Andy (and list) - I  found my notebook dealing with determining the speed of
my Hobie Cat via a pitot tube.  It seems that most of the information I used
came from SenSym  handbook Applications Note SSAN-7.  The basic formula is:
 delta P = p(v*v/2g)
 where:
  delta P is the difference in pressure between a reference pressure and the
pressure in the tube cause by the flow of water (or air)
  p is the fluid density in lbs/square inch (fresh water is 1g/cm cubed or
.036 lbs/inch cubed - sea water is 1.025 gm/cm3 or .037 lbs/in3)
  v is the velocity of the stream in inch/second
  g is the gravitational constant in inch/second/second (386.088
inch/sec/sec)

In playing what I found was that delta P plotted against velocity would give
what looked like a parabolic curve.  But if you took the square root of
delta P you would get a straight line.  I wrote a quick program that would
give me speed in knots for a given PSI, square root of PSI and slope of
square root PSI.

Since all I was interested in was delta P, I had chosen an absolute
pressure sensor that was one closed chamber and one open chamber.  The delta
P with the Hobie sitting in the water at rest was used as an offset for 0
knots.

SenSym Application Note SSAN-18 shows a low cost precision amplifier for one
of their pressure sensors.  It consist of 4 op amps and a few other
components.

One last comment for those who did not know - knots are used in sea and air
navigation.  It is speed in nautical miles per hour, where a nautical mile
is 6000 feet rather than the typical mile of 5280 feet.  The 6000 feet is
based on an equal division of longitude or latitude.  Gee - I wonder if
airline frequent flyer miles are based on nautical miles?  If so then we are
getting shorted!

Hope this helps.

David V. Fansler
Network Administrator
TriPath Imaging, Inc. (Formerly AutoCyte, Inc)
336-222-9707 Ext. 261
dfansler@autocyte.com <mailto:dfansler@autocyte.com>
Now Showing! www.dv-fansler.com <http://www.dv-fansler.com/>
Updated March 31,2000
Ann's Cancer, David's Observatory, Disney World



#include <stdio.h>
#include <math.h>
main()
{
        double sqrt();
        double a,b=0.0,c,d,e,f,v;
        double k=20.2537, g=386.088, p=.036;
        FILE *prtout;

                if(!(prtout = fopen("prn","w"))){
                    printf("\n Cannot open printer!\n");
                        exit(0);
                }
fprintf(prtout,"knots\tPSI\tsqrt(PSI)\tslope\t\t");
fprintf(prtout,"knots\tPSI\tsqrt(PSI)\tslope\n");
        for(v=0.1; v<40.0; v=v+.1){
            a=p*(((v*k)*(v*k))/(2*g));
            c=sqrt(a);
            fprintf(prtout," %3.1f\t%5.3f\t%5.4f\t\t%4.3f\t\t",v,a,c,c-b);
            b=c;
            v=v+.1;
            a=p*(((v*k)*(v*k))/(2*g));
            c=sqrt(a);
            fprintf(prtout," %3.1f\t%5.3f\t%5.4f\t\t%4.3f\n",v,a,c,c-b);
            b=c;

if((v<10.05&&v>9.95)||(v<20.05&&v>19.95)||(v<30.05&&v>29.95)){
                    fprintf(prtout,"%c",0x0c);
                    fprintf(prtout,"knots\tPSI\tsqrt(PSI)\tslope\t\t");
                    fprintf(prtout,"knots\tPSI\tsqrt(PSI)\tslope\n");
                }
        }
                fflush(prtout);
        exit(0);
}