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This article appeared in EAA Sport Aviation in August 2006

MANOMETER or HOW FAST ARE WE GOING?

Manometer Guage Built for Airspeed CalibrationsHave you ever wondered just how accurate your airspeed indicator is? Does your pitot system have more holes in it than a slice of Swiss cheese? Can you pressure test a fuel tank, without worrying about putting to much pressure in it. Your answer lies in a Manometer.

A Manometer gives you the ability to accurately measure very small pressures. The theory behind a Manometer is simple, how much pressure does it take to push a column of water straight up, through a tube.

We use a simple form of a manometer almost every day of our lives. Every time we sip a drink through a soda straw, we use differential air pressure to push a liquid up the straw. We’re going to do exactly the same thing here, except our straw is going to be much larger, and it has a gauge to indicate exactly how far the liquid moves.


THE SCALE:

The Scale can be printed from the file "Manometer.PDF".

Print or copy the Pressure Scale sheet. Use a tape measure to verify that the scale sheet didn’t shrink or expand in the printing process. The distance between the 0 and 19 inch marks should be exactly 9 ½ inches, This is critical for the accuracy of the Manometer.

Manometer Guage Built for Airspeed Calibrations Cut out the 0 – 19 inch scale and slide it under the Left leg of the "U-Tube". The "0" or "ZERO LEVEL" should be approximately in the center of the U-tube.

If you plan to use the Manometer to measure over 19 inches, then cut out the 19 – 38 scale. Tape it so the 19 is directly over the 19 on the 0 – 19 scale. Use a tape measure to verify that the full scale, 0 – 38, reads exactly 19 inches.

The scale has two sets of numbers, Inches on the left, and Knots on the right. Upon initial inspection, you may think that the scale is wrong, because each ½ inch is labeled as an Inch. The following demonstration will answer this question, plus provide you with a good idea of how small a pressure the Manometer is capable of measuring.


DEMONSTRATION: Fill the "U" tube to about the ½ point with water. Blow very gently into the one side of the "U" tube. As the pressure that your applying pushes the water level down on one side, the water level on the other side of the tube goes up an equal amount. If we measured the difference between the two sides, this is pressure that we’re exerting, in "inches of water".


CONSTRUCTION:

Follow the general layout shown in the diagram.

Most, if not all the parts can be purchased at a hardware or department store. Look in the plumbing and automotive departments.

         10 ft. of 3/8 inch Clear Vinyl tubing for the "U" tube
         15 ft. of 3/8 inch "Surgical" tubing for connecting to pitot systems
         1 "T" fittings for 3/8 inch tube
         FRAM G2 Gasoline Filter
         1 x 10 pine board 48 inches long
         Electrical staples for Romex wire. For securing the tube to the board
         Golf "T"s

The height of the "U" will depend on the pressure you want to measure. If the maximum pressure is 19 inches, then make each leg of the "U-Tube" at least 25 inches. If the maximum pressure is 38 inches, then make the legs at least 44 inches. This will give a 6 inch safety margin at the maximum pressures.

When securing the "U-Tube" to the board, Pay particular attention to the location of the staples. The spacing of the staples leaves enough room to allow the paper scale to be inserted between the tube and the board on the left side. The scale paper may also need to slide up and down to adjust the ZERO LEVEL

DO NOT omit the FRAM filter. This filter does two things. 1) It reduces the risk of accidentally getting water or other contaminants into the pitot system during testing. 2) It acts as a baffle or shock absorber, thus reducing pressure pulses going to the Pitot system.

The Pine board needs to be straight up and down. Drill a hole in the top and hang it from a nail.


FILLING THE MANOMETER

Manometer Guage Built for Airspeed CalibrationsPrepare 1 pint of water, preferably distilled, but soft tap water will work. Mix food coloring into the water. Make it as dark as you can, this makes the water level very easy to see. Mix 1 teaspoon of Dishwasher Softener into the water. This reduces the surface tension of the water so it doesn’t stick to the inside of the tube. Detergent will also work but you’ll have to allow the bubbles to disperse before you can make any measurements.

Stir the mixture, do not shake. If you shake, you’ll be waiting for hours for all the bubbles to settle

Fill the "U" tube with the color water mixture. The exact level isn’t critical. Fill so the level is about an inch above the middle staple, This will be the ZERO ADJUST point

The Pine board needs to be straight up and down. Drill a hole in the top and hang it from a nail.

When the bubbles settle and the two sides equalize, slide the paper scale up or down so the ZERO ADJUST point on the scale is lined up with the water level.


VERIFY OPERATION and TEST FOR LEAKS

Pinch off the tube that goes to the Pitot system by using a Golf "T", or some other plug.

Manometer Guage Built for Airspeed CalibrationsBlow gently into the AIR SUPPLY hose. You’ll see the water level change. Get a feel for how much pressure it takes to move the water.

Practice pinching the AIR SUPPLY hose to hold a pressure. Release the pinch just a little to generate a small leak. When testing an airspeed indicator, a leak of approximately 1 inch of water every 5 seconds provides enough time to compare the indicator with the Manometer scale.

Blow a pressure equal to about 120 knots, then pinch the AIR SUPPLY tube with your fingers to hold the pressure. Put a Golf "T" into the end of the AIR SUPPLY tube, then release your pinch. Once the level stabilizes, it should remain at this level, indefinitely. This method is ideal for testing pitot systems or fuel tanks for leaks.


PITOT LEAK TEST

Before getting serious about calibrating airspeed indicators, make sure the system is not leaking. Apply enough pressure to get about ½ of you red line airspeed, then pinch off the AIR SUPPLY line. Once the Manometer stabilizes, the water level should not change. If it does, you have a leak.


PITOT AIRSPEED CALIBRATION

NOTE – Airspeed indicators respond to pressure changes a lot fast than the Manometer. When blowing into the AIR SUPPLY, do so gently so as not to cause sudden, jerky movement on the Airspeed indicator. The FRAM G2 filter will provide some protection, but the airspeed indicator can be permanently damaged by sudden pressure changes. DO NOT peg the Airspeed needle.

Verify that the water level is at the ZERO LEVEL on the paper scale.

Gently blow into the AIR SUPPLY while watching the Airspeed Gauge. DO NOT peg the airspeed indicator.

When the airspeed indicator is at its maximum reading, pinch the AIR SUPPLY tube. Once the Manometer stabilizes, the water level should not change, there should be no leaks.

Reduce the pinch on the AIR SUPPLY to generate a constant 1 inch every five seconds. The Airspeed indicator should read the same as the airspeed on the Manometer gauge. The Airspeed / Pressure table is provided for your convince.

You may need to gently tap on the airspeed indicator to get it to move, this is normal.


LEAK CHECK FUEL TANK

Cap off all known ports in the fuel tank. Pressurize the tank in the same manner as the PITOT LEAK TEST, but DO NOT EXCEED 100 knots. The tank should hold pressure indefinitely.


TEST FOR VACUUM

The Manometer can just as easily test for Vacuum. Move the paper scale to the right leg of the "U", and adjust the "0" ZERO ADJUST to the water level. Instead of blowing into the AIR SUPPLY, sucking will generate a small vacuum and "pull" the water column up.

Connect the Manometer to the Static system. While watching the Paper Scale, Suck on the AIR SUPPLY tube to pull the water column to approximately 35 inches. Pinch off the AIR SUPPLY. Once the Manometer stabilizes, the water level should not change, there should be no leaks.

35 inches of Water (vacuum) represents approximately 2,500 ft altitude



Knots

In H2O

PSI

Knots

In H2O

PSI

0

0.0000

0.0000

230

35.4855

1.2819

10

0.0180

0.0025

240

38.7350

1.3993

20

0.2583

0.0100

250

42.1476

1.5226

30

0.5846

0.0210

260

45.7234

1.6518

40

1.0469

0.0380

270

49.4486

1.7863

50

1.6315

0.0590

280

53.3507

1.9273

60

2.3521

0.0850

290

57.4159

2.0742

70

3.1590

0.1154

300

61.6442

2.2269

80

4.1740

0.1508

310

66.0493

2.3861

90

5.3024

0.1916

320

70.6312

2.5516

100

6.5397

0.2362

330

75.3898

2.7235

110

7.9265

0.2863

340

80.3387

2.9023

120

9.4492

0.3414

350

85.4644

3.0874

130

11.1079

0.4013

360

90.7804

3.2795

140

12.9026

0.4661

370

96.2868

3.4784

150

14.8332

0.5359

380

101.9972

3.6847

160

16.8998

0.6105

390

107.8979

3.8978

170

19.1159

0.6906

400

114.0025

4.1184

180

21.4816

0.7760

410

120.3246

4.3468

190

23.9833

0.8664

420

126.8507

4.5825

200

26.6345

0.9622

430

133.5943

4.8261

210

29.4353

1.0634

440

140.5554

5.0776

220

32.3856

1.1699

450

147.7613

5.3379






























































































































































































































































































































































































































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