I’m trying to measure pressure differential using a glass manometer (using water) in increments of 0.1 iwg. It should be a simple matter. As you know, the U tube is filled with water (dyed red), and the water level in the two halves of the U are level with the glass tube held vertical. Then, with one nipple connection (at one top end of the U) attached to plastic tubing and inserted into a chamber, and the other nipple connection (at the other top end of the U) open to atmosphere, the fluid should naturally displace, and one simply records the delta from the zero point.
But, it’s not working out to be so simple. Either each nipple connection has a one-way valve (to assure the liquid won’t spill out) that is sticking and/or the nipple connections have to be slightly loosened to get air into the glass tube! Yet, the latter is contradictory because the base of each nipple connection has an O-ring to assure a seal. It seems I do need to crack open the seal (by turning one nipple connection) so that the liquid in the tube can move! …akin to placing your finger over a soda straw.
The only time I’ve used one of these is in a college lab where everything was set up for me and ready to go. I never had to think of all these details, and the little instruction sheet is no help. Have others had similar issues with a glass manometer (as opposed to a digital one)? What is going on? What would you recommend I try?
To measure pressure differences as small at .1" of water you may want an inclined manometer to amplify the distance of fluid movement. Usually this would be just a straight length of tubing at an incline with one of the connecting tubes forming one side of the “U”. Also a good manometer has overflow chambers at each end to prevent overpressure from blowing the fluid out of the system.
Try googling “homemade automotive flow bench” (for racing engine air flow measurement) to get some good ideas for accurate homebrew manometer systems.
Thanks for the thoughts, Nefario, but the pressure drop I was expecting should have been measurable. My main concern is if air is really able to enter the U tube, or not. Should it be this difficult to use? In short, once the plastic tubing is attached and you think you’re ready to roll, should one have to twist open anything?
People are already doing what you want to do. What if you’re wrong when you think you’re ready to open the valves? Do you have time to reload and recalibrate? Why water? Why do you want the hassle of glass tubing at all? Have you googled anything yet?
Yuh, yeah… because this is the show-string budget I’ve been given to work with. Yes, Googled to no avail for what I seek. It must be that the device has a defect, perhaps the one-way valve is intermittent. It shouldn’t be this hard.
You don’t want any one-way valve.
Do you you put a screw driver in and open the valve or something like that ?
Do you remove the nipple and insert it the other way around ? Is there a plug inside that you remove?
Your trouble is in the nipple at the ends of the U tubes, remove them, inspect them, test them out…
SInce these things may be used in the field, eg climbing up a mountain, they have a way to seal in the water… so you don’t have to carry an extra container…
I re-read JINX’s post and now understand this is an existing manometer.
For measurements in increments of 1/10" of water you don’t want any check valves. This is a tiny amount of pressure and may not crack the valve.
1/10" of water (1/20" per side) may not be distinguishable in the meniscus of the fluid.
It’s not clear what pressures you’re measuring. For small differences you could make an inclined manometer from aquarium tubing taped to a sheet of cardboard.
I agree w/ the others that the checkvalve is probably the culprit, but what are you trying to measure? What are going to do with the numbers/values you are obtaining from the manometer?