2.18.2014

Waterproofing a Tru-Stability Pressure Sensor w/ Arduino

Our spooling mechanism is going to unspool/spool our tether based on the depth of the submersible (+/- a margin of error). Therefore, as part of the float we need to design a depth sensor capable of traveling up to 250 feet underwater (roughly 105 psi). After much searching, we settled on using Honeywell's Tru-Stability Force Sensor to do this. We purchased the SSC 150psi model with a 5v logic and analog output. Below is a quick example of how to use the Tru-Stability Sensor, which is followed with how we waterproofed the sensor. This is the data-sheet we used.

SSC 150 psi


1. Hook up the Sensor using the 5v, A0, and Ground pins. Our sensor is pictured below. The pin-out is also pictured (analog version -- Table 9). If you have a different sensor just use this data sheet to find the drawing of your sensor and the tables below to find the correct pin-out.

SSC 150PSI (take note of pin locations)

Sensor Pin-Outs
















2. Upload the SSC150.ino sketch located in the "SSC150" folder here. The code is based off of the data table and equations found on Page 6 of the data sheet (and pictured below). A shout of to a forum post (which I have sadly lost...) which mentioned the equation needed to calculate pressure.



B.N. -- Our sensor is based off of 5v logic


3. Upload the sketch to your Arduino to make sure it's working. If you are around 1 atm (sea level) the PSI should read around 14.70, the feet around 0.00.

B.N -- This sketch does not currently calibrate itself when powered. This is because our submersible might lose power 250 feet underwater and we don't want the sensor to automatically calibrate when rebooted at that depth. Therefore, in the finished product there will be a specific boot sequence to run through before using the submersible.


4. We then chose a sturdy PVC pipe drilled some holes, added the sensor (with soldered wire extensions), and siliconed everything to waterproof the connections.




We then had an issue deciding on how to have the sensor measure pressure. Should we use a homemade manometer or a balloon to sense the variation in pressure? What about a flexible latex covering? PSI increases in increments around .4 psi/foot of H20  and because our submersibles tether is going to be based on depth, we needed a setup which gave us the most accurate readings. In the end we decided on using a manometer because we were able to read a large enough pressure difference per foot.

The sensor is siliconed to a tube which exits the PVC and is exposed to the water. Our sensor was sealed to this small diameter tube. Not water can enter the tube when submerged because of the cohesions of the water and the fact that one end of the tube is sealed As the sensor descends, the water pressure increases the air pressure in the tube. Below is a picture of the finished sensor:


Sensor w/tube

Finished sensor

I tested this sensor at the local pool with my sister (thanks Mackie L.!). We attached it to the top of a weight a dropped it roughly 3.8 feet (4 feet minus the width of the weight). Below are the readings from the sensor and a picture of it in the pool (H2 in the Arduino code should be H20).


Pressure submerged in pool



Sensor at 3.81 feet


Although this setup worked, it still needs to be tested to depths up to 250 feet in salt water. Hopefully it will continue to work, but you never know. Stay updated for more posts about completing the float!

13 comments:

  1. Since this has been posted, have you been able to test the sensor at depth. How well did it work? Di you have any issues? How was the reaction time?

    ReplyDelete
    Replies
    1. Sadly we were never able to test this sensor at depth. In the pool it had a good reaction time and worked well. It was a good prototype, but due to its size we never put it in the submersible. We are currently working on a second design that'll hopefully be done by the end of year. It'll follow the same principles for cost reasons.

      For a relatively accurate sensor (+-.3psi...if I remember correctly) it's definitely an option looking into. What are the specs you're looking for? Are you also building a submersible?

      Thanks for the comment!

      Delete
  2. Hi, I am curious how can you prevent the sensor from water. Saying "one end of the tube is sealed", I guess you seal one side of the tube that is not connected to the sensor. This means water will not come into the tube but squeeze the tube and the air inside, then the sensor will be able to detect the pressure? Thank you.

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  3. The idea behind the sensor is based on a closed system. One end of the plastic tube is sealed to the depth sensor (which is sealed in the PVC container) while the other end is open in the water. When the sensor is submerged the water forces the air inside of the tube into the sealed PVC container. This causes a constant increase in pressure in the PVC container which is then read by the pressure sensor thus giving us depth.

    Does that answer your question?

    Thanks for the comment.

    ReplyDelete
  4. What was the reaction time of the sensor?

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    Replies
    1. On the SSC data sheet it says that

      "Calibrated output values for pressure are updated at approximately 1 kHz."

      If you want to know more, you can read over the data-sheet.

      (http://www.mouser.com/pdfdocs/HoneywellSSC.pdf)

      We also recommend looking at an MS5803-14BA (https://www.google.com/webhp?hl=en#hl=en&q=MS5803-14ba) breakout board. Theses sensors are already waterproofed, you so only need to epoxy the wire leads. If you stayed updated with the blog this summer we will be testing these sensors more thoroughly and posting our results.

      Thanks for the question!

      Delete
  5. Did you test temperature compensation?
    I use ABP series which must be temperature compensated like SSC.
    But I found direct correlation between sensor temperature and returned pressure value.
    more info

    ReplyDelete
    Replies
    1. Hey hello,

      Sorry this is totally not related to this post, but could you tell me if you are able to use ABP sensor with arduino?

      Delete
  6. This comment has been removed by the author.

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  7. Hi, is there any website for us to find honeywell pressure sensor arduino code?
    What's the difference between analog and digital output? Which one easier if I just wanna measure the the value of pressure across time?

    ReplyDelete
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