# Liquid pressure differential gage? **URL:** \ **Category:** Factual Questions\ **Created:** [March 6, 2025, 8:34pm UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221 "2025-03-06T20:34:03Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [March 6, 2025, 8:34pm UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/1 "2025-03-06T20:34:03Z")
Going to make a little pressure differential gage for measuring low air pressures ( below 2" - and +) I will be using clear plastic hose with no vent. I would like it to express itself over a longer distance than would read using the 1/2" hose. Where I am stumped is I assume I will have to have the liquid being into the smaller hose with static pressures about the middle. How do I handle the blank air space in the smaller tube? I don’t think it would change the length of the expression without using vents? Any tips? Double check valve? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [March 6, 2025, 8:45pm UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/2 "2025-03-06T20:45:58Z")
A sketch would help. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [March 6, 2025, 9:08pm UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/3 "2025-03-06T21:08:34Z")
I will try a sketch but it is a simple thing to explain. think of a tube 1/2" diameter by 6 ft long open on one end and closed on the other end. Somewhere in that line it would droop down maybe 12". At the bottom of that loop was a few inches of colored was allowed to settle at the bottom of that loop. so if one end of the loop was a different pressure than the other the water would move until it equalized. I would simply monitor the difference and calibrate it to pressure source. I would like to lengthen the distance that the water moved but can’t figure out how to make that work. ---
**Author:** ![ThisOneGuy](https://avatars.discourse-cdn.com/v4/letter/t/c0e974/32.png) [@ThisOneGuy](https://boards.straightdope.com/u/ThisOneGuy)\ **Post date:** [March 6, 2025, 9:11pm UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/4 "2025-03-06T21:11:21Z")
You can’t change the vertical distance as that will be set by the pressure difference. But if you put the liquid section on a slope you could increase the tubing length associated with the vertical distance. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [March 6, 2025, 9:14pm UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/5 "2025-03-06T21:14:58Z")
That makes good sense! Is this simply based on ramp theory and the weight of the water? Actually that makes very good sense, I would simply hardwire the Loop section on a bard using clear plastic tubing and the angle would be adjustable\> I really like that! ---
**Author:** ![ThisOneGuy](https://avatars.discourse-cdn.com/v4/letter/t/c0e974/32.png) [@ThisOneGuy](https://boards.straightdope.com/u/ThisOneGuy)\ **Post date:** [March 6, 2025, 9:17pm UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/6 "2025-03-06T21:17:46Z")
Just that the pressure difference in a liquid is based on depth, not the width. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [March 6, 2025, 9:22pm UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/7 "2025-03-06T21:22:26Z")
Yes I realized that when I started thinking of how much liquid to put in the loop. Would just having them level at the loop nullify the weight factor? Could I simply change the angle on one side of the loop for a longer reading but make sure they were at the same height when static? ---
**Author:** ![ThisOneGuy](https://avatars.discourse-cdn.com/v4/letter/t/c0e974/32.png) [@ThisOneGuy](https://boards.straightdope.com/u/ThisOneGuy)\ **Post date:** [March 6, 2025, 9:25pm UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/8 "2025-03-06T21:25:33Z")
Thinking about it some more, the other thing that is changed is the density of the fluid. Water is normally used for cheapness and lower pressure differences, mercury used for high pressure differences because of its’ high density. The amount of water in the loop doesn’t matter (for reasonable amounts), the height difference will still be set by the pressure. Tubes like this are used for “water levels” to set things to the same height when they are a long distance apart so a standard level can’t reach. The key measurement is the difference in height between the two sides, so I would put both on the same slope. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [March 6, 2025, 9:29pm UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/9 "2025-03-06T21:29:14Z")
That’s what I thought. I am expecting about a 1" rise in height, if I simply spread that 1" over about and 8" length it should work pretty good. A little care will be needed setting up maybe but sounds excellent. ---
**Author:** ![mixdenny](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mixdenny/32/2962_2.png) [@mixdenny](https://boards.straightdope.com/u/mixdenny)\ **Post date:** [March 7, 2025, 7:03am UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/10 "2025-03-07T07:03:38Z")
Yes, you just build an inclined manometer. You can buy them also. I have built manometers to measure natural gas pressures. You need to have both ends open and connect one end to the pressure source. > **[Inclined Tube Manometer](https://www.indiamart.com/proddetail/inclined-tube-manometer-12948691030.html)** > > Spear Exim Private Limited - Offering Acrylic Inclined Tube Manometer, 0 to 500 mm H2O at ₹ 3000/piece in Mumbai, Maharashtra. Get Inclined Manometer at lowest price | ID: 12948691030 ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [March 7, 2025, 8:52am UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/11 "2025-03-07T08:52:58Z")
> [@mixdenny](#): > > u need to have both ends open and connect one end to the pressure source. I need one end closed because I will also be measuring suction ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [March 7, 2025, 1:50pm UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/12 "2025-03-07T13:50:54Z")
> [@HoneyBadgerDC](#): > > I need one end closed Your device is going to measure pressure difference between the open end and the captive space inside the closed end. That is a difference, but what significance is there for the pressure inside the captive space? You’re not connecting it to anything interesting. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [March 7, 2025, 4:12pm UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/13 "2025-03-07T16:12:01Z")
> [@Napier](#): > > Your device is going to measure pressure difference between the open end and the captive space inside the closed end. That is a difference, but what significance is there for the pressure inside the captive space? You’re not connecting it to anything interesting. Hopefully I can calibrate it to measure applied force. I would prefer the open end measure but I think it would give me problems on the suction side, also I get a little too much movement of the water with the open end ---
**Author:** ![mixdenny](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mixdenny/32/2962_2.png) [@mixdenny](https://boards.straightdope.com/u/mixdenny)\ **Post date:** [March 7, 2025, 6:11pm UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/14 "2025-03-07T18:11:54Z")
It won’t work with a closed end. That would be a barometer that wasn’t built properly. Suction works just fine on open manometers. Just make sure the active end connected to the source loops high enough to allow for the maximum suction from your device. ---
**Author:** ![ZurBob](https://avatars.discourse-cdn.com/v4/letter/z/71e660/32.png) [@ZurBob](https://boards.straightdope.com/u/ZurBob)\ **Post date:** [March 11, 2025, 2:18am UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/15 "2025-03-11T02:18:57Z")
> [@HoneyBadgerDC](#): > > **Hopefully I can calibrate it** to measure applied force. I would prefer the open end measure but I think it would give me problems on the suction side, also I get a little **too much movement of the water** with the open end (Bolding mine) Hope you’ve solved your dilemma over the weekend. If not, go here: [Amazon.com: ULTECHNOVO Easy Read Manometer, U-Tube Air Pressure Indicator Pressure Gauge for Radon System Monitoring Measuring Gas and Water Column Pressure : Industrial & Scientific](https://www.amazon.com/ULTECHNOVO-Manometer-Indicator-Monitoring-Measuring/dp/B0DMW1RFWL/ref=sr_1_5?crid=2L9QUPPXK3EN4&dib=eyJ2IjoiMSJ9.f8RnzxNm9sAyGRV4DuSgQNqyYVXctiYoBHGGNDeWM3rEclKbNQnpMojDanfpVyY9kEVO_d50SkHUSXNLRgxy_zeS6IgmWN1bApo4jqFaioryBNMPbPbWIhuyLvn2crLF3EjAOHYeiYyNBjPZLvBSw7dbVP-6QvODT32envOi9DCI-HLv4voKpiLJatOMybzigbmb9nKlLZEiBdSObbH4bdqwZVRbeRav35XVtUTF5mw.H12WY149LdO7mOh9CJQlMz8-j0kY7yk8mVWKawj_-A4&dib_tag=se&keywords=water+column+manometer&qid=1741653172&sprefix=manometer+water+column%2Caps%2C310&sr=8-5) Buy this thing for $12.79 and save yourself multiple headaches. Calibrating a homemade inclined manometer might not be worthwhile, especially when you require such a high degree of resolution (fractions of an inch H2O \<2") not to mention accuracy. Don’t get me wrong, a (storebought) inclined meter would be great, but perhaps a simple U-tube meter would be simpler to comprehend. The referenced meter is capable of measuring 12"WC @ 0.04" per graduation. Certainly 4/100 of an inch H2O will get you where you want to be? While the size of _your_ tubing makes no difference, if the reading is unstable or “jittery” try increasing the tube diameter to smooth things out. BTW: What the heck are you measuring? Indoor/outdoor D/P? The force of a butterfly wing? ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [March 11, 2025, 2:53am UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/16 "2025-03-11T02:53:15Z")
> [@ZurBob](#): > > BTW: What the heck are you measuring? Indoor/outdoor D/P? The force of a butterfly I am using a large diameter bellow as a source of resistance in strength training. Its about 1/100 PSI of the applied force. So with 100# force I only have 1 PSI. I like the closed system because t doesn’t move much but I would like at least 4" to read 6" would be better. Just using my breath on a closed system I get about 1" movement. .But if I can buy it for the price above I would much rather do that. ---
**Author:** ![ZurBob](https://avatars.discourse-cdn.com/v4/letter/z/71e660/32.png) [@ZurBob](https://boards.straightdope.com/u/ZurBob)\ **Post date:** [March 11, 2025, 3:57am UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/17 "2025-03-11T03:57:08Z")
I’m a little unclear as to your application. However, keep in mind 1psi = 28" H2O. That is way higher than the max range of the manometer mentioned above by a factor of 2.3 so it may not be a good choice for you. What unit (psi or inches of water) do you wish to measure…and over what range? ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [March 11, 2025, 4:43am UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/18 "2025-03-11T04:43:08Z")
> [@ZurBob](#): > > What unit (psi or inches of water) do you wish to measure…and over what range When I was originally set up with an open hand unit to measure 2 lb per square inch which reflected at 56 in and that was just too much so I decided to build it like a differential gauge with a closed in and that cut it down below an inch. That might suggested just putting it on an incline and then I can stretch it out for six or eight inches probably using alcohol because the surface tension of water affects the reading too much, I also would like to measure the vacuum side when you’re sucking the air back in and that’s why I went to the closed loop because the water all gets stuck back into the bag ---
**Author:** ![ZurBob](https://avatars.discourse-cdn.com/v4/letter/z/71e660/32.png) [@ZurBob](https://boards.straightdope.com/u/ZurBob)\ **Post date:** [March 11, 2025, 5:56am UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/19 "2025-03-11T05:56:36Z")
> [@HoneyBadgerDC](#): > > I went to the closed loop because the water all gets stuck back into the bag Understood, but in your case, dead-ending either lead will give erroneous readings. Let’s shift gears…goto: [Amazon.com: YZTMETER 5/16 Inches Diameter Pressure Port Digital Manometer, YZT Meter Dual Port Air Pressure Meter Pressure Gauge HVAC Gas Tester, Differential Pressure Manometer : Industrial & Scientific](https://www.amazon.com/dp/B0C6V1Q3CK/ref=pe_386300_440135490_TE_simp_item_image?th=1) $32 will get you a digital meter with a 3psi/83"H2O range that may be what you need. It has a decent size illuminated screen that you could place right in front of you, while exercising and monitoring pressures. 1. Hook one lead to bellows discharge, let other lead hang in air = discharge pressure -or- 2. Hook one lead to bellows suction, let other lead hang in air = vacuum pressure -or- 3. (Perhaps most helpful) Hook one to suction, the other to discharge = differential pressure (D/P) for each repetition. **Be aware** if you exceed 3psi you’ll blow the guts out of this meter! ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [March 11, 2025, 12:55pm UTC](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221/20 "2025-03-11T12:55:40Z")
> [@ZurBob](#): > > **Be aware** if you exceed 3psi you’ll blow the guts out of this meter! It sounds like it would be in the perfect range I will check it out. [Next page](https://boards.straightdope.com/t/liquid-pressure-differential-gage/1015221.md?page=2)