waterproof digital thermometers

[erics1997]

Hi! I have been looking online for 2 hours for inexpensive waterproof digital thermometers. I need to buy 6 of them for my science fair project. I am looking for a kind that would allow me to measure the temperature of waterbottles in the fridge over a period of time (without having to open the door throughout the day). Each bottle will have different types of fabric wrapped around them so I won't be able to see inside the bottle. It would need to be one I could submerge with a cord that I could have sticking up through the cap and then ideally outside on the door of the fridge. Could anyone help me find them. I am no supposed to spend more than $50 according to my teacher. Thank you! Eric

[rmarz]

erics1997 - You might want to look into Thermistors as a remote sensor. Thermistors will change resistance with temperature and would allow electrical wires to lead out the door to the outside where the resistance (temperature) could be measured remotely with an ohmmeter or similar setup (Wheatstone bridge, meter). These are quite inexpensive, many under $1-2 each. One caveat, the lower price units are not hermetically sealed, so you might need to dip them in an epoxy or similar material to insure they still function over time in water. Higher resistance levels, like 1-10K ohms will minimize copper losses in the wiring. You may have to calibrate each one with an accurate thermometer to give you valid measurements. I don't know if your experiment is over hours or days. Here is a link to a source.

Rick Marz

http://www.newark.com/thermistors?isRedirect=true

[erics1997]

Thank you Mr. Marz. I will look into that. Is it difficult to get a temperature reading off of one of them? Eric

[deleted-71588]

These are RTD (Resistance Temperature Devices). The resistance of the device is a function of its temperature. You would use a DVM (Digital Volt Meter) on the Ohms scale connected to the leads to measure the resistance. You then have to convert the resistance to temperature using an equation like: T = a * Ohms - b (where a and b are calibration constances for a specific RTD device). As Rick mentioned, you might have to do your own calibration of the devices you end up with depending on the accuracy you require. Typical RTD devices used in digital thermostats from the same manufacturing lot vary less than a couple of degrees in the 40 to 90 degree F range. Depending on the accuracy, you maybe able to come up with a single set of calibration constants for all the RTDs.

I would caution you that water expands when it freezes so imersing an RTD in water and then freezing it may cause physical damage to the RTD unless the container will easily expand without crushing the RTD case or connections.

[rmarz]

Eric - I don't think it is too difficult to get a reading using a digital multimeter or ohmmeter. Attached is a typical data sheet for a Thermistor, and one of the sheets shows a table of resistance values compared to the resistance at 25 degrees C. I would guess your temperature range of interest is actually pretty narrow, around freezing, so that this approach may actually be more accurate than a graduated thermometer. You might even be able to use extrapolation to get accurate measurements to one tenth of a degree C. An inexpensive digital multimeter will probably give you accuracy to 3 significant digits, but that would seem to be accurate enough given the rates of resistance changes with temperature suggested by these data sheets. If you have studied any calculus, this would be dR/dT, and seems to be a pretty measurable number from reviewing this data sheet.

Rick Marz

http://rocky.digikey.com/WebLib/BC%20Co ... istors.pdf

[deleted-71588]

http://rocky.digikey.com/WebLib/BC%20Co ... istors.pdf

Beware, the ones in this link are "glass envelope" which I definitely would NOT recommend for imersion in freezing water.