Measuring the temperature of salt water

[tero999]

Hello everyone from a new forum user. I watch eevblog videos a lot but I never actually post anything to this forum and also  sorry for my not so perfect english.

So I live very close to the sea and almost everyone is allways curious what is the temperature of the water. Is it warm enough to swim in it or not?  So I came up with an idea to show the temperature on some big screen on the beach and maybe also show on the internet with a webcam. The idea is that I can connect a raspberry pi or something similiar to a sensor. But what kind of sensor? Does "normal" water temperature sensors work in salt water? Or the saltiness "confuses" it somehow? And anyone know on what debth shoud I measure it anyway? I try googling for some salt water temperature sensors and I did not found any intersting (or cheap) Any ideas or pointers in the right direction?

Thank you.

[Marco]

The tide makes things a bit hard.

Maybe you could you use an IR camera or an IR thermometer with a servo to determine the temperature near the waterline?

[han]

Normal water sensor like thermocouple or pt 100 will do. just protect it from salty water corrosion, and water Ingres too

[Rerouter]

you can buy IR thermometers with quite small spot areas, hack into it, and add that to your webcam feed.

[Kalvin]

Rerouter was a bit faster with his suggestion. I would check whether the IR sensor would work. For example the MLX90614 is quite cheap and the accuracy is good enough for the application. Combined with an ESP8266 Wifi-module, you can get a very inexpensive wireless temperature measurement sensor quite easily.

Edit: The ESP8266 consumes around 10 uA when in the deep sleep mode. The on-chip real time clock can wake the module periodically to make the measurement, send the measurement results over the Wifi and then go back to deep sleep mode. The gadget can easily run with a battery over a year when the measurement period is made to match the battery capacity. Of course, you can also use the solar power to charge the battery or super capacitor.

[george graves]

Bake sure you look into how inaccurate IR temps sensor are on a glossy surface.  I know a guy that works for a earth moving equipment company and they spray yellow primer on parts when they need a accurate temp using IR guns.

[Kleinstein]

There is not just one water temperature to measure. So it would make sense to use more than one sensor, so maybe 5 in different water depths. The temperature right at to top can be quite different as is is influenced by wind and humidity relatively fast.

[Ian.M]

Also, you  don't want to measure the water temperature too close to shore due to sunlight heating rocks etc. so to do the job right, you need a buoy to support the sensor string, with a battery + solar panels to keep it charged and a wireless link back to shore for the data.   

Placing sensor buoys without following the formal procedures of your local navigation authority is likely to result in their removal and possible legal action, and getting appropriate permissions as a private individual may be difficult or expensive so this sort of project is best done in collaboration with a nearby university or technical college.


The only exception would be if there is an existing open framework pier or a pontoon that you could get permission from the owner to let you hang the sensor string from in deep enough water.

[wraper]

Ir sensor won't work. Surface of the water have significantly different temp compared to 1cm under the surface.

[PA0PBZ]

It would be easier to use an existing service:

[nctnico]

There is not just one water temperature to measure. So it would make sense to use more than one sensor, so maybe 5 in different water depths. The temperature right at to top can be quite different as is is influenced by wind and humidity relatively fast.

I agree. Even in a shallow lake (2 meters / 6 feet depth at most) you can feel different water temperatures while swimming. I'd try and attach several sensors to a structure which is already in the water (note the remarks from Ian.M above!). PT1000 sensors are nice because they are easy to interface and still relatively cheap including stainless steel housing and silicone cable.

[nowlan]

I have a regular fishtank thermometer in my marine tank. The sensor is plastic coated so it doesnt rust. I dont see why you cannot use one of these. The lcd/battery section isnt water tight, but you could repurpose the probe or just seal the display more.

[Marco]

Ir sensor won't work. Surface of the water have significantly different temp compared to 1cm under the surface.

Lots of churn near a sea beach though ... can't really compare that to open ocean, pool or lake measurements.

The advantage of IR is that you can simply put it on a pole (and maybe scan it with a servo). With subsurface thermometers you'll need a radio buoy anchored relatively far from shore (but close enough to be representative). Which I assume will require a permit.

[Marco]

It would be easier to use an existing service:

Where can you get up to date AVHRR data? (ie. latest scan with minimal delay, measured in minutes, not days.) I'm sure the national meteorological services have it, but they don't generally seem to be sharing it. I guess it would be worth it to inquire with his national service to see if the up to date data for his local beach could be made available somehow.

[TheDirty]

Unfortunately I can't answer much about the specifics of salt water.

This guy makes several DIY environmental monitors including an underwater version that logs several variables including temp.  He uses arduino's and several e-bay modules for everything.  There's a good explanation of how it all goes together.  Made for logging, but it would be pretty easy to adapt it to reporting every 10/30/60 minutes.  It talks over an NRF24l01 module.  Not certain the range through salt water, but it should be pretty decent to get it out there otherwise and RF chain might be needed.

https://edwardmallon.wordpress.com/

[Ian.M]

*VERY* interesting blog, and a fully submerged sensor pod would be a possibility, but the effective range of a low power RF datalink in seawater will be measured in centimeters, not meters.  See: http://users.tpg.com.au/users/ldbutler/Underwater_Communication.pdf

Also, a submerged sensor pod will need to rely on primary batteries for a seasons endurance as there is no possibility of using solar panels,  must be fully hermetically sealed against significant pressure and would need to be positioned and retrieved by a diver.

[Marco]

You want a temperature just below the surface, not just above the bottom ... in the sea you can't do both at the same time all of the time.

[Kalvin]

*VERY* interesting blog, and a fully submerged sensor pod would be a possibility, but the effective range of a low power RF datalink in seawater will be measured in centimeters, not meters.  See: http://users.tpg.com.au/users/ldbutler/Underwater_Communication.pdf

With magnetic fields the range can be few meters:

http://www.academia.edu/11614868/Near_Field_Magnetic_Induction_for_Underwater_Wireless_Communication_Networks

Using repeaters or magnetic wave guides, the range can be extended.

[TheDirty]

*VERY* interesting blog, and a fully submerged sensor pod would be a possibility, but the effective range of a low power RF datalink in seawater will be measured in centimeters, not meters.  See: http://users.tpg.com.au/users/ldbutler/Underwater_Communication.pdf

Also, a submerged sensor pod will need to rely on primary batteries for a seasons endurance as there is no possibility of using solar panels,  must be fully hermetically sealed against significant pressure and would need to be positioned and retrieved by a diver.

Thanks.  I was trying to think of the best way to do this, but I'm at a loss.  I was thinking a tethered buoy application would be better, where the sensor itself measures 'swim' temperature maybe 1/2 a meter under the water and still have RF range.  Assuming this is an ocean beach, there would be waves to contend with closer in, and farther out would be a hazard for small craft.  Either or, you have to worry about people just grabbing it and taking off with it.

Anyway, probably straying off the original topic.  He probably has some method.