How to Measure Very Small Differential Pressure in a Subsonic Wind Tunnel

In summary: You may need to have your manometer calibrated periodically to ensure it is still accurate. Additionally, you can use a pressure standard to ensure the accuracy of your manometer and make adjustments if necessary.
  • #1
agnimusayoti
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TL;DR Summary
Currently I conduct an education tools for my university project. I build an simple subsonic-open circuit wind tunnel. I want to measure differential pressure in object inside the wind tunnel which under influence of low speed wind.
I assume that air have ##1 kg/m^3## density.
Therefore, using Bernoulli equation, on upside and downside of my test object, there is a differential pressure ##\Delta P##:
$$\Delta P=0.5*(v_2^2 - v_1^2)$$
From cases:
(a) ##v_1 = 1 m/s## and ##v_2 = 2 m/s##, then ##\Delta P = 1,5 Pa##
(b) ##v_1 = 10 m/s## and ##v_2 = 20 m/s##, then ##\Delta P=150 Pa##
I conclude that I need an instrument which can measure differential pressure from 0 up to 200 Pa with 0,1 Pa accuracy.

Could you suggest an instrumentation system that can cover my tasks? The constraints are in low production cost and availability sensors or tools in my country.

[Mentor Note -- Poster's Profile Page lists Indonesia as his country]
 
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  • #2
Yes, I am from Indonesia. Thanks dear mentor.
 
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  • #3
agnimusayoti said:
Thanks dear mentor.
We need more posts like this. Lol. Mentor appreciation.
 
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  • #4
Search 'digital manometer' and you will find; https://www.ebay.com.au/itm/283893957464

But you are asking for something with about 100 times better resolution.
200 Pa ± 0.1 Pa
0.002 bar ± 0.000001 bar
2 mbar ± 0.001 mbar

You will probably have to make a symmetrical differential sensor for the job.
I expect it will need a large diaphragm with an electrical or optical measuring system.
 
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  • #5
You could get close to your desired accuracy with an inclined tube water manometer. Dwyer Instruments has a good discussion of these: https://www.dwyer-inst.com/ApplicationGuides/?ID=11. This diagram is from that link:
Manometer.jpg

200 Pa is about 20 mm of water column, so 0.1 Pa would be 0.01 mm water. Multiply the water displacement by 10 for a tube inclined at 5.7 degrees. Note that the diagram is slightly misleading - you need to measure the water level change horizontally, not vertically. Measuring to 0.1 Pa is unrealistic, but you might be able to measure to 0.5 Pa. And the possibility of a good lecture about how measurement accuracy sets the minimum velocity in the wind tunnel.
 
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  • #6
BME280 has a resolution around 0.2 Pa - while not ideal for your needs it is very cheap and easy to buy.
 
  • #7
BME280 has operation range of 300 to 1100 hPa, which is not compatible for my need.

Are there any sensors that can measure small differential pressure? Or should I use manometer for my instrument?
 
  • #8
jrmichler said:
You could get close to your desired accuracy with an inclined tube water manometer. Dwyer Instruments has a good discussion of these: https://www.dwyer-inst.com/ApplicationGuides/?ID=11. This diagram is from that link:
View attachment 285917
200 Pa is about 20 mm of water column, so 0.1 Pa would be 0.01 mm water. Multiply the water displacement by 10 for a tube inclined at 5.7 degrees. Note that the diagram is slightly misleading - you need to measure the water level change horizontally, not vertically. Measuring to 0.1 Pa is unrealistic, but you might be able to measure to 0.5 Pa. And the possibility of a good lecture about how measurement accuracy sets the minimum velocity in the wind tunnel.
If I choose alcohol and 5 degree inclination, so I need 0,15 mm water displacement. Here is my calculation:
$$P_A = (\rho)(g)(d sin \theta) + P_B$$
So, differential pressure between leftside and right side is:
$$\Delta P = P_A - P_B = (\rho) (g) (d sin\theta) $$

For alcohol, the density is ##\rho = 789 kg/m^3## and for ##\theta = 5\deg##. Using ##g = 9,81 m/s^2##I get:
$$d = \frac{P_A - P_B}{674.59 kg/(m^2 s^2)}$$

Is my model true?

But, How can I measure 0.15 mm water displacement? Could I use camera analysis?
 
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  • #9
agnimusayoti said:
How can I measure 0.15 mm water displacement?
Very carefully. Try putting a mirror behind the manometer tube, try using a magnifying glass, and experiment.

Another possibility is a low range differential pressure transducer. I found these by using the Digikey (https://www.digikey.com/) search function:

Differential pressure as low as +/- 0.5 inches water full scale: https://www.allsensors.com/datasheets/DS-0376_Rev_A.pdf. These have digital outputs.

This one has analog output at 1.0 inches water full scale: https://www.sensata.com/sites/defau...ferential pressure mount sensor-datasheet.pdf

Other distributors include Mouser and Newark. I have had good experiences with all three of these distributors. Digikey lists shipping times to the Philippines, so they should be able to ship to Indonesia.
 
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  • #10
It's really all a matter of what you are willing and able to spend. There are plenty of pressure transducers that can measure pressures and pressure differences that low. They just aren't generally the cheapest devices on the market. You can also get an analog transducer that can be amplified provided your background noise isn't too high.
 
  • #11
Well, I decided to try the inclined manometer for my instrument. Do I need to calibrate the manometer? I mean, how I believe my instrument measure air pressure accurately?
 
  • #12
Manometers are inherently accurate, so can be used to calibrate mechanical pressure gauges and electronic pressure sensors. An inclined manometer is as accurate as the sources of error:

Gravity
Fluid density
Inclination angle
Your measurements

At low pressures, measurement error will be the largest error.
 
  • #13
Okayy Thanks all.
 
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  • #14
agnimusayoti said:
how I believe my instrument measure air pressure accurately?
You could perhaps get an idea of the accuracy of gauge pressure measurements with your inclined manometer by connecting it to a rigid reservoir of air and connecting that to a small syringe. By making small volume changes of the air in the syringe, and knowing the total volume of air in the reservoir and syringe (and any connecting tubing), you should be able to predict what the change in gauge pressure should be. You could use some quick calculations to see how small you can make the gauge pressure changes in order to figure out if this will be a valuable test for you.

1626620634274.png

https://www.perkinelmer.com/product/250-ul-syringe-kit-w-250-ul-buffer-tbg-n2936052
 
  • #15
Flow50_1.jpg

Guys, after a few trial, I decided to use simple U tube manometer as an inclined manometer. I used oil as working fluid which has ##\rho = 890 kg/m^3##. The angle between oil column and horizontal approximately as large as 4 degree. Theoretically, the oil surface will drop because P outside (connected to column under) is bigger than P inside wind tunnel. But, I didn't observe any oil surface movements. What do you think I can do to improve this simple instrument? Thanks.
 
  • #16
I placed my manometer under the wind tunnel. Does the location of manometer from propeller affect the measurement?
 
  • #17
agnimusayoti said:
What do you think I can do to improve this simple instrument? Thanks.
Stepwise refinements.

In the post #15, upper picture, it looks to me like you have a bubble in your liquid column. The column must be bubble free in the loop.

You have mounted the 'U' loop on the vertical plane. I would have mounted it on a horizontal surface, then tilted the surface by about 4°.

I'm not sure of your material, if the tube was hard and straight, like acrylic or glass tube, there would be less wandering error in the reading scale.

If you can use a fitting to reduce the radius of curvature, the two legs can be closer together, that will reduce the reading error. Place the tubes right next to each other, not separated by the width of the scale.

Rather than a thick clear oil, consider using an alcohol or kerosene that will flow faster and accept a strong coloured food dye.
 
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  • #18
Which bubble do you mean? Does it located beside the 9 scale?

My U manometer is made from glass, like another standard laboratory equipment. Therefore the two legs is stiff and can't be moved closer to each other.

Well then, I will try to place the manometer on horizontal plane and tilt it 4 degree.
 
  • #19
How if I use two open capilary pipes, connected with sillicon hose? Is it same with U manometer?
 
  • #20
agnimusayoti said:
Which bubble do you mean? Does it located beside the 9 scale?
Yes, there is liquid between 9.0 and 9.6; The bubble is between that and the main liquid.
A manometer is not calibrated if there are bubbles in the liquid.

agnimusayoti said:
How if I use two open capilary pipes, connected with sillicon hose? Is it same with U manometer?
Yes. If you use two tubes they can be very close together and can be joined at the bend. That will reduce reading errors.
 
  • #21
Uh I see. Well I'll try to eliminate the buble first. first I think at that column, fluid's surface must be located above the joint beside the 9 scale. Problem is the column length is not that long to cover fluid's height.

But if can't, I will try to use to open capilary pipes. Thankss.
 
  • #22
Baluncore said:
Yes, there is liquid between 9.0 and 9.6; The bubble is between that and the main liquid.
A manometer is not calibrated if there are bubbles in the liquid.Yes. If you use two tubes they can be very close together and can be joined at the bend. That will reduce reading errors.
One more, I think if the manometer is on horizontal plane, there will be a difficulty to take the picture. What should I do if the manometer is still on vertikal plane?
 
  • #23
agnimusayoti said:
What should I do if the manometer is still on vertikal plane?
Experiment with many alternatives.
You will have several manometers all in the same picture.
Keep one manometer disconnected, so it can act as a level reference and control.
 
  • #24
How many manometer do I need? 3? 2 manometer connected to the object and 1 as reference?
 
  • #25
The closer you can get the camera to the tubes, the better the pixel and pressure resolution.
That suggests all tubes should be close together. The best way to do that is to have camera axis close to vertical with all tubes parallel, on a sloping surface.

You must have some way to identify the horizontal and zero pressure in the picture. Place the two reference manometer tubes in the outer positions to give a good estimate of cross-slope.

How many cameras will you use?
How will you make measurements from the pictures?
Will you count pixels?
 
  • #26
Is this incline U manometer or simply U manometer?
 

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  • #27
Baluncore said:
The closer you can get the camera to the tubes, the better the pixel and pressure resolution.
That suggests all tubes should be close together. The best way to do that is to have camera axis close to vertical with all tubes parallel, on a sloping surface.

You must have some way to identify the horizontal and zero pressure in the picture. Place the two reference manometer tubes in the outer positions to give a good estimate of cross-slope.

How many cameras will you use?
How will you make measurements from the pictures?
Will you count pixels?
i think I just use 1 camera (phone's camera) and using picture analyzer like Tracker.

What is sloping surface? What is cross slope? Sorry I don't understand.
 
  • #28
agnimusayoti said:
What is sloping surface?
An inclined plane or a ramp. https://en.wikipedia.org/wiki/Inclined_plane

agnimusayoti said:
What is cross slope?
When the manometer is placed at an angle on the inclined plane.
 
  • #29
Baluncore said:
The closer you can get the camera to the tubes, the better the pixel and pressure resolution.
That suggests all tubes should be close together. The best way to do that is to have camera axis close to vertical with all tubes parallel, on a sloping surface.

You must have some way to identify the horizontal and zero pressure in the picture. Place the two reference manometer tubes in the outer positions to give a good estimate of cross-slope.

How many cameras will you use?
How will you make measurements from the pictures?
Will you count pixels?
Ohh i see. So, if I use a ruler beside the column and the ruler is parallel with horizontal line, can I use the vertical one? Then I need to place two manometer close each other?

I prefer vertical one because I still don't understand the hydrostatics prinsipal in the ramp manometer. Do you have any idea about this? Or literature.. thanks again
 
  • #30
agnimusayoti said:
I prefer vertical one because I still don't understand the hydrostatics prinsipal in the ramp manometer. Do you have any idea about this? Or literature.. thanks again
You must search 'manometer' and 'hydrostatics'.
Until you understand what you are doing and the principles of how it works, you will be unable to measure small differences of pressure.
 

FAQ: How to Measure Very Small Differential Pressure in a Subsonic Wind Tunnel

How do you measure very small differential pressure in a subsonic wind tunnel?

To measure very small differential pressure in a subsonic wind tunnel, specialized instruments called pressure transducers are used. These transducers are designed to accurately measure small changes in pressure and convert them into an electrical signal that can be read and recorded by a computer or data acquisition system.

What is the range of differential pressure that can be measured in a subsonic wind tunnel?

The range of differential pressure that can be measured in a subsonic wind tunnel depends on the specific pressure transducer being used. However, most modern pressure transducers have a range of 0-5 psi, which is sufficient for measuring small changes in pressure in a subsonic wind tunnel.

What factors can affect the accuracy of differential pressure measurements in a subsonic wind tunnel?

Several factors can affect the accuracy of differential pressure measurements in a subsonic wind tunnel, including the calibration of the pressure transducer, the air flow conditions in the wind tunnel, and any external sources of pressure or interference.

How often should the pressure transducer be calibrated for accurate measurements?

The pressure transducer should be calibrated regularly, ideally before each use, to ensure accurate measurements. The calibration process involves comparing the transducer's readings to a known standard and making any necessary adjustments to ensure accuracy.

Can differential pressure measurements be affected by the shape or design of the wind tunnel?

Yes, the shape and design of the wind tunnel can affect differential pressure measurements. This is because the air flow patterns and turbulence within the wind tunnel can create variations in pressure that may impact the accuracy of the measurements. It is important to carefully design and calibrate the wind tunnel to minimize these effects.

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