Calibrating a Raymarine Wind Vane: fluid dynamics

nigelmercier

nigelmercier

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Following on from this post about Relating Vane RPM to Speed on an ST60 Wind Instrument:

nigelmercier said:
The Yellow wind speed circuit ... produces a square wave, at a frequency of twice the shaft rotation speed... Rotating the shaft at 600RPM (10 per second) produces a 7V square wave at 20Hz, and on an ST60 Wind with factory defaults this displays a wind speed of 20kn...
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My plan is to mount the wind vane on a "mast" sticking out of the sun-roof on a car. Connect the new vane to an oscilloscope or frequency counter. Drive at 20kn (23MPH) measured on a GPS on a still evening. I will then have a reference point to calibrate the wind instrument.

My only concern is that the airflow above the car will cause a false reading, so what would be a reasonable height of mast above the roof?
 
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Can't wait for the results on this one!
I guess it depends on the car involved. My preferred option would be a pole from the sunroof to the back seat as long as sensible, to get the instrument over the bonnet, in front of the normal turbulence. or if you've got roofbars, get the instrument in front of the car.
It would be interesting to get a reading from the ST60 as well as the 'scope results...
 
Not sure why you need the osilloscope.
If you know the actual wind speed then you can see if the ST60 calibration setting of 0.7 is correct. If not you could change it from the recommended setting so that the indicated wind speed matches the GPS speed.
In any event the wind speed at the transducer is just that. you only have to look at rotating anemometers in a marina to see the different rates at varying mast heights.
If you can measure the wind speed accurately it would be interesting to see if the setting of 0.7 as suggested by Raymarine is correct. Hard to imagine that it's not.
 
nigelmercier said:
Following on from this post about Relating Vane RPM to Speed on an ST60 Wind Instrument:



My plan is to mount the wind vane on a "mast" sticking out of the sun-roof on a car. Connect the new vane to an oscilloscope or frequency counter. Drive at 20kn (23MPH) measured on a GPS on a still evening. I will then have a reference point to calibrate the wind instrument.

My only concern is that the airflow above the car will cause a false reading, so what would be a reasonable height of mast above the roof?
Click to expand...

20 or 30 ft to be completely clear of the vehicle air flow or at least to give similar issues to when its mounted at the mast head.

why would you want to calibrate the wind guage anyway? Just suppose that in a given wind it was now reading 20kn and after your calibration it read 22kn - what would you change on your boat? Given all the other variables in a sailing siuituation, you would just be playing with numbers
 
foeu said:
Not sure why you need the osilloscope.
If you know the actual wind speed then you can see if the ST60 calibration setting of 0.7 is correct.
Click to expand...

He's not trying to calibrate the ST60, he's building something (not sure what) that works directly with the analogue signals from the sensor. So he needs to know what those signals look like when the wind is blowing.

Pete
 
£10 on ebay buys you a multi-featured anenometer with a claimed accuracy of +/- 5%.
If that's near correct, I think it would be as good as you would get from a car with a pole through the sun-roof on a "calm" day.
 
prv said:
He's not trying to calibrate the ST60, he's building something (not sure what) that works directly with the analogue signals from the sensor. So he needs to know what those signals look like when the wind is blowing.
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Actually, I am doing this to calibrate my ST60, with the new type wind vane. Yes, I could just set it to 70% and assume it is correct, but I'd like to check. A post above gave me an idea, place the mast base in the boot, with the head forward. The car isn't a hatchback, but it has a ski door between the seats.
 
birdseye said:
why would you want to calibrate the wind guage(sic) anyway? Just suppose that in a given wind it was now reading 20kn and after your calibration it read 22kn - what would you change on your boat? Given all the other variables in a sailing siuituation, you would just be playing with numbers
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The calculation of TWS and TWA is dependent on accurate input of boat and wind speeds. For example, polar diagrams are conventionally generated for TWS/TWA. Whether you want or need such accuracy is a matter of choice (and money).
 
Don't forget that once you have your perfectly calibrated MHU, you're likely to then put it on top of the mast where it will get all sorts of wind effect from antennas, lights etc etc that are mounted in the same area. you'l also get not inconsiderable upwash from the sails. That's why top end race boats fit vertical MHU so that the MHU is above any mast head effects.
:)
 
Have you got a variable speed fan in the office, use that and set the speed to give 20 kn reading on a hand held anenometer held in front of the fan. then replace it with the windvane you want to calibrate.
 
awol said:
The calculation of TWS and TWA is dependent on accurate input of boat and wind speeds. For example, polar diagrams are conventionally generated for TWS/TWA. Whether you want or need such accuracy is a matter of choice (and money).
Click to expand...

Sometimes the best thing to do is fiddle with the windspeed cal until you can tack without the alleged true wind changing...
 
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