Except for wet-bulbs, RH is what all humidity meters/hygrometers will read. (Wet-bulbs just give a temperature reading). RH is relative humidity. Relative to what? It is a comparison of the water content of air relative to the water content of the same air at the same temperature when it can hold no more vapour - when it is saturated, when it has an RH of 100% (it is affected a little by atmospheric pressure). Saturated air at 99F will contain a little over 40g (a little short of an ounce and a half) of water vapour for every Kg of air (one Kg of air is around 8000 litres, 280 cubic feet). Incubator air at 50% RH will contain half that amount. Cool that incubator air to 77F and the RH rises to 100% - it is fully saturated. Fully saturated (100% RH) air at 65F contains around a third the amount of water as saturated air at 99F - warm it to 99F and the RH will be around 33%. Fully saturated air at 50F contains around one fifth the amount of water as saturated air at 99F - warm it to 99F and the RH will be around 20%. So what does that mean for running an incubator? Although the water loss from the eggs in an incubator that is full of eggs will make some contribution to the RH in the incubator, unless the ambient temperature is above 77F, you are near certain to need to add water to the incubator and the greater the ventilation rate, the more you will need to add. Even at temperatures over 77F, the RH would have to be very high to avoid the need for adding some water. As you can see, water content, temperature and RH are not simply linked or predictable, one from/to the other. None of the methods used to measure RH use any property/sensor that is simple either - in electronic meters, software calculates RH from a temperature input and the output of the humidity sensor (usually a capacitor) and the changes in the capacitor that the meter measures are not simply linked to RH. For wet-bulb readings it is normal to use simple tables to look-up RH - the complicated maths has been done for you. In mathematical speak - none of these relationships are linear. What does that mean for humidity gauges (including wet-bulbs)? First, it means that calibration at any temperature other than the one you are interested in, means very little - an RH reading at 70F might be correct, but an RH reading at 99F could be way out, or not - one is not a predictor of the other. Second, reading correctly at one RH does not mean that the meter will read correctly at another RH at the same temperature - it may do, it may not. Can life get more complicated? Unfortunately, yes. In electronic RH meters, the software must have a temperature reading and that comes from a thermistor, or in expensive machines, usually an RTD such as a Pt100. They too are not linear devices either, so calibration at a temperature away from where you want to work does not necessarily mean much either. The same applies to other devices used to measure temperature - even mercury thermometers - anyone who has seen a professionally calibrated thermometer will tell you that they can read too high at low and high temperatures and be accurate somewhere between, or some similarly unhelpful and apparently random errors. If incubated at the correct RH (and temperature), eggs loose 12-16% of their fresh weight by the time the chicks break into the air cell (I use 13% and that seems to work fine here). To use weight loss to set humidity, you will need a scale that weighs accurately to 0.1g , 0.01g would be better. If you do use weight loss, you will find that a few eggs will loose too much or too little weight - often not by much - these are usually thin and thick shelled eggs respectively, the eggs that everyone gets that will either struggle or not hatch at all. Many people incubating very valuable eggs - birds of prey and parrots for instance - will have several small machines running at different RH, varying in setting by perhaps 5%. If they find an egg that is loosing too much or too little weight, they move it to another machine with another RH. That way they hatch most of the odd-ball eggs too. Incubated to correct weight-loss, poultry (and all Galliforme) eggs, hatch absolutely fine at incubation conditions. RH control in an incubator is more important than temperature control - if the incubator is a relaible, manufactured machine, the temperature control should be good or better than good due to design. RH control is down to you. Variation of 5%RH is a lot in an incubator. Imagine that the pores in the egg-shell are a series of gates in the perimeter of a sports stadium, and that the water molecules in the egg are a whole group of identical people inside the stadium that want to leave. Can I alter the pores/gates? Not really, not easily. Can I alter the water molecules/people? Not really. So how do I alter the rate that those people can exit the stadium? I alter the number of people waiting outside the gates - I alter the RH. Does it matter where the stadium is? No - the effects are exactly the same anywhere in the world. Does the RH needed for correct water loss change/vary? No - for "normal" eggs, the RH required for successful incubation does not alter anywhere in the world.