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This article appeared in EAA Sport Aviation in August 2006
MANOMETER or HOW FAST ARE WE GOING?
 Have 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.
 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.
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10 ft. of 3/8 inch Clear Vinyl tubing for the "U" tube |
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15 ft. of 3/8 inch "Surgical" tubing for connecting to pitot systems |
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1 "T" fittings for 3/8 inch tube |
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FRAM G2 Gasoline Filter |
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1 x 10 pine board 48 inches long |
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Electrical staples for Romex wire. For securing the tube to the board |
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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
 Prepare 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.
 Blow 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
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Knots |
In H2O |
PSI |
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Knots |
In H2O |
PSI |
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0 |
0.0000 |
0.0000 |
|
230 |
35.4855 |
1.2819 |
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10 |
0.0180 |
0.0025 |
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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 |
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50 |
1.6315 |
0.0590 |
|
280 |
53.3507 |
1.9273 |
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60 |
2.3521 |
0.0850 |
|
290 |
57.4159 |
2.0742 |
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70 |
3.1590 |
0.1154 |
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300 |
61.6442 |
2.2269 |
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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 |
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350 |
85.4644 |
3.0874 |
|
130 |
11.1079 |
0.4013 |
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360 |
90.7804 |
3.2795 |
|
140 |
12.9026 |
0.4661 |
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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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