# Water Pressure Calculation with atmospheric pressure corrections

1. Jul 17, 2009

### jfowler

I am looking for a formula to let me obtain a pressure reading comparison for two different tests based upon the following information:

I have a given flow rate in liters / minute. (same for each test)
I know the size of the orifice that the water is flowing through. (same for each test)

My question is the following. Would there be a pressure difference between the two tests due to the location of the testing laboratory. One lab is very high above sea level (I can get the exact information) and the other lab is almost at sea level. I realize that this is probably a very ignorant question, but I am a biologist trying to fit into a physicists world.

2. Jul 17, 2009

### Q_Goest

Hi jfowler, welcome to the board. The flow rate of water through an orifice is a function of the pressure difference across the orifice, not the absolute pressure, so a difference in altitude won't change anything unless there's a change in pressure difference. That said, there could be other variables that will affect it.
- State of the liquid (ie: absolute pressure doesn't matter unless the water is close to boiling or temperature changes such that density changes are significant.)
- The geometry of any fluid conduit upstream or downstream of the orifice must also be identical, including where the pressure is being measured.
- There is a spot immediately downstream of your orifice you don't want to try and take a measurement from because that's where the flow stream contracts before expanding again and becoming steady. As a rule of thumb, about 10 pipe diameters downstream should be sufficient. Also, measuring less than about 5 pipe diameters upstream of the orifice could skew results.

3. Jul 17, 2009

### jfowler

Thank you for the quick response. A little more information about the tests. These tests are performed using identical pressure washer units (lab units, not your everyday pressure washer) to check for surface coating adhesion. Our lab here in the US will test a sample and get different results than our overseas laboratory testing the exact same sample sent to them after our test is complete. The test is performed at 60 degrees celcius in a cabinet washer that is identical as far as the nozzle and supply tubing to the nozzle for at least the last 15 feet. The atmoshpheric pressure questions was just raised so I thought that I would ask. Thanks again for the quick response.

4. Jul 17, 2009

### FredGarvin

I agree with Q. It sounds like, to me, that there are differences in experimental set up that are inducing errors. There are specifications that spell out requirements for certain levels of flow measurement accuracy. I would suggest looking into the set ups in both locations and comparing to what is accepted in industry for proper orifice measurement.

I would also look at the choice of instrumentation as well. I can think of a few transducer/instrumentation gaffes that can cause varying results.

By the way, just how far apart are the results?

5. Jul 17, 2009

### jfowler

The results are very different. The units as I said are identical from the same supplier in europe. The water flow is obtained by actually collecting the water for 60 seconds (actual test time) and measuring it using certified glassware, so there is no mechanical room for error in the amount of flow. The tubing inside the machine is the same leading up to the nozzle.

6. Jul 17, 2009

### Staff: Mentor

Where is the water coming from?

There are some scenarios where atmospheric pressure would matter, ie if the water was supplied by a pressurized vessel and was released to atmosphere.

7. Jul 17, 2009

### Q_Goest

Can you provide a bit more information on the flow rate? What is causing the flow, a pump or pressurized vessel? Pumps for example, typically vary in flow depending on the head pressure they have to produce, so if the system they are in has more restriction, the flow will decrease.

Are there any differences in other parts of the system such as the tubing? Different lengths of tubing, different valves, elbows, etc.. are all restrictions that create a pressure drop.

8. Jul 18, 2009

### FredGarvin

You have to be setting up the flow with some form of pressure setting/readings. Where are these being taken?