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To check ASI performance, one can make a simple water manometer to safely pressurize the dynamic pressure port to see if the gage is operating within acceptable limits. The table to the left shows the calculations required by using the ideal gas law to come up with the water heights that correspond to the respective air speeds. One has to pick a base line air temperature and atmospheric pressure. Test conditions may not meet these picks but a plot of the measured data against the calculated data will show a constant offset or trend, that will lead to confidence in the ASI.
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The manometer was constructed out of small bore Tygon tubing, a "Y" connector for an automobile windshield washer hose, an animal hypodermic syringe, a tubing pinch off, a scrap piece of aluminum and electrian's tape, plus some water. A little food coloring helps too.
No bubbles allowed!
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The syringe plunger has been pressed in to pressurize the dynamic port. Here one can pick off the inches of water difference by referring to the yellow scale that is mounted on another water manometer that I use to measure pressures in flight. It looks like one inch for an indication of 45 mphi. Checking the chart above, it is a reasonable value.
If the pressure leaks down, it is an indication that there is an air leak between the probe and the ASI. |
Here is a view of the whole set up. One tube from the top of the "Y" goes to the air probe dynamic port. Another shorter tube is connected to the other leg of the "Y" and ends at the top of the aluminum. Electrician's tape was used to hold the tubing to the scrap aluminum channel. Another short tube connects the bottom of the "Y" to the syringe. A squeeze tubing clamp is placed over the tube in between the syringe and the "Y". This can be used to prevent the water pressure above the syringe from forcing the plunger back out of the syringe.
In my case I could slip the tubing inside the dynamic air probe and seal it with tape. If your ASI is not mounted you can make an adapter and bench test it. |
There are several options about how to measure the water column difference. A pair of calipers, a six inch machinist rule, or in this case a graphical plot.
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