Reptile & Amphibian Forums

This is always a hot topic with many different views. In reality temperature and humidity is purely a matter of physics. As Bigworm said in previous posts, temperature and humidity are linked- inextricably. The humidity is a function of the air temperature and water temperature. The air temperature is fairly straightforward, the water temperature would be the temperature of the water in the water bowl, or the temperature of damp cypress mulch or whatever. Misting will increase humidity because much of the moisture will be heated through the effects of bulbs or UTHs on the damp mulch, but it is a temporary effect and not controllable.
Add to this that relative humidity probes are notoriously unreliable. Typical accuracies of inexpensive ($100.00) good quality probes are within 5% to 10% mid range decreasing at higher or lower humidities. Expensive ones (>$500.00) can get within 2%. All need to be recalibrated against saturated salt solutions at least once a year.
As most folks can't stretch to this expense how can humidity be controlled and monitored properly?
Temperature probes are way better than humidity probes with respect to accuracy, precision and robustness.
As temperature is a fuction of water temperature and air temperature, simply put the water-bowl on a heat pad and dunk a thermostat in the water. Control the air temperature with another thermostat as normal and your good to go.
Use the following link to work out what temperatures to use- it's free! Note: Dry bulb = air temperature, wet bulb = water temperature.
http://www.gpengineeringsoft.com/pages/pdtpsychrocalc.html

happy herping

The wet bulb temperature cannot be measured in liquid water. When determining percent relative humidity by temperature, it is calculated from the difference between the air temperature (dry bulb) and the temperature of evaporation from a wet surface such as a wet cloth material (wet bulb) in that same air. Both temperature measurements need to be taken at the same air temperature within the cage, and so you would not want any additional heat on the wet surface above that of the air temperature of the cage or the calculations will be thrown off. Cage air must be passed over both the dry thermometer and the one with the wet cloth wrapped around it within the cage, ideally using a small fan, and both temperatures recorded at the point when the wet thermometer has ceased dropping. The comparison of the two readings is used to calculate the relative humidity. Or these two wet bulb and dry bulb temperatures can be plugged into the psychrocalc software that you supplied, and the relative humidity determined much easier. That is some really great software you found and definitely covers all parameters. And you are also correct in that this temperature procedure, when done carefully, is an extremely accurate way of determining percent relative humidity. Thanks for passing on the software.

Kelly

By the way both post did a fantastic job at articulating the facts.
-----

Hi Kelly,
Nearly right. You can only measure the wet bulb temperature in liquid water. This can either be a pot of water or a damp cloth as you suggest, as long as the damp cloth doesn't dry out. The reason is that the lower temperature of the wet bulb compared to the dry bulb 'wet bulb depression' is caused by the latent heat of vaporisation. This is the energy used to turn the liquid water into water vapour. The dryer the air, the more capacity for evaporation and the greater the wet bulb depression. Some hygrometers use a standard thermometer wrapped in a wick which is dunked in a reservoir, others simply put the thermometer bulb itself in a reservoir. In either case the wick or reservoir must be in contact with the air under test. With the situation I described, the temperature in the water bowl will NOT be the same as the wet bulb temperature INITIALLY, for the reasons that you point out. BUT only initially: If the water in the bowl is cooler than the wet bulb temperature water will condense from the air into the bowl and the wet bulb temperature will fall as the air becomes dryer (and the water level will rise very slightly); if the water is warmer than the wet bulb temperature, water will evaporate from the bowl into the air and the air will become more moist and the wet bulb temperature will rise. After a short period (say, 1 hour) the water and surrounding air reach a steady state and the water temperature and the wet bulb temperature equilibrate. Try it, it works I promise! Control and monitoring in a 1er. I use the same principle to control process air in some manufacturing facilities I work with.

Don't forget that the dry bulb temperature will be unaffected as it is controlled on a separate circuit.

Cheers, and happy herping.

The wet bulb temperature cannot be measured in a container of water for the results to be anywhere near accurate. It can only be measured with the wick and reservoir system (thermometer is never placed in the reservoir), or the damp cloth system you mentioned. A container of water has too much mass to cool the entire volume to the same temperature efficiently by evaporation, and evaporation is the key to these temperature measurements for determining relative humidity. The same air must be passed over the wet bulb and dry bulb thermometers at the same air temperature for the cooling differential, and the calculations for determining relative humidity, to have any meaning. The damp wick or cloth around the thermometer will cool completely in less than a minute and is the basis for the sling psychrometer method of humidity measurement. It is a direct relationship. The drier the air, the higher the rate of evaporation and subsequent cooling of the wick or cloth, and the greater the temperature differential, or depression. This shows that the greater the temperature differential, the less humidity there is in the air. If the wet bulb and dry bulb temperatures ever equalize as you stated, you would be at the dew point, or 100% humidity for a given temperature, and you would no longer have evaporation from the wet bulb to have the cooling effect. This is what your calculations would show when you use the water bowl as the wet bulb temperature, if done correctly, with it at the same temp as the air. Without the efficient evaporative cooling, the wet and dry bulb temps would be nearly equal no matter what the actual humidity and the calculations would show close to 100% relative humidity every time, no matter what the actual humidity is in the enclosure. In process air control, you are changing humidity by adding or removing heat (using the latent heat of vaporization or cooling with condensers), but the actual measurement of that humidity is unrelated to these and still follows the methods I stated above. Great topic for discussion.

Kelly

Hi Kelly,

Sorry my fault, you are absolutely right- but look at it this way. In the case I described the wet bulb temperature is not being measured in the container of water as such, rather it is being *dictated* by the temperature of the container of water. The temperature of the water is being controlled by a water heater and thermostat to a particular temperature. The air temperature is also being controlled to another temperature by a separate heater.
So.- If the wet-bulb temperature is the same as the temperature of the water, there will be no net change; evaporation from the water and condensation from the water vapour in the air will be equivalent. If the wet bulb temperature is less than the temperature of the water, evaporation from the water will exceed condensation from the air until they are the same (and the water heater will have to work a little bit to maintain the water temperature). If the wet bulb temperature is more than the water temperature, condensation from the air will exceed evaporation from the water until they are equivalent once again (and everything in the vivarium will get damp! n.b. this situation should not arise).
It is the moisture in the air that is being adjusted, not the temperature of the water in the container, which is thermostatically controlled and constant.
I think you are saying that the wet and dry bulb temperature of the air is unlikely to change the temperature of a chunky reservoir of water to any great degree. I agree with you. It is the other way around, the reservoir is changing the wet bulb temperature of the air. You are also right about the actual measurement of humidity; it is unrelated to the process control, which is why I don't measure it. I don't need to measure the humidity to know what it is, just as I dont need to measure the temperature of boiling water at atmospheric pressure to know it is 100 °C.

In my previous post I did not mean that the wet bulb and dry bulb temperatures equalise, I meant the wet bulb and *water container* temperatures equalise. The dry bulb temperature will be different (greater).

This is an interesting debate 'cos I think we agree with each other!

Best regards,
Cooljosh's Dad

You would only need the real-time bulb temps if you were either going to calculate relative humidity from a graph or plug those temps into your psychrocalc software to calculate it. I guess all I was saying was that it wouldn't work to use the water bowl temp as the wet bulb temp for making the calculations for the actual relative humidity. What you are doing is changing the dew point by adjusting the temperature differential between the air and the water bowl to either increase or decrease the relative humidity within the cage. The bulb temps really wouldn't mean anything in that case unless you just wanted to know the actual percent relative humidity. I think we just had a confusion in definitions so to speak. Thanks again for the great discussion.

Kelly

Thanks Kelly, I enjoyed the discussion. Sorry for the confusion, but it helped as a useful reality check for me and it's good to bounce ideas around. It's a very simple technique if anyone is concerned about controlling temperature and humidity.

best regards,
Cooljosh's Dad

Now, what dry bulb and wet bulb temperatures should I pick? Is there a definitive answer, or even a consensus on the forum?