antique diver wrote:Wow, at first that hurt my brain. Thank you, Herman, for pointing that out!Herman wrote:Nice to see you Greg.
A note on the manometer, they are accurate and work well but it is important to note that if you use a standard ruler as your scale you must double the reading (or add both sides together). For example, if you inhale and the water column moves up by 1 inch, the actual value is 2 inches (1x2).
In the past I had always used a straight tube dipped into a glass of water, which works properly, and reads comparable to my Magnehelic gauge. For the illustration I built it with a "U", thinking it would be nice to also easily read the exhalation pressure. I didn't give a thought to the effect of the weight of water on the other side of the U. Now I see that the U worked in the same manner as a Pulley attached at a load, reducing the lifting effort by one-half!
My apology to all for not thinking that out better, and disseminating faulty information. So, if using a U-shaped tube, do as Herman suggests, and mark your own graduations. However, if you just drop a tube straight into the water, it works fine with a ruler or true markings on the tube. Easier to carry around a little coil of 3' of tubing anyway!
The pressure differential is always the distance from the top of the water column to the bottom of the water column.
With a U tube manometer, when the water goes up by one inch on one side, the water will go down by one inch the other side (to keep the same volume). To create that situation, you need 2 inches of water column of pressure differential (positive or negative). Which is the distance from the low end to the high side.
Basically a similar thing happens when you draw water in a small tube from a glass of water. The difference is that the area (and therefore the volume) of the glass of water is so much bigger than the small tube. That is why you do not see the water level on the glass of water going down. But you still measure the pressure differential by measuring the same distance: from the low side to the high side.
If your glass of water is small and your tube is large, you will see the water level on the glass go down as you pull suction on the tube.
I hope that my explanation is clear.
Note: there are some specialized U tube manometers that do have different cross section (and therefore volume) on the two sides, but they have specific applications. You still measure the difference in the fluid levels.