Post by thread:[OT} Pressure to MPH

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 spam_OUTdfanslerTakeThisOuTautocyte.com <.....dfanslerKILLspam@spam@autocyte.com> Now Showing! http://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); }

Just got this in from my brother the physics teacher. Thought it might be helpful to pass on. Andy Andy, Thanks for getting this to me. I was just getting out my thermo book. Potential problem here though. Go to the line in the note from the other guy where is says p is the fluid density in lbs/square inch (fresh water is 1g/cm cubed or little p, (actually the Greek letter rho), is density and must be lbs/cubic inch since square inch is surface area not volume Also, 6000 feet is NOT one nautical mile. That's the useful approximation they teach you in airplane and marine navigation. There are a couple of exact figures given in the Handbook of Chemistry and Physics. One nautical mile (British) is 6080 feet. In US feet its 6076.1. Only about 1% but you might as well program it as accurately as possible. Mike "Fansler, David" <DFANSLERKILLspamAUTOCYTE.COM> on 05/11/2000 08:04:20 AM Please respond to pic microcontroller discussion list <.....PICLISTKILLspam.....MITVMA.MIT.EDU> To: EraseMEPICLISTspam_OUTTakeThisOuTMITVMA.MIT.EDU cc: (bcc: Andrew Kunz/TDI_NOTES) Subject: [OT} Pressure to MPH 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 dfanslerspam_OUTautocyte.com <@spam@dfanslerKILLspamautocyte.com> Now Showing! http://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); }

Well - I guess "the other guy" has been told! :-) Your brother is right Andy - but it was easier and foolproof to use a p rather than r (lower case rho) not knowing if your email reader would handle it. Yes it should have been volume instead of area. Darn - and I thought I was going to make through the year without a single mistake. As for the 76 or 80 foot error, I was too lazy to look that one up! I taught high school physics for a little over 3 years before bailing out for more money - but that was about 25 years ago. David V. Fansler Network Administrator TriPath Imaging, Inc. (Formerly AutoCyte, Inc) 336-222-9707 Ext. 261 KILLspamdfanslerKILLspamautocyte.com <RemoveMEdfanslerTakeThisOuTautocyte.com> Now Showing! http://www.dv-fansler.com <http://www.dv-fansler.com/> Updated March 31,2000 Ann's Cancer, David's Observatory, Disney World {Original Message removed}

Jim - I never finished the project - but after we purchased our 27' sailboat I received a GPS for an anniversary gift. Being "waterproof", I strapped the GPS to the mast to see how fast I went. Of few days I tried it in the ocean I hit 18 knots. I feel I have been faster, but it just takes the right combination of wind and lack of waves. A protected bay would be ideal for speed trials. It is stated that a 16' Hobie with no sails - just the mast - will do 8 knots in a 25 knot wind. David V. Fansler Network Administrator TriPath Imaging, Inc. (formerly AutoCyte, Inc.) spamBeGoneDfanslerspamBeGoneautocyte.com <TakeThisOuTDfanslerEraseMEspam_OUTautocyte.com> 336-222-9707 Ext. 261 Now Showing! http://www.dv-fansler.com <http://www.dv-fansler.com> Last updated March 31, 2000 Ann's Cancer, David's Observatory, Trip to Disney {Original Message removed}