Water/pressure proofing a pressure sensor


#1

Hi All,

I'm using a Rapsberry Pi with BMP085 pressure sensor to measure depth, however the sensor isn't waterproof. I have considered a two options, the first being; housing the sensor in a case with a small drilled hole which is covered by a flexible membrane, which in turn will deform as pressure increases.

Second option would be to buy an IMU depth module from the OpenRov store which is already water proofed, but £45 ($80) IMO seems expensive. Also I'm a beginner with Python so if I did consider buying the module I'd like to be able to play change the code to suit my other Python programs.

I'd like to know if anyone can offer any advice on materials for the flexible membrane, or whether anyone has any other suggestions regarding to waterproofing my BMP085?

Thanks in advance


#2

Hey Jamie!

There are certainly quite a few options for you, but I'm nervous that the pressure sensor you're using is only meant to measure the kind of pressure differences you'd get from changes in (air) altitude- not (water) depth. According to this spec sheet, the BMP085 only measures up to 1100 hPa which translates to just slightly over one atmosphere of pressure.

That being said, assuming the sensor can handle being in a liquid as long as the liquid is not conductive, I'd recommend placing the sensor in a zip-lock bag, rubber glove, or other flexible container filled with mineral oil. The oil won't short out any of the electronics, but will equalize to the pressure of the water around it. Surgical gloves (such as latex or nitrile gloves) also are a great source for the sort of membrane material you're describing. Keep in mind that the sensor you have seems to have a hole in it which allows air to enter the chamber where the sensor is, so a membrane may not be able to push directly against the sensor as you describe. If you were planning to keep the sensor inside a container filled with air, and you were hoping the movement of the membrane from water pressure would pressurize the air to reflect depth, you should also remember that air is compressible, so there may be a hard-to-calculate relationship between the pressure you see and the depth you're at.

Anyway, you seem to be working on a cool project, and I hope it works out. Please let us know how it works out, and don't hesitate to ask of you have any other questions or ideas to bounce off of us!

Good luck!

Eric


#3

Hi Eric,

Thanks for the quick reply. I guess I should of mentioned that the ROV I'm building is to dive no deeper than probably 5m to demonstrate it works. I thought the BMP085 would of been a good sensor to use since it's easy to setup with the Rpi and it seems to have good resolution, also does say on the spec sheet -500hPa (which I assumed was to indicate below sea level?). Does anyone have any suggestions for alternative sensors I could consider?

You raised a fair point about air being compressable, if I was to go down this route, taking into consideration the sensor resolution and air compression. How much harder would it be to calculate depth from the pressure sensor?

I wouldn't of considered putting the sensor in mineral oil are you sure that's a good idea? I'll look into it a bit more, sounds interesting!

Thanks

Jamie


#4

Hi,

I do not think you can put BMP085 under water. First of all, the pressure range of the sensor is 300hPa - 1100hPa (0.3 - 1.1 bar). Also, its datasheet does not mention underwater operation for its typical application, so I suspect the materials it is built from would not sustain the underwater pressure or the corrosive water or both.

The sensor sold by OpenROV is built by www.meas-spec.com (one of sensors at http://www.meas-spec.com/pressure-sensors/board-level-pressure-sensors/digital-pressure-sensor-modules.aspx). You might find a supplier providing just the sensor for under 30USD (it was possible last year :)). You will have to invent how to waterproof it by yourself, not to mention other issues like soldering.

There is new sensor made by the same company, which is built as ready to mount package: http://meas-spec.com/product/pressure/89BSD.aspx - I have not found anyone who sells it at appropriate price, unfortunately. (If anyone have found one... let me know! :)))


#5

I have already had success with the MS5803 series (as used in the open ROV sensor). But they are a little pricey ($30) so I will be experimenting with some MS5805-02 ($10) units soon, even though they are not designed to go under water. I will simply be completely covering them with QSil 216 Clear Liquid Silicone (Amazon.com) , using a needle to try to get the silicone right into the pressure sensing housing. That and a rubber membrane(ie a really thick balloon) might just be enough to change them into water pressure sensors.


#6

Dear all,
Iam planning to use bmp 085 as a depth sensor under water using arduino with windows as operating system.
Can anybody guide me with arduino ide code and circuiting Arduino
Regards
Ajith.


#7

Hi bujji99dpi,
Adafruit has some really comprehensive and useful guides on using the BMP085 and BMP180 for Rpi and Arduino. Check them out.

Kind Regards,
smooth_jamie


#8

Hi Jamie,
Thanks for your help.
I went through adafruit manuals if BMP 180 and 085.There nothing related to
under water operations in the manual.

Kind Regards
Ajith


#9

Hi there,
The sensor works exactly the same under water as it does in ‘free air’. What exactly about underwater operations do you need to know that isn’t on the adafruit site? If you need technical details Bosch have a datasheet too, otherwise please be a little more specific about what you need.

Kind regards
Smooth_jamie


#10

Hello
just buy an old digital depth meter,dive computer for cheap, look for aqulung uwatec
in france you can find it at 15 usd

http://www.leboncoin.fr/sports_hobbies/790083585.htm?ca=12_s

Dismantel it and take the sensor


#12

I just sealed the BMP085 inside a balloon to waterproof it if that’s what you mean, otherwise it works exactly the same. Did you get something sorted out?