Sail pressure measurement

Instrumentation for measurements, and instrumentation for anything else; also control systems including self-steering gears
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John Perry
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Joined: Tue Nov 08, 2016 6:39 pm

Sail pressure measurement

Post by John Perry » Tue Feb 11, 2020 9:18 pm

Measuring the distribution of pressure over the windward and leeward surfaces of sails is one of many potential projects I have had in mind although I havent yet been able to make practical progress due to time spent going sailing/rowing and carrying out routine boat maintenance etc.

A basic difficulty in measuring the pressure on a sail is that the pressures that need to be measured are quite low - too low to be adequately resolved by most industrial pressure sensors. Pressure measurement on aerofoils in low speed wind tunnels was traditionally carried out using liquid manometers, the sensitivity of which can be improved by incling the tube so that it is near horizontal and/or by measuring the position of the meniscus between two immisible fluids rather than between one fluid and air. Liquid manometers when set up in an on-shore laboratory can be sensitive and accurate but I can see real difficulties in using a sensitive liquid manometer on board a sailing boat!

At one of the recent AYRS Thorpe meetings I made a short presentation of an idea for an electronic sensor with a flap of shim metal, say about 0.05 to 0.1mm thick stainless steel, that is deflected by an air pressure differential, the deflection being measured by strain gauges mounted on a narrow ''neck' about which the flap hinges. I did some finite element analysis that showed that it should be possible to measure strains in the region of 1Mpa for a pressure differential of 1Pa - futher optimisation of the geometry might even improve on this. I had it in mind to make up an instrumented hollow (i.e. tubular) sail batten that could be fitted into a fully battened sail with sensors measuring the pressure differential between the windward and leeward sides of the sail and the space inside the tubular batten, the pressure within this space acting as a reference pressure which, if the batten is open at the aft end, would be essentially the wake pressure, so I think similar to free stream pressure. The data from all the sensors along the batten could be monitored by a microcontroller in a pouch sewn onto the sail, then transmited by wireless data transfer to a computer below decks. The idea seemed quite promising but so far I havent made practical progress, for reasons given above.

Meanwhile, I wondered about commercially available pressure sensors for low pressures, these are generally piezo resitive sensors. I happen to have such a sensor, an MPL 3115A2, built into a small 'breakout board' from Adafruit Inc. I think one of the main applications for such devices is for use as an altimeter to stabilise the height of drones. I have had this device lying around for some time and this afternoon I got round to hooking it up to an Arduino and logging the readings from it. I would assume that sail pressure measurements would be carried out in fairly light winds, at least to begin with, so the requirement would be to measure pressures up to a few tens of Pascals with a resolution of 1Pa or better. I have to say that from my quick trial I dont think the MPL 3115A2 is good enough for this. I attach a graph (see end of this post) straight from the 'Serial Plotter' facility in the Arduino IDE, this graph showing the readings taken over a few minutes, each reading being relative to a mean pressure determined from a series of initial readings. For part of the graph I blew cool air from a hairdryer (with the heating element off) onto the sensor to simulate a gentle breeze. The sensor did respond to this but the response was hardly more than the variation and drift that occurred between readings with the sensor sitting in still air on a desk indoors.

A couple of weeks ago I happened to meet AYRS member Richard Fish who is also interested in measuring pressure distribution on sails. I wonder if anyone else reading this has such an interest, it would be interesting to compare notes and maybe do something collaborative. I think Richard said that he was thinking about using Bosch piezo resitive sensors, so I wonder how well these will work. Adafruit make breakout boards for various types of pressure sensor including Bosch ones and I summarise some of the manufacturers specs below. It is a bit difficult to make comparisons since different manufacturers specify accuracy and sensitivity in different ways, probably the best way to compare is to get samples (they are only a few pounds each from Pimeroni) and hook them up to an Arduino. On the face of it the DPS310 looks worth a try.

DPS310 (Infineon Technologies)
Sensor precision 0.2Pa
Relative accuracy +/- 6Pa
Pressure temperature sensitivity 0.5Pa/K

BMP 388 (Bosch)
Absolute accuracy +/- 40 Pa
Relative accuracy +/-8 Pa
Noise 0.03 Pa
Long term stability +/- 33 Pa

BMP 380 (Bosch)
Absolute accuracy +/- 50 Pa
Relative accuracy +/- 6 Pa
Noise 0.03 Pa
Long term stability +/- 33 Pa

BMP 280 (Bosch)
Absolute accuracy +/- 100 Pa
Relative accuracy +/- 12 Pa
Resolution of data 1 Pa

MPL3115A2 (NXP Semiconductors)
Absolute accuracy +/- 400 Pa
Pressure resolution 1.5 Pa
Long term drift +/- 100 Pa

In addition to the above sensors, long time AYRS member Graham Ward recently sent me details of another piezo resistive sensor - see here - ... +28.06.pdf
These sensors have actually been used for aerofoil pressure measurement but I think for aircraft work where the pressure differentials are higher than for sailing boats. I have emailed this company for more info.

20-02-11 MPL 3115A2 Pressure readings.png
(89.84 KiB) Not downloaded yet
20-02-11 MPL 3115A2 Pressure readings.png
(89.84 KiB) Not downloaded yet

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Re: Sail pressure measurement

Post by AlexQ23 » Sun Feb 16, 2020 3:20 pm

I don't know if it can help, but Dwyer makes some Magnehilic differential sensors very accurate rated as low as 10 mm of water. One tube can be set in the extrados for example and the other one inside the mast. Hope you the best!

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