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Poor HRM connection under drysuit

nradov
over 3 years ago

Out of curiosity I wore my HRM-Tri strap under my drysuit for two scuba dives to see what my heart rate was doing. When I got back on the boat I selected the "Download Heart Rate" option on the dive activity. It seemed to work or at least didn't give any errors, but when I looked at the activities in Garmin Connect later I could see that chunks of heart rate data were missing. And on the boat the Descent was intermittently connecting and disconnecting from the HRM-Tri. That's understandable considering that I was wearing a drysuit and thick undergarment which probably blocked the ANT+ radio signal.

So anyway if you want to get heart rate data on a dive I recommend waiting to download until after you remove your suit. That way you can get a reliable sensor connection.

  • 0 over 2 years ago

    Hi,

    Did you get it to work properly. I got a HRM PRO specifically to use under my drysuit. I put it on well before putting my undergarments and drysuit on. I wait till I'm undressed to download it and it also says it downloads fine but then there is massive sections missing. It works fine for other activities. I wet the sensors, I put it on very snugly and its not moving around.

  • 0 over 2 years ago

    This is far less likely to be caused by the drysuit, more the fact that you are using it underwater.

    The simple fact is radio waves propogate very poorly underwater.

    Firstly, the losses are very high due to the fact that water is conductive, sea water more so.

    Also water molecules directly absorb radio waves.  To make matters worse, this effect is large at 2.4GHz (which is why microwave ovens work).The ISM band at 2.4GHz is where ANT+ lives.  ANT+ is very low power so more susceptable to a lossy link and really very unsuitable for underwater use.

    Lastly, the antenna design has to be altered to compensate for the fact that the wavelength of radio underwater is not the same as in air due to the fact that electromagnetic waves propogate much slower in water. As all these effects change with pressure (depth), temperature and purity (i.e. salinity) so the antenna will only ever be at best a compromise that may not match the actual conditions very well. An antenna that is imperfectly matched to the frequency is lossy, and has low gain, making things worse again.

    There are so many variables that it's difficult to put a proper number to this but it is likely to be in the order of more than 100dB loss over 10cm.

    A typical ANT+ has a receiver chips has a sensitivity of -93dBm, so to work with the 100dB path loss suggested above, you'd need to transmit more than +7dBm. ANT+ typically uses +3dBm as ANT is designed for long battery life.  I hope now you can see the problem.

    It takes about 1.5 wavelengths for an EM wave to form from the antenna. 2.4GHz is 12cm wavelength in a vacuum and the water is much closer than that.  So even though the antenna is not actually in water, (it is within in the device), it behaves as if it is.

    Perhaps Garmin will make a version of the HRM for underwater use that makes use of the Sonar communications the MK2i/T1 has.

  • 0 over 2 years ago in reply to Jason_Morgan

    You're really missing the point. The watch is supposed to be able to download the heart rate data stored on the HRM-Tri when you're back on the surface and saving the activity. Not underwater. But the surface download doesn't work reliably. Fortunately the HRM-Pro does work.