Building a Manometer

[c-snake]

I have searched this forum and the general Internet via Google and am just getting more confused.

I thought I would build a simple manometer for testing purposes, but am having difficulty finding consistent information.

Here are my questions;

Does the size of the tube matter?

Can a reading in inches be taken directly from either column? Regardless of tubing size?

Is the reading taken from one column only or is it the difference between the two columns?

I have found answers to these questions on several sites, only to find contradictory answers on the next.

I am considering just ordering a Magnehelic or two from E-Bay, but then I have a slew of questions on these also.

It seems like for the price of the Magnehelic, that is the way to go. They can be found so inexpensively, maybe it can still be a DIY project in the assembly and building of the stand etc.

Any and all experience, help etc is appreciated.

 

[herman]

I have searched this forum and the general Internet via Google and am just getting more confused.

I thought I would build a simple manometer for testing purposes, but am having difficulty finding consistent information.

Here are my questions;

Does the size of the tube matter?

Can a reading in inches be taken directly from either column? Regardless of tubing size?

Is the reading taken from one column only or is it the difference between the two columns?

I have found answers to these questions on several sites, only to find contradictory answers on the next.

I am considering just ordering a Magnehelic or two from E-Bay, but then I have a slew of questions on these also.

It seems like for the price of the Magnehelic, that is the way to go. They can be found so inexpensively, maybe it can still be a DIY project in the assembly and building of the stand etc.

Any and all experience, help etc is appreciated.

Tube size is not critical but too small and it's hard to read, too large and it becomes less responsive...just as accurate however. I found 1/4 in ID tubing to be a good compromise. You should keep the inlet hose length to a mimimun (I like around 2 1/2 ft) to reduce the amount of air that has to be moved. If you want a longer inlet hose you can splice in an 1/8 section of tubing to increase the responsiveness. IMO this is more trouble than it's worth.

You are reading the differance the water moves in total, you read across the water column. For example, you slowly constantly inhale on a reg under test and the water column rises from 6 inches to 7 on one side (and 6 to 5 on the other), your inhalation pressure is 2 inchs of water column (or IWC). Or you can double the reading on one side - 1 in rise= 2 in of water. I like set the water at 6 inches just because it's in the center of the scale. The real beauty of the manometer is it can read vaccum and pressure without changing anything and it never needs any kind of calibration. It's dead on accurate as long as you put plain (colored is OK too) water in it and is the standard that magnahelics are calibrated to. Plus if you drop it, the worst thing you can do to it is spill the water.

Did you see the photos on the one I made in the DIY section??

 

[c-snake]

Hi herman, thanks for the reply.

I think I did see your picture in another thread, was it a green ruler used for a scale?

OK, so the reading is taken only from one side (or the other) and it is an accurate indication in water column change by the inch.

One of the articles I found said that the reading was taken between the high and low columns together, this is where I started to get confused.

It seems odd that larger tubing with a greater weight of water to move will read the same. I guess I will just accept the fact. This site seems to say that the reading is taken between the two tubes. (scroll down to diagram 4) Maybe I am misunderstanding it.

Pressure Measurement By Manometer

I will go with the 1/4" tubing and see how it goes. One nice thing is that it is so inexpensive I don't mind experimenting.

It's a fun and inexpensive project, my favorite kind.

 

[HenrikBP]

never mind

 

[Luis H]

The reading is the total difference between the levels on both sides of the “U”.


The size/ diameter of the tube doesn’t mater.
You are measuring pressure in force per unit area.


Added:
Example 4 in your link even shows that the tube size can change and it doesn’t matter…you still only need to read the difference from the low point to the high point.


For regulator testing I do own several Magnehelic gauges that I bought on eBay.


I hope this helps.

 

[c-snake]

I found this page from Dwyer that confirms the reading. It must be taken as a measurement across both tubes, meaning the reading will be taken from two different places at the same time.

Measurement of Pressure With The Manometer

The way to overcome this would be to make one side of the "U" tube larger so that the true reading in inches of water column can be taken from only one side of the tube.

Sorry for the confusion on my part, but there are some "how to" pages that show the reading being taken from one side only, in inches of water. I hope those weren't important to anyone's safety!

I think you could make your own measurement graph, in 50% scale that would take a true reading from one side. Is this good logic? Would that be too small to get an accurate reading in the increments we use in scuba?

The other options are to do the math and build one side of the u-tube larger than the other, or make a scale that is easy to read across both sides of the tube.

Hmmmmm

 

[Luis H]

The pressure differential is always the difference between the high side and the low side. What you accomplish by making one side much larger than the other is that the fluid level in the very large side tends to move very little compared to the small side…therefore, this movement can be ignored since it only introduces a small reading error.

That is a good link.

 

[c-snake]

Again, assuming I am understanding the theory, this is what I have come up with;

I will use a 1/4" tubing on one side and a tube with twice the water surface area on the other. I came up with a value of .353DIA for the large side. This way a direct reading in inches can be taken from the 1/4" side of the gauge.

The only way I can think of getting a tube with an ID of .353 is to find a drill bit as close to that as possible and bore out a smaller pipe or a solid block. I would then have to figure a way to make the connections.

This is getting more complicated as I go, and the fun is sliding away fast.

Using a straight tubing of one size would work just fine, it just makes getting an accurate reading a little more difficult. As I am breathing on the reg, I would have to watch the gauge and get the readings from both sides of the tube at that brief instant. Then figure the remainder from the difference in the two values.

Anyone have any feedback on this?

I just missed a 0 to 5 magnehelic on E-Bay that went for $3.77.

 

[james croft]

I have gotten a magnehellic guage on the bay but I also used a DIY one out of tubing that works just fine. The whole point of a DIY project is to do it yourself so i think you should make one just for the knowlege and satisfaction gained. Then you can use it to amaze your buddies who will think you are a genius.

 

[Luis H]

I haven’t checked lately, but I have bought several Magnehelic gauges from eBay for about $20 on a buy it now option. I like the 0 to 3 inWC for better precision and the +5 to -5 inWC since it is convenient if I want to check the exhaust.

Using a tube that has twice the area will not accomplish mush. To ignore the large side it needs a reservoir that has a surface area much larger than the measuring small tube.

For example if it is exactly twice the area that means that if the fluid in the small tube goes up by an inch, the fluid in the large tube will go down by ½ inch. You still need to read both sides for a total of 1.5 inWC. The error would be too much if you ignore the larger tube.

Again, you can only ignore the large side if it is many times larger than the smaller tube.

The “U” tubes are good, but a Magnehelic is super convenient…I have used both professionally (I am an engineer) and for my regulator repair hobby for a long time (I have also worked at several dive shops in the past).