The table of ASI reading vs water manometer height in the "How Fast is Fast" article in the April 2007 issue has errors. The errors are small at low speed, but there is about a 4 kt error at the highest speeds in the table. I'm not sure where those numbers came from.

The CAS for a given pressure is equal to asl*SQRT(5*((P/Psl+1)^(2/7)-1)) [reference test pilot school notes].

where:
asl = speed of sound at sea level (661.48 kt or 761.22 mph)
Psl = atmospheric pressure at sea level (407.513" H2O at 20 deg C, or 10350.8 mm H2O)

The above values are from NASA Reference Paper 1046, Measurement of Aircraft Speed and Altitude, Tables A19 and A27.

Corrected ASI vs water manometer height:

Speed in. mm | Speed in. mm
(mph) H2O H2O | (kt) H2O H2O
15 0.11 2.8 | 15 0.15 3.7
20 0.20 5.0 | 20 0.26 6.6
25 0.31 7.8 | 25 0.41 10.4
30 0.44 11.3 | 30 0.59 14.9
35 0.60 15.3 | 35 0.80 20.3
40 0.79 20.0 | 40 1.04 26.5
45 1.00 25.3 | 45 1.32 33.6
50 1.23 31.3 | 50 1.63 41.5
55 1.49 37.9 | 55 1.98 50.2
60 1.77 45.1 | 60 2.35 59.7
65 2.08 52.9 | 65 2.76 70.1
70 2.42 61.4 | 70 3.20 81.4
75 2.78 70.5 | 75 3.68 93.4
80 3.16 80.2 | 80 4.19 106.4
85 3.57 90.6 | 85 4.73 120.1
90 4.00 101.6 | 90 5.31 134.8
95 4.46 113.3 | 95 5.91 150.2
100 4.94 125.6 | 100 6.56 166.5
105 5.45 138.5 | 105 7.23 183.7
110 5.99 152.1 | 110 7.94 201.8
115 6.55 166.3 | 115 8.69 220.7
120 7.13 181.2 | 120 9.47 240.4
125 7.74 196.7 | 125 10.28 261.1
130 8.38 212.9 | 130 11.12 282.6
135 9.04 229.7 | 135 12.01 304.9
140 9.73 247.2 | 140 12.92 328.2
145 10.44 265.3 | 145 13.87 352.4
150 11.18 284.1 | 150 14.86 377.4
155 11.95 303.5 | 155 15.88 403.3
160 12.74 323.7 | 160 16.94 430.2
170 14.41 365.9 | 170 19.15 486.5
180 16.17 410.8 | 180 21.52 546.5
190 18.05 458.5 | 190 24.02 610.2
200 20.03 508.9 | 200 26.68 677.6
210 22.13 562.0 | 210 29.48 748.8
220 24.33 617.9 | 220 32.44 823.9
230 26.64 676.7 | 230 35.54 902.8
240 29.07 738.3 | 240 38.80 985.6
250 31.61 802.8 | 250 42.22 1072.4

As a sanity check on the above calculated values, NASA Reference Paper 1046 has tables of pressure vs ASI readings, with the pressure in units of mm Hg. At 250 kt, table A23 has a pressure of 78.744 in Hg. At 20 deg C, 1 mm Hg is equal to 0.53577 " H2O. So, 250 kt would be 42.19" of H2O. This is within 0.1 kt of the values I have provided above. NASA Reference Paper 1046 is available for free download from NASA.

http://hdl.handle.net/2060/19800015804 (14 MB PDF file)

I have created Excel and OpenOffice spreadsheets that will convert back and forth between ASI reading and water manometer height. They are available at:

http://www.kilohotel.com/rv8/filemgmt/visit.php?lid=3