How good is Suunto Cobra

[Charlie99]

ShoalDiverSA once bubbled...
Being an Electronic Engineer, I am all to aware of the vagaries of radio communication:bolt: . As you say, it may well be too risky to entrust my life to a radio link (

Though, having said that, I don't blindly trust the info I am getting now - but there is the peace of mind knowing that the instrument is getting *every* sample.

What makes you think the wireless link is radio, rather than IR or ultrasonic? Does anybody have the details as to what method and frequencies are used?

All wireless AI units I know of have the pressure sensor in the wrist unit, so the instrument gets *every* depth/time sample even if the wireless link is not functioning. The wireless link is just for gas pressure --- hopefully that is changing as a very slow rate.

 

[ShoalDiverSA]

Good point, Charlie.

IR = Line of sight, so this one is definitely out.

Ultrasonic = Good underwater. Gut feel that the power requirements might be quite high. From what I have read, the underwater communications systems that utilise ultrasonics are quite heavy on batteries, but this may be due to the earphones/piezo-electric transducers.

Over short distances I suspect that radio would be the medium of choice, particularly at lower frequencies. I would be interested to hear from someone who knows.

Maybe one day I'll get a rush of blood to the head and test radio comms underwater. But then again, who wants to hear someone babbling over the reef noise?

Cheers,

Andrew

 

[Charlie99]

ShoalDiverSA once bubbled...
IR = Line of sight, so this one is definitely out.

Ultrasonic = Good underwater. Gut feel that the power requirements might be quite high. From what I have read, the underwater communications systems that utilise ultrasonics are quite heavy on batteries, but this may be due to the earphones/piezo-electric transducers.

Over short distances I suspect that radio would be the medium of choice, particularly at lower frequencies. I would be interested to hear from someone who knows.

Maybe one day I'll get a rush of blood to the head and test radio comms underwater. But then again, who wants to hear someone babbling over the reef noise?

My guess is that the link is in that borderline zone of "inductive coupling", using a couple hundred kHz, similar to some smartcard systems.

Charlie Allen
formerly an electronic engineer, now a retired bum

 

[scubasean]

ShoalDiverSA once bubbled...


Maybe one day I'll get a rush of blood to the head and test radio comms underwater. But then again, who wants to hear someone babbling over the reef noise?

Isn't seawater conductive due to the salt? I would think that would make it necessary to make underwater comm sets be based on ultrasound.

Radio might be good for very short range ocean (underwater) communications, or communications that are extremely close to the surface (within a few feet), but I would think more than a few feet is impractical.

What am I missing?

 

[Charlie99]

scubasean once bubbled...


Isn't seawater conductive due to the salt? I would think that would make it necessary to make underwater comm sets be based on ultrasound.

Radio might be good for very short range ocean (underwater) communications, or communications that are extremely close to the surface (within a few feet), but I would think more than a few feet is impractical.

What am I missing?

The lower the frequency, the further it will penetrate. GPS signals up in the GHz range don't even get a foot. US Navy radio system such as at Jim Creek at very low frequencies (14kHz IIRC) can penetrate the ocean down to the crusing depths of ballistic submarines, although the data rate is extremely slow.

I don't have reliable info, but I believe that wireless transmitters for dive computers use inductive coupling (kind of like a widely spaced transformer) in the 200kHz range ---- just a guess, but it would have peformance like described in the manual as far as range and directionality.

 

[ShoalDiverSA]

Makes sense, the induction idea.

The application is quite suited to inductive coupling. I suppose the improved permeability of salt water over fresh would help here, but it has to work in both environments. The advantage of this system is that you can make one end of the link passive and power it entirely from the radiated power of the master unit. This would mean less battery changes on one end of the link. You would still probably need batteries on the pressure transmitter for the pressure sensor, A to D's and microprocessors.

But with the way that transponder systems are developing at the moment (see www.microchip.com - RFID), a batteryless transmitter is a probable development.

[I started work on a similar passive RFID system for timing of Radio Controlled car racing, but work caught up with me. Yet another project on ice.]

It would need some energy prioritisation in the programme for the master (wrist) unit, because I would be concerned that the wrist unit utilised a dwindling battery supply (didn't do the pre-dive check again, huh? ) to continue with the comms.

Interesting topic, nice one to brainstorm.

Anyone have some insider information? Put me out of my misery. I promise I won't tell...

Cheers,

Andrew