Calibrating an anemometer - practical advice?

[Oen]

Greetings all,

Having just fitted a new ST60 instrument (but not changed the masthead unit), I'm pondering how to calibrate the unit correctly. Lacking appropriate meteorological kit, what does the team suggest, please?

Thanks in advance...

 

[DJE]

Anchor near Bramble Bank and compare readings with Bramblemet ?

 

[gettingready]

Alignment of wind angle is relatively easy especially if you've got a windex at the mast head. Find a place where you can steer head to wind and tweak until you get the needle at zero. Wind speed is tough and most wind systems don't allow an adjustment factor but if you do have one then I suggest you leave it at factory default as it should be boat independent unlike paddle wheels for logs which often need corection for hull effects.

 

[Oen]

Thanks so far...

The direction is no problem. I first carried out the factory calibration, then used the autohelm wind vane function to motor head to wind and adjusted out the errors.

The speed side of things is troubling me, though. The ST60 allows for calibration, and though I've set it to the correct 'factory' setting for my mast head anemometer, I'd like to get it spot on.

From my met days, I remember that anemometers were technically calibrated (the kit was designed and built to a standard that didn't need calibration on site) but there are variables in the yot context which need to be addressed.

Has anyone tried taking a hand-held anemomoeter aloft and comparing the readings?

 

[BAtoo]

Other than for knowing its "spot-on" why does it need to be that accurate?? Within a knot or 3 is surely good enough?
However 2 tips - in a flat calm motor @ a set speed whilst you fiddle/adjust & this should give you a reasonably accurate low figure;
Ask you neighbour in the mrina/mooring what his is reading.

Have done both & still not certain how accurate these things are - ST60 too.....

 

[reeac]

The easy way to calibrate for wind speed is to use your GPS SOG. Motor directly into the wind and note wind speed and SOG. Then turn around and repeat in opposite direction. Into the wind your instrument should read SOG plus true wind speed whilst with the wind it should read true wind minus SOG. If you keep the two SOG s the same by appropriate use of throttle then the difference between the two apparent wind speed readings will be equal to twice the SOG. Some wind instruments have a calibration facility which requires input of a percentage correction in which case use the above calculated difference expressed as a percentage. You can use the same method to calibrate a conventional log when in a tidal situation. Great things, GPS s!

 

[CSFenwick]

Are you sure the ST60 wind speed calibration can be set by the user?? I have an ST50 that reads wrongly and was told by Raymarine that the factory pre-set can't be altered and hence it will have to go back to them for checking. On the ST50 the only change you can make to the ‘calibration’ is wind angle as already mentioned. I'd be intrigued if the ST60 wind speed can be adjusted/re-calibrated by the end user after comments made by the Raymarine engineer.

 

[Oen]

Yes, the ST60 can be calibrated.

reeac's technique would only work assuming a constant wind... A rare day, that!

 

[reeac]

[ QUOTE ]
Yes, the ST60 can be calibrated.

reeac's technique would only work assuming a constant wind... A rare day, that!

[/ QUOTE ]

That's true in principle but if you want to be fussy then you do a third run and compare wind reading with the first one. If it's the same [at same SOG] then you're OK. My wife and I thought up this method and tried it and it worked first time. Maybe we'll repeat it next year and see if anything's changed. After all, what's the alternative?