Heat conduction from air to water

[Anonymous]

This is Pedro from Tenerife, my greatings to everyone.

I have recently built my own forced air incubator out of plywood and polystyrene (a 60W light bulb as heat source and a computer fan for air circulation). Temperature is controlled with an Eliwell thermostat and humidity monitored with an Exo Terra hygrometer for terrariums. I am about to try it for the first time with fertile chicken eggs and a Hova Bator automatic egg turner which fits perfectly well inside. I have been comparing temperature readings from a clinical thermometer, aquarium thermometer and the digital display of the thermostat to check accuracy, the three of them dipped in a glass of water inside the functioning incubator set to 37.7 ºC. The fluctuation of temperature observed was 37.6 – 37.8 in short intervals (with probe outside the water). I noticed the following:

I observed that the glass of water never reached 37.7 º after operating for more than 12 hours (the whole night through) if the thermostat’s probe was not immersed in it. In other words, air at 37.7 ºC did not warm the water up to room temperature (incubator temperature). The immersed clinical thermometer (with a range of 35-42ºC) didn´t respond, the immersed aquarium thermometer showed a slight temperature rise. Only when I immersed the probe in the glass of water it reached 37.7 by means of excess in air temperature rise. It took about 15 minutes to warm up, and when the thermostat switched the power off the temperature kept rising up to 38.5 as some form of inertia.

Does anyone know if this does happen in any incubator? I fear that the eggs may act in a similar way as water and never warm up to the ideal temperature due to the poor conduction of heat from air to water.

Thanks.

 

[binghamd]

If the thermometers when immersed in the water do not reach 37.7 deg then I think that the heating bulb has not got enough power to heat it up. Could you use something smaller about the size of an egg, to put the water in?
I compared 3 thermometers, a cheap digital, a clinical and a scientific. The digital differed from the other 2 by about 1 degree. Not very reassuring. I was surprised how cheap the clinical thermometer was on e-bay.
I chickened out of building an incubator and bought a second hand one on e-bay.

Just a thought, Is the fan blowing the air from the heater onto the thermostat?

Good luck with the project.

 

[Anonymous]

The thermostat is fixed outside the incubator and only the probe which signals it lays inside and can be shifted around. The fan is placed right behind the bulb and forces air to circulate inside (it does not force air through vent-holes nore directly onto the probe). I am sure that the temperature inside the entire incubator is even. What I find surprising is that the glass of water does not warm up to reach the same temperature after hours.

I will follow your advice and repeat the test using a smaller glass. Perhaps a small spice container will do, they are about the size of an egg. I will also use a 100W light bulb instead of the 60W, though I guess the thermostat will keep the temperature at 37.7º no matter what kind of light bulb I use. I first used a flexible silicone rubber heater because it was supposed to be more reliable (it doesn’t blow), but it took too long to cool down after the relay switched the power off and the temperature kept rising over 38ºC. This may happen because the box is well insulated and the heat source is too powerful for the ammount of air space. A 40w light bulb also worked well apparently.

Thank you binghamd, you are very kind.