Incubator Humidity Converter: Wet Bulb to Relative Humidity and Back

Type in your incubator’s temperature and its wet-bulb reading and the converter gives the relative humidity. Or type the humidity you want and it gives the wet-bulb reading to aim for. Either way it lays the duck egg targets from Cornell, Kansas State, Mississippi State, Metzer Farms and Texas A&M alongside, each turned into a wet-bulb reading at your temperature.

Key takeaways

• A wet bulb reads lower than the incubator temperature, and the gap is the humidity: the drier the air, the more the wet wick cools. At 99.5 °F a wet bulb of 85.0 °F means about 55%, 86.9 °F about 60%, 90.4 °F about 70% and 93.6 °F about 80%.
• The published duck targets differ. Across the seven sources here, setting figures (days one to twenty-five) run from 45% to about 63%, and lockdown-to-hatch figures from 60% to 81%.
• Several sources print a humidity and a wet-bulb reading side by side that do not quite agree. The converter shows each source’s own figure and says where the pair is out.
• A wet bulb needs moving air. In a still-air incubator treat any reading as rough.
• Weighing the eggs is the better check. Cornell’s target is about 14% of the setting weight lost by day 25.

What a wet bulb measures

A wet-bulb thermometer is an ordinary thermometer with its bulb wrapped in a wet wick. Water evaporating from the wick cools it, and the drier the air, the faster the water evaporates and the lower the reading. In saturated air nothing evaporates and the wet bulb reads the same as the dry one; in dry air the gap between them is large.

Kansas State University’s incubation guide describes making one: cover the bulb with a piece of cloth, dip it in water, place it in the incubator level with the eggs, and watch the temperature until it stabilises before taking the reading. Oklahoma State’s extension service describes the same thing with a wet cotton cloth around the bulb.

Many older incubation guides give humidity only as a wet-bulb reading, in degrees, not as a percentage. That is why this converter exists: to put Kansas State’s 86–88° wet bulb and Cornell’s 55% on the same scale.

How the conversion works

The converter uses the psychrometer formula the US National Weather Service publishes for working out relative humidity from a dry-bulb and a wet-bulb temperature. It finds the water vapour the air is holding from the wet-bulb reading and the gap between the two thermometers, then divides by the most the air could hold at the incubator’s temperature. A wet bulb only reads correctly with air moving over the wick, as a fan provides in a forced-air incubator — more on that below.

Before relying on it we checked it against published incubator tables. It agrees with every cell of Mississippi State’s wet-bulb table — 24 values at 99 to 102 °F — to within about 0.6 of a percentage point, with Brinsea’s table at 99.5 °F to within about 0.8 of a point, and with Oklahoma State’s table, which prints whole numbers, to within about a point. It also matches the University of Missouri’s worked example: 88 °F wet bulb at 101 °F is 60%.

Going the other way, from a humidity to a wet-bulb reading, the converter solves the same formula backwards, so the two directions always agree exactly.

The duck targets, source by source

Duck incubation humidity is one of the places the published advice genuinely differs, and the converter shows each of the seven sources below rather than an average of them.

• Cornell University’s Duck Research Laboratory sets eggs at 99.5 °F and 55% humidity, which it prints as 84.5 °F on the wet bulb. On day 25 the eggs move to a hatcher at 99 °F and 65% (88 °F wet bulb), raised to 80% (93 °F wet bulb) once pipping starts.
• Kansas State University gives wet-bulb readings only: 86 to 88 °F during incubation and 92 °F at hatching, for large forced-air incubators at 99¼ to 99½ °F. By the formula that is about 58 to 63% while setting and about 75% at hatching.
• Mississippi State University‘s species table gives ducks a wet bulb of 85 to 86 °F at 100 °F — about 54 to 57% — and 90 °F, about 68%, for the last three days. Its general text for all eggs puts setting at 58 to 60% and hatching at 65% or more.
• Metzer Farms uses 99.5 °F with a wet bulb of 86 °F, or 57 to 58%, for days one to twenty-five, and 98.5 °F with a wet bulb of 94 °F, or 81%, for the hatch.
• Texas A&M AgriLife gives 58 to 62% at 99.5 °F while setting and 66 to 75% at 98.5 °F for hatching, and the same for Muscovy ducks, whose eggs move to the hatcher on day 31 to 33.
• The British Waterfowl Association gives 55%, rising to 60 to 70% for hatching, at 37.4 °C (99.3 °F).
• Brinsea, the incubator maker, gives 45 to 55% for duck eggs at 37.5 °C (99.5 °F) in its handbook — the lowest published range here.

Put side by side, setting humidity runs from the bottom of Brinsea’s range, 45%, to about 63% at the top of Kansas State’s, and hatching humidity from the BWA’s 60% to Metzer’s 81%. Most of the duck-specific figures cluster at 55 to 58% for setting, which is what Quackoria’s hatch date calculator shows.

When a source’s two numbers disagree

Checking every published pair with the same formula turns up a few that do not line up, and it is worth knowing why before you chase a reading that cannot be reached.

Cornell’s 55% and 84.5 °F agree at 99 °F rather than at its 99.5 °F setting temperature — 84.5 °F is exactly Mississippi State’s table figure for 55% at 99 °F. At 99.5 °F the same wet bulb works out at about 54%. The difference is small and does not matter to the eggs.

Metzer’s hatching pair, 94 °F and 81%, agrees at 99.5 °F, not at its 98.5 °F hatching temperature, where 94 °F works out nearer 84%.

Mississippi State’s is the largest gap. Its general text gives 58 to 60% or 84 to 86 °F wet bulb as if they were the same thing, but by Mississippi State’s own conversion table 84 to 86 °F at 100 °F is about 52 to 57%, and its duck row’s 85 to 86 °F about 54 to 57%. The converter uses the duck row’s wet bulbs for Mississippi State. Where a source prints both a percentage and a wet bulb, it uses the percentage and says so, and it turns each source’s figure into the wet-bulb reading to aim for at your temperature.

Height above sea level

The psychrometer formula depends on air pressure, and air pressure falls with height. Put in your height above sea level and the converter works out the pressure with the National Weather Service’s station-pressure formula and uses it. The effect is modest but real: at about 5,000 feet, a wet bulb of 84.4 °F at 99.5 °F means 55%, where at sea level it takes 85.0 °F. If you live high up and follow a sea-level wet-bulb figure, your air is a point or two wetter than you think.

Still-air incubators

A wet bulb only reads correctly with air moving over the wick. Brinsea, the incubator maker, says that both wet bulbs and hygrometers need air flow and do not give accurate results in still-air machines. Kansas State’s and Mississippi State’s duck settings are for forced-air incubators; Mississippi State adds 2 to 3 °F to the temperature for still air. If your incubator has no fan, use the converter as a rough guide only and lean on weighing.

Weigh the eggs

Humidity is only a means. What matters is that the egg loses the right amount of water, so the air cell is the right size when the duckling needs it. Cornell’s figure is about 14% of the weight at setting lost by day 25 for common duck eggs; the British Waterfowl Association gives about 15% across incubation. Too little loss means the air is too humid; too much, too dry. The egg weight-loss tracker plots your weighings against those targets and tells you which way to move the humidity.

For the whole timetable — candling, lockdown and the hatch — use the hatch date calculator and duck egg development day by day, and incubating duck eggs for the practical guide. If the eggs are from a broody duck rather than a machine, the broody duck guide is the one to read.