volume versus pressure 1

[ColinScowen]

We have a vacuum device that is specified in l/min from our design centre, but our line testing unit is measuring in mmH2O.

Is there a conversion between these two units?

We are seeing drift in the pressure measurements that give the impression that the units are out of spec on the low side, however, if I send some sample failed units to the design centre, the l/min measurements are all in spec.

The pressure measurement system is using differential pressure. The voltage and speed of the motors has been checked and these are in spec.

What, if any, outside influences could be causing the variation?

Colin.

(The design centre's response to this problem was, 'buy a different tester that measures l/min', which, while it is a solution, isn't likely to happen unless the current tester explodes.)

 

[IFRs]

liters per minute is a flow rate (length cubed per time)while mmH2O is a pressure (height of a fluid column). They are not directly convertable without some more information.

 

[ColinScowen]

What further information is required?

Colin

 

[katmar]

The operating point of a pumping device is specified by giving a flowrate and a pressure. If the flowrate varies then so does the pressure. There are an infinite number of combinations of flowrate and pressure, and taken all together they make up the pump curve. If you have the pump curve you can read off either the flowrate or the pressure if you are given the other one.

It is very similar to specifying where you are on earth by giving an Easting and a Northing co-ordinate. What your design centre is doing is giving you only the Easting, which means you could be anywhere between the North Pole and the South Pole on that line of longitude. Not very helpful if you are trying to plan your wardrobe.

They must specify the pressure that goes with the l/min value they have given you, and you need to measure both to be sure that you are achieving the desired performance.


Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[ColinScowen]

I guess I could approximate the pump curve from the data I get from the test units I sent to the design centre (l/min) which I will test on our tester (mm H2O), and the upper and lower limits.

Any thoughts on possible external influences on the pressure readings. The sensor assembly is an Omega PX275-05D1, with the readings being taken off the low side of the differential sensor.

Colin.

 

[budt]

You don't say what work you are trying to perform with the vacuum.

If there are differing flow rates then pressure will fluctuate downward each time flow is required and will increase to normal when the flow requirement ceases.

An Accumulator, (Vacuum Chamber, the bigger the better)in the line can reduce the pressure fluctuations but will not eliminate them. Locate the Accumulator as close as poossible to the reason for the fluctuating flow.


Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us

 

[Ataloss]

Are you supposed to be measuring pressure or leakage? The design center's response of "buy a different tester that measures l/min" sounds like you are supposed be measure leakage while you're just measuring the pressure. You do see the difference don't you? Imagine you've blown up a balloon. The air inside is at a certain pressure. Now imagine the balloon has a small leak. The air inside is still at a certain pressure, but as air leaks out the pressure will drop. The design center is measuring how much air leaks out in a certain amount of time while you're just measuring the pressure inside the balloon at different times. This is why you're seeing a drift in pressure readings, the air is leaking out (or in this case it's leaking in since it's a vacuum) so your pressure is changing as more air gets in. Find out exactly what you're supposed to be measuring then get test equipment that will measure it properly.

 

[Ataloss]

I forgot to add:
This is assuming that the design center has all the specs and knows what needs to be tested and what the specs are. Having worked in several design centers, I know this is not always the case. If the design center is measuring leak rate but is supposed to be measuring pressure, tell them to buy a new tester.

 

[ColinScowen]

It's a dust extraction system for power tools. Basically a vacuum cleaner that you stick on your drill.

In the tester, we connect the drill bit end of the unit, via some tubing, to the low side of the sensor. We then run the unit, this sucks air from the drill end, through a filter, then out the side of the unit.
The test cycle goes as follows.
Run units for 10 seconds, measuring pressure and amps. If pressure is too low, unit is rejected. Repeat 5 times.

Uses a dc motor rotating at 21 - 24 k rpm, to spin a fan, to get the airflow. We buy the motors in, and they are already run in.

Colin.

 

[TheTick]

You need to calibrate your tester to relate pressure to flow.

Ideally, measure differential pressure across a nozzle with known characteristics. Make the nozzle part of your test appartatus.

 

[notnats]

I think you will need to know the pressure drop across the filter when it has collected the required amount of dust, measured with your water gauge across the filter, and the flow rate required to keep on picking up dust as the filter becomes loaded.

Then set up a test rig with 2 orifices, one to simulate the loaded filter, and one with the water gauge connected across it to measure the flow rate as described by The Tick.

Jeff

 

[Chris0164]

Colin,

flow and speed are related to each other by the formula of Bernouilli which sounds

P = Rho*g*h + 1/2*Rho*v²

Here the speed component is v²/2g if you devide all the terms by Rho*g and gives you meters H2O

Success



Kind Regard,

Chris

 

[panduru]

Thanks Chris, for the valuable post. That's the right solution, I am sure.

Hope ColinScowen has his problem resolved.

 

[Chris0164]

Thanks Panduru,

always feels good helping brothers in arms

Kind Regard,

Chris