Temperature Probe in water

[nathanknz]

Hi there.

I've been poking around with the temperature probe feature board.

It reads a good steady value in air, but I noticed that in our tropical fish tank the temperature can bounce around a bit, sometimes varying by up to 3 degrees (worse case) in a 10 second interval.

I was thinking about the different possible reasons, and I believe I have ruled out electrical interference. The other possibility was the fact that the water is moving around the tank (due to the water filter).

We've got a regular off-the-shelf digital thermometer and that seems pretty stable. The difference is that the temperature sensor for the off-the-shelf thermometer is coated in some form of epoxy which I'm thinking would smooth the result.

I was just wondering how other people on the forum have gotten on with measuring the temperature of water?

[iobridge]

Electrical interference is possible. The guys at TankedCAM.com or SecureCove.com had ran into this issue. There are tank grounding systems. I think this improves the fluctuations.

http://www.marinedepot.com/miscellaneous_titanium_grounding_probe-ap.html
http://www.tankedcam.com/

Let us know if you find anything out.

[nathanknz]

Ahhh, I see. I was thinking more electrical interference from wiring in the walls, not through the water in the tank.

Thanks for the links. I'll investigate and let you know how I get on.

[nick]

The temperature sensor has a resistor that varies in resistance with temperature.  If water is leaking into it and providing a bypass the resistance could vary, which would show up as a fluctuating temperature.  The first thing I would try is to waterproof the sensor.  Try putting it in a ziplock bag with the opening above water.  If that works try sealing it in epoxy or silicone.

Nick

[nathanknz]

Thanks for the ideas Nick - I'll have a play this weekend.

I've been doing some Googling on the ground probe idea, and I've read both good and bad things about the idea. The 'bad' idea being that creating a path to ground could cause current to pass through the water and harm the fish (a comparison was made to a bird sitting on a power line without being harmed, unless it touches another wire at the same time).

I was unable to find anywhere in New Zealand that sells grounding probes. I'm wondering if there is a reason for that? Perhaps the NZ groundings are different than in the US? I'm going to research more before I attempt anything in that respect.

I'll report back on my findings.

[nick]

My wife used to keep tropical fish and I spent some time dealing with grounding issues.  What happened in our tank was that a heater started leaking and pretty high voltages -- 90V or so -- were present between the tank and ground.  A grounding rod would eliminate the voltage, but I found there was a lot of controversy in the aquariast community as to whether this was a good idea or not.  The real fix was to replace the heater.  This kind of stray voltage is very common whenever you use electricity around water, hence the GFI outlets in your bathroom (in the US at least).

With the iobridge it's not such a concern because it only operates at 5v, and it's not grounded, so a path to ground is not an issue.  However, it's not good for either the components or the fish to have the sensor exposed to water.

Nick

[jason]

The end of temperature probe is coated with enamel to keep out moisture.  I've had them in my aquariums for a long time now without any problems.

[fillibar]

I am assuming this is for a freshwater aquarium. While the probe may be enameled it did not hold up for a long time in saltwater. However this appears to have been more due to leakage and corrosion of the leads rather than the probe itself.