Question for an EE: How do the transmitters work in salt water?

[pieter3d]

I am an EE (electrical engineer), and I was just curious how the transmitters for hoseless air-integrated computers manage to communicate in salt water, a conductor. I know submarines do it using very large & powerful very low frequency (VLF, 20-ish kHz) transmitter. No way that fits in a little tank transmitter.

 

[Anti-Hero]

You're on the mark, it would likely be a VLF transmitter / receiver. What frequency I have no idea, it probably varies from model to model. You can't compare the usage model of a submarine to that of a wireless dive computer.

 

[pieter3d]

I can compare it somewhat, both need to transmit data through salt water. It takes submarines tons power and huge antennas to do so. Of course they go much further, so I am guessing the low range requirement of the dive computers must simplify it. You still need a huge antenna to transmit VLF though. Maybe the whole tank is used as an antenna? Seems like charging that sucker up could def kill a battery quickly...

 

[SkipperJohn]

Isn't it a blue tooth-like protocol?

Or witchcraft.

 

[knotical]

If you go to this page:
Diving and Marine Services Padi
and select in turn computers, then Suunto, then scroll down to the Vyper Air, you'll find a statement that the Vyper uses 5.3 kHz.
Recall that cylinder pressure changes slowly, so telemetering does not have to be highly responsive.

 

[pieter3d]

If you go to this page:
Diving and Marine Services Padi
and select in turn computers, then Suunto, then scroll down to the Vyper Air, you'll find a statement that the Vyper uses 5.3 kHz.
Recall that cylinder pressure changes slowly, so telemetering does not have to be highly responsive.

Cool, that's interesting. Still don't understand how the transmitter can be so small though.

 

[herman]

This is really not a big deal if you think about it, the required tx range is only 6 to 8 ft and considering the only data that is being transmitted is a tank pressure with a little overhead, the power requirements are not much at all both in an rf and battery sence. A little rf goes a long way....had many a conversation half way around the world with a few hundred milliwatts. Optimal antennas have to be big at these frequencies but for short range pretty much anything can be pressed into service. A poor performer can be pretty much anything, including a trace on a circuit board or a flat plate or maybe the body of the reg is used but my guess is there is a loaded coil inside of the transmitter assembly. The data being sent is only few bytes sent every 5 to 10 seconds so the duty cycle is fairly low allowing a battery to last quite a while.

 

[Anti-Hero]

I can compare it somewhat, both need to transmit data through salt water.

That is like comparing the power needs of a small city, to the power needs of your cell phone. No EE worth their paper would make such a comparison.

If you want to understand the antenna design, take one apart. I bet it's some kind of RF coil inside the transmitters cylindrical design.

 

[GrandpaScuba]

Look at this: Underwater Radio Communication

 

[Hoomi]

You see, what they did was to hire some old Navy technicians who didn't have EE degrees, and therefore weren't smart enough to know that they couldn't transmit RF through salt water. These old techs just put the stuff together, and tinkered with it until it started working.



(A little tongue-in-cheek, but I suspect a whole lot of our modern innovations were a result of someone not being "smart enough" to realize it couldn't be done...)