Water Pressure Calculation with atmospheric pressure corrections

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Discussion Overview

The discussion revolves around the impact of atmospheric pressure and altitude on water pressure measurements in a testing scenario involving pressure washers. Participants explore the relationship between flow rate, orifice size, and environmental conditions, particularly focusing on two laboratories located at different altitudes.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether altitude differences between two testing laboratories could affect pressure readings, given identical flow rates and orifice sizes.
  • Another participant suggests that flow rate through an orifice depends on the pressure difference rather than absolute pressure, implying altitude may not be a significant factor unless it alters the pressure difference.
  • Concerns are raised regarding other variables that could influence results, such as the state of the liquid, upstream and downstream geometry, and measurement locations relative to the orifice.
  • One participant notes that differences in experimental setups may be introducing errors and recommends comparing setups to industry standards for orifice measurement.
  • There is a request for more information about the flow rate and the source of water, with a suggestion that variations in pump performance due to head pressure could affect flow.
  • Another participant emphasizes the importance of consistent measurement practices and instrumentation to avoid discrepancies in results.

Areas of Agreement / Disagreement

Participants express differing views on the significance of atmospheric pressure and altitude in the context of their experiments. While some suggest that these factors may not be critical, others point out potential experimental errors and the need for further investigation into the setups used in both laboratories. The discussion remains unresolved regarding the exact causes of the observed differences in results.

Contextual Notes

Participants note that the accuracy of flow measurements can be influenced by various factors, including the choice of instrumentation, the configuration of the fluid system, and the specific conditions under which tests are conducted. There are also mentions of potential measurement errors due to the positioning of pressure readings.

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

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