I am building a sea scooter and looking to keep everything sealed. Amazon has a 400 mhz remote 12v switch. It states the max transmitting distance is approx 90 feet. I am looking at having the key fob about 15" away as the on off switch and the receiver in the body of the scooter. Will a 400mhz freq travel underwater a short distance or does water completely block it? I am looking at encasing the key fob in a sealed bag like one from the vacuum sealing machines. I plan on leaving air sealed in it but do you think the water pressure will compress the air and it will constantly press the switch? Any other ideas how to encase it without being too bulky?
Will 433 mhz radio freq work under water?
- Thread starter Seville
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Gone for diving
Contributor
I highly doubt it will work,
15inch away? That's a long way,
Maybe if the fob was against the scooter body,
I do know subs use ultra low frequency to transmit,
15inch away? That's a long way,
Maybe if the fob was against the scooter body,
I do know subs use ultra low frequency to transmit,
Yeah, low frequency is needed. The specs I found for Shearwater's air-integration transmitter state the frequency is 38kHz. That's pretty low--if you think about it, well below the old AM radio frequency band that starts at 550kHz. I'm not an RF engineer, but my gut tells me 400MHz is way too high to be received through water at any useful distance.
Russjstewart
Contributor
Without trying to be funny, why not just test it. Amazon in Australia is selling similar switches for $aud 27 or about $usd20, so they are not a huge investment
Seal one end of a pvc tube, wire up the switch, a battery and a light put them in the tube, put the tube vertically in the water, and tie it off so the open end is dry and visible. Seal the transmitter in your vacuum sealed bag. Stick your hand in the water and test, see how close you have to get before the light turns on. That will give you a best case distance. You will get different results in salt and fresh water.
Seal one end of a pvc tube, wire up the switch, a battery and a light put them in the tube, put the tube vertically in the water, and tie it off so the open end is dry and visible. Seal the transmitter in your vacuum sealed bag. Stick your hand in the water and test, see how close you have to get before the light turns on. That will give you a best case distance. You will get different results in salt and fresh water.
OTF
Coney Island Whitefish Biologist
You won't get reliable transmission underwater at any distance, but very close range will likely work.
Scooter switches need to be very reliable because you REALLY don't want them to stick on unintentionally. So a proper sealed shaft like the buttons on a camera housing (which isn't actually that hard to achieve) is your best bet. Take apart some obsolete or broken camera housings to scavange parts or learn how they construct them. There are lots of online guides for designing o-ring seals.
You could look into magnetic reed switches (you will need a reed switch+relay or mosfet for controlling high current) which can work great, actuated by a magnet outside the housing, but again they can stick on and off which may be dangerous on a dive scooter specifically.
As for the key fob thing - or even a wired switch in a sealed bag - pressure on the bag pushing the button could definitely be an issue. But if you fill the bag with mineral oil and bleed out all the air it might work pretty well.
Scooter switches need to be very reliable because you REALLY don't want them to stick on unintentionally. So a proper sealed shaft like the buttons on a camera housing (which isn't actually that hard to achieve) is your best bet. Take apart some obsolete or broken camera housings to scavange parts or learn how they construct them. There are lots of online guides for designing o-ring seals.
You could look into magnetic reed switches (you will need a reed switch+relay or mosfet for controlling high current) which can work great, actuated by a magnet outside the housing, but again they can stick on and off which may be dangerous on a dive scooter specifically.
As for the key fob thing - or even a wired switch in a sealed bag - pressure on the bag pushing the button could definitely be an issue. But if you fill the bag with mineral oil and bleed out all the air it might work pretty well.
moose_grunt
Contributor
Yes, you can successfully use 433 mhz underwater. Not 15", though. I use adafruit 433 lora modules to transmit battery information through the hull of the scooter, with the antennas lined up on either side, and it works. Move the receiver a couple of inches away, though, and it drops the signal.
As mentioned earlier, magnetic switches (reed switches or hall effect sensors) are probably a more reliable way to go.
As mentioned earlier, magnetic switches (reed switches or hall effect sensors) are probably a more reliable way to go.
gvnwst
Registered
Temporarily ignoring all the other issues and proposals...
To get some intuition on what frequencies will and won't work, we can look at some helpful charts from places like this.
It doesn't actually go up to the correct frequency (we'll see why) but it provides simple equations we can evaluate ourselves.
If the transmitter works in air to about 90ft/27m, we can estimate the minimum detectable signal strength in dB, just assuming spherical emission -- it's about -29dB (the signal strength decreases as 1/distance^2 --> based on a reference position of 1m from transmitter the relative strength is 1^2/27^2 --> 10*log10(1^2/27^2) = -29 dB). For science-y reasons, spherical emission for this sort of device is a super good approximation.
Based on the webpage above, the attenuation at 400 MHz should be 0.0173*sqrt(frequency*conductivity)= 774 dB/m, assuming the worse stated conductivity value as a conservative bound (5 Siemens).
At 774 dB/m attenuation, you reach 29 dB of attenuation after only 3.75 cm.
Even if the exact conductivity values are substantially different in the water you're diving, it's unlikely you'll have the reliability you want.
It's likely you could have wireless transmission if you glued your fob to the exterior casing, but not if you want to hold it in your hand.
Going back to that plot in the link above, frequencies on the order of 100 kHz give you a few meters of propagation, you can get at least 1m propagation up to about 700 kHz. As @Lorenzoid pointed out, this is probably why Shearwater uses that frequency band for its transmitters. If you want to go the wireless route, find something low frequency.
To get some intuition on what frequencies will and won't work, we can look at some helpful charts from places like this.
It doesn't actually go up to the correct frequency (we'll see why) but it provides simple equations we can evaluate ourselves.
If the transmitter works in air to about 90ft/27m, we can estimate the minimum detectable signal strength in dB, just assuming spherical emission -- it's about -29dB (the signal strength decreases as 1/distance^2 --> based on a reference position of 1m from transmitter the relative strength is 1^2/27^2 --> 10*log10(1^2/27^2) = -29 dB). For science-y reasons, spherical emission for this sort of device is a super good approximation.
Based on the webpage above, the attenuation at 400 MHz should be 0.0173*sqrt(frequency*conductivity)= 774 dB/m, assuming the worse stated conductivity value as a conservative bound (5 Siemens).
At 774 dB/m attenuation, you reach 29 dB of attenuation after only 3.75 cm.
Even if the exact conductivity values are substantially different in the water you're diving, it's unlikely you'll have the reliability you want.
It's likely you could have wireless transmission if you glued your fob to the exterior casing, but not if you want to hold it in your hand.
Going back to that plot in the link above, frequencies on the order of 100 kHz give you a few meters of propagation, you can get at least 1m propagation up to about 700 kHz. As @Lorenzoid pointed out, this is probably why Shearwater uses that frequency band for its transmitters. If you want to go the wireless route, find something low frequency.
@gvnwst is right, you wont have much range. I know even the 38khz pressure transmitters sometimes struggle to be heard in bad conditions. Hall Effect/ Reed switches would be the way to go. If you really wanted wireless It would be reasonably straight forward to make a ~30 khz underwater radio rx and tx, if you can be liberal with power usage.
Interesting. I'm curious if anyone tried the experiments above.
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