I found a website that sort of described this, but still left some to the imagination. It also had a bunch of calculations that would make most mere mortals shrink into the corner, but since I have a minor in math, I only wasted 2 hours trying to make the numbers work. I eventually gave up and quickly found a couple of web resources that simply have a chart that gets "close enough for KR work". This picture tells the story. Clamp or tape some clear flexible tubing (1/4" up to 1/2") to the wall or a board or something, as long as it's vertical. This particular board is the framing for my garage door. Start by filling up the "U" with colored water, but NOT with the ASI connected, so you'll end up with a level "U". It doesn't matter what the bottom of the U looks like, as long as the tubing where you start measuring is vertical.

Now hook up the ASI, and start adding colored water (slowly!) with something like this ear washer deal, and record the level of the miniscus of both columns of water by marking it, and labeling it something like "40" for 40 mph or knots, or whatever. Add more water 'til you get to 60, and mark them both again, repeating for as far as you want to go, or have enough wall. 48" will go 250 mph. I was only checking the low end, so 18" was plenty for me. Then measure the distance between each pair of points to get a "delta", and use that delta in the chart below to see how far and in what range your ASI is off.

I measured the 100 mph mark and got 9.5", rather than 4.94", so I'm off by 52%! I realize that the end of the tape is not exactly at the 100 mph mark, but it was close enough to determine that this UMA ASI is junk! I'd try to return it, but 7 years since purchased is certainly past the statute of limitations.

Check your results with the chart below (plagiarized from the web)

knots/mph  inches
17.4/20          0.20
26/30             0.44
34.8/40          0.79
43.5/50          1.23
50/57.5          1.63
69.6/80          3.16
70/80             3.20
86.9/100        4.94
100/115         6.56
104.3/120      7.13
110/126.5      7.95
120/138         9.48
130/149.5      11.14
130/150         11.18
150/172.5      14.87
173.9/200      20.04
200/230         26.71
217.4/250     31.63

The first column is the airspeed in knots/MPH, the second column is inches of water difference. This also works fine without removing the airspeed indicator. Just hook the tubing to your pitot tube and proceed. If your pitot system leaks it will show you, but then you can't calibrate the ASI until you fix the leak.

Contact Mark Langford (if you must) at N56ML "at" hiwaay.net (replace the "at" with @)

Return to Mark Langford's KR2S N56ML.