Gents, for who is interested,

I do have some communication about the airspeed measurement and of course the question, how to calibrate the Taurus airspeed transmitter?
More inportant, what is the pressure level of a Prandtl tube?

When we look for values of dynamic pressure, the pressure of a Prandtl tube, we find for example in Bennewitz Flugzeuginstrumente (1921), the values of picture 1. It is a part of a “Staudrucktabelle”. These values does not change, only the Barometric pressure can be some different. We also can calculate the values.

We see the dynamic pressure of an airspeed of 50 km/h is 12,3 mmws (Millimeter Wassersäule) is mm water column, and that is about ½ inch water culumn.
Measuring the important stall airspeed of the Taurus isn’t easy. these pressure levels are lower than ½ inch wc.

To calibrate the airspeed transmitters of the Taurusses I use a Thommen (Switzerland) Micro pressure gauge (age about 40 years, so classic pressure indicator) . Max range of this pressure indicator is 10 mm water column and has a scale to read in tenths of mm. Dimensions front is 95 x 95 mm.
10 mmwc (so max scale) is generated at 45,1 km/h.
Lowest value to check the transmitter of the Taurus has to be about a half of this speed.
The dynamic pressure of a speed of 22,5 km/h is a quarter of the 10 mmwc is 2,5 mmwc generated at a speed of 13,7 mph!

Of course the Thommen is calibrated too (with a German Askania Minimeter).

Speeds above 45 km/h I check with a U tube water gauge, see again values picture 1 red square, flight level 0 km.
There are some tricks to couple the instruments and “generate” these low pressures by hand with a rubber hose.
The intrument indications are compared on a few speed levels, for example 20 -50 - 80 % (is 4 - 25 - 64 % of pressure level of max speed pressure)
Deviations are noted and can be used to correct measurements after flights.

Cees.