertical manometer too, you just enter the same number in the cells marked "Scale Length" and Vertical Scale Height"

Posted: **Tue Mar 31, 2009 10:40 am**

Replys most appreciated. Maybe you can both help me. I know only basic math. Using a 3/16 id tube, I get .02757 sq. in. x 1000 as you stated equal's 27.60 sq. in. of surface area of fluid. Is my math correct or not? Rocco81 says I should use the formula. Why? Besides I really don,t understand it. Help! I have another question or thought also. This one is directed to Bruce too. Rather than using a U bend tube for testing velocity, can't you use a straight tube and test vacuum in the port for testing air speed? There would be way less air disturbance with a straight tube I would think. It sounds like members are using the U shape pitot with a manometer and converting the pressure to fps. Couldn't you do the same with vacuum? Just food for thought. Would like some answers on that one. Thanks all, Jim

Posted: **Tue Mar 31, 2009 10:45 am**

Sorry for the misprint 86Rocco1. I wasn't looking at your post when I wrote this. Jim

Posted: **Tue Mar 31, 2009 3:25 pm**

Your math is correct. If for example your tubing has an 3/16" id and your reservoir has a 3/4" id, you WILL need to use the formula to adjust the scale to give you proper readings but if the surface area of the reservoir is VERY large compared to the cross sectional area of the tubing as Tony is suggesting, the error introduced is so minimal and you can just ignore and uses something like a yardstick as a scale with no mathematical adjustments required.

As for the Pitot tube, I think it requires the direct impact pressure of the air to get a proper reading, I don't think a straight Pitot will work for vacuum, but I could be wrong, I've never tried it and don't know anyone that has.

Posted: **Tue Mar 31, 2009 3:36 pm**

Your math is correct, but 3/16 ID tube is quite large. Many people here use 1/4 OD tube, which is a very common size. The bore of that is quite small. Another way to look at the area of circles is to say, a minimum area ratio of 1,000:1 requires a minimum diameter ratio of very roughly 32:1

It is a most curious thing, but a column of coloured fluid in a transparent tube looks a lot larger than it actually is. The curvature of the tube wall magnifies the optical size. So don't assume that a 1/4 OD (thick wall) tube will be far too small to see clearly. Try it, I think you will be quite surprised how well it works. Your fluid well might then only need to be a very few square inches.

Posted: **Tue Mar 31, 2009 3:59 pm**

FWIW, my tubing is rigid acrylic aquarium tubing .200" od, .125"id with red coloured water in it, it's very easy to read

Posted: **Tue Mar 31, 2009 4:01 pm**

My velocity probes (pitots) have a velocity and static pickup so they measure airspeed and need to face the airflow. A straight single tube probe would allow you to see pressure but you would not be able to relate that back to any air speed just a number.

Posted: **Tue Mar 31, 2009 4:32 pm**

Posted: **Tue Mar 31, 2009 8:17 pm**

Thanks much for the reply's AND the help. I'll go with smaller tubing as you suggest. Bruce, I have noticed in your pictures the extra fitting on your pitot's and have wondered what it is.If this is the static fitting, maybe you can tell me how it works. Thanks,Jim

Posted: **Tue Mar 31, 2009 8:47 pm**

Posted: **Tue Mar 31, 2009 9:14 pm**

Here's a schematic of what Bruce's Pitot tube looks like, this image is from .

Posted: **Tue Mar 31, 2009 11:12 pm**

What type of manometer is needed for Bruces pitot. Jim

Posted: **Tue Mar 31, 2009 11:17 pm**

Just a simple U-tube works. Needs to have a scale longer than your static depression manometer scale.

Edited By bruce on 1238555882

Tractorsport Flowbench Forum Archive

Vertical manometer help