How do I calibrate an incubator thermometer?

Compare it to a known reference. The simplest is an **ice bath**: crushed ice plus a little water should read **32°F** — the difference is your offset. Because incubation happens near 100°F, it's even better to cross-check against a **fever-checked medical thermometer** or a trusted known-good thermometer at egg height, then adjust your incubator setting so the true temperature lands in the **99.5–100°F** forced-air band.

How do I do the salt test for a hygrometer?

Mix about **1/2 cup of table salt** with just enough water to make a thick, wet slush (all the salt still visible). Seal the cup and your hygrometer together in a zip-top bag or clear container — not touching — and leave it at stable room temperature for **8–12 hours**. The air over saturated salt sits at **75% relative humidity**, so a correct hygrometer reads 75%. Whatever it reads, the gap from 75% is your offset.

Why shouldn't I trust the incubator's built-in thermometer?

Built-in displays and cheap dial thermometers can be off by more than a degree or two — and at incubation temperatures, a degree or two is the difference between a healthy hatch and cooked or stalled embryos. The readout is frequently the reason a hatch fails. Calibrate a separate instrument against a known reference so the number you set is the temperature the eggs actually experience.

Where should I put the thermometer and hygrometer in the incubator?

At **egg height** — the level the eggs actually occupy. In a **still-air incubator** measure at the **top of the eggs**, where you want 100.5–102°F, because warm air layers upward. In a forced-air unit the fan evens things out, but still keep sensors in the egg zone and away from the heater, walls, and water tray.

How long should I run the incubator before setting eggs?

Run it **empty for at least 24 hours** with your calibrated instruments in place. This lets the temperature and humidity settle and reveals a drifting thermostat or a room that swings warm before eggs are at risk. Confirm it holds 99.5–100°F (forced-air) or 100.5–102°F at egg-top (still-air) before you set.

How often should I recalibrate?

Re-verify before **every set**. Thermometers can drift and hygrometers especially lose accuracy over time and with humidity exposure. A quick ice-point check and salt test between hatches keeps you honest and takes an evening.

How to Calibrate an Incubator Thermometer and Hygrometer

Verified September 2026

The short version

Don't trust the incubator's built-in readout. Before you set eggs, calibrate a separate thermometer against a known reference (an ice-point bath or a fever-checked medical thermometer) and check your hygrometer with the salt test — the air over a saturated salt solution settles at 75% relative humidity, so a correct hygrometer should read 75% ± a couple of points. Only once your instruments are proven should you dial the incubator to the forced-air target of 99.5–100°F (100.5–102°F at the top of the eggs in a still-air unit). Place the sensors at egg height and let the whole thing stabilize for 24 hours empty before eggs go in.

Why calibration matters: a 1–2°F error ruins a hatch

Chicken embryos develop in a very narrow thermal window. Mississippi State University Extension puts the forced-air target at 99.5–100°F and warns that temperatures only a degree or two above that can kill embryos within 15–30 minutes. A cheap dial thermometer or the incubator's built-in display can easily be off by more than that — which means an incubator that reads a perfect '99.5' can actually be sitting at 101–102°F and cooking eggs, or at 97–98°F and stretching incubation into weak, late hatches.

The instrument, not the incubator, is usually the thing that lies. Trusting the built-in readout without checking it is one of the classic reasons for a failed hatch. Calibration is the cheap insurance that the number you set is the temperature the eggs actually feel. The same logic applies to humidity: a hygrometer reading 45% when the real value is 30% will have you fighting a bone-dry incubator with no idea why the air cells are growing too fast.

Calibrating a thermometer against a known reference

You have three reliable references, from most to least precise. Ice point (32°F / 0°C): fill a glass with crushed ice, add just enough cold water to fill the gaps, stir, and let it sit a minute. Insert the thermometer probe into the slush without touching the sides or bottom. A correct thermometer reads 32°F; note how far off it is and apply that offset at incubation temperature.

A fever-checked medical thermometer: a digital medical thermometer is manufactured to be accurate near body temperature (about 98–100°F), which is conveniently close to incubation temperature. Seal it in a small plastic bag, rest it beside your incubator thermometer at egg height, let both stabilize, and compare — the medical thermometer is your reference in exactly the range that matters.

A known-good reference thermometer: if you already own a thermometer you trust (a lab or NIST-traceable unit, or a second instrument you have ice-point checked), set it next to the one you are testing, let them equalize for 20–30 minutes, and read both. Whichever method you use, write down the offset — for example, 'reads 1°F low' — and adjust your incubator setting so the true egg temperature lands in the 99.5–100°F band.

The salt test for hygrometer calibration

Hygrometers drift, and there is a simple physics-based reference to check them against: the air above a saturated salt solution stabilizes at 75% relative humidity at room temperature. This is a known property of sodium chloride, not an incubation setpoint, which is what makes it a trustworthy calibration target.

To run it: put about 1/2 cup of table salt in a small cup or jar lid and add water a little at a time until you have a thick, wet slush — damp sand texture, with all the salt still visibly present, not dissolved into clear water. Place the cup and your hygrometer together inside a sealed clear container or a zip-top bag so they share the same air but the hygrometer does not touch the salt. Leave it undisturbed at stable room temperature for 8–12 hours.

Read the hygrometer through the bag without opening it. A correct instrument reads 75% RH. If it reads 70%, it is 5 points low — add 5 to every future reading (or adjust it if the unit allows). If it reads 80%, it is 5 points high. Now you know your hygrometer's offset and can trust it to hold 45–55% during incubation and 65–75% at lockdown, the ranges Mississippi State Extension cites for chicken eggs.

Placing sensors at egg height

Temperature is not uniform inside an incubator, and it matters most where the eggs are. In a still-air incubator (no fan) warm air layers toward the top, so you measure at the top of the eggs and run the higher 100.5–102°F band there. A sensor sitting on the floor of a still-air unit will read cold and tempt you to overheat everything above it.

In a forced-air incubator a fan mixes the air, so the cabinet is closer to one temperature — but you should still position the probe and hygrometer sensor at the level the eggs occupy, not resting against a wall, the heating element, or the water tray. Suspend or prop the sensor so it reads the same air the embryos are in, and keep the display readable through the window so you never have to open the lid to check it.

Stabilize 24 hours before setting eggs

A freshly assembled incubator does not hold its final temperature the moment you plug it in. Run it empty for at least 24 hours before setting eggs, with your calibrated thermometer and salt-checked hygrometer in place, and watch that it holds steady through a full day-and-night cycle. This catches a drifting thermostat, a room that swings warm in the afternoon, or a unit sited too close to a window — all common causes of a temperature spike.

Once it holds 99.5–100°F (forced-air) or 100.5–102°F at egg-top (still-air) with humidity in the 45–55% range, and you have applied your instrument offsets, it is ready. Re-verify the calibration before every set — thermometers and especially hygrometers drift over a season. Then set your eggs and follow the day-by-day schedule.

Frequently asked questions

How do I calibrate an incubator thermometer?: Compare it to a known reference. The simplest is an ice bath: crushed ice plus a little water should read 32°F — the difference is your offset. Because incubation happens near 100°F, it's even better to cross-check against a fever-checked medical thermometer or a trusted known-good thermometer at egg height, then adjust your incubator setting so the true temperature lands in the 99.5–100°F forced-air band.
How do I do the salt test for a hygrometer?: Mix about 1/2 cup of table salt with just enough water to make a thick, wet slush (all the salt still visible). Seal the cup and your hygrometer together in a zip-top bag or clear container — not touching — and leave it at stable room temperature for 8–12 hours. The air over saturated salt sits at 75% relative humidity, so a correct hygrometer reads 75%. Whatever it reads, the gap from 75% is your offset.
Why shouldn't I trust the incubator's built-in thermometer?: Built-in displays and cheap dial thermometers can be off by more than a degree or two — and at incubation temperatures, a degree or two is the difference between a healthy hatch and cooked or stalled embryos. The readout is frequently the reason a hatch fails. Calibrate a separate instrument against a known reference so the number you set is the temperature the eggs actually experience.
Where should I put the thermometer and hygrometer in the incubator?: At egg height — the level the eggs actually occupy. In a still-air incubator measure at the top of the eggs, where you want 100.5–102°F, because warm air layers upward. In a forced-air unit the fan evens things out, but still keep sensors in the egg zone and away from the heater, walls, and water tray.
How long should I run the incubator before setting eggs?: Run it empty for at least 24 hours with your calibrated instruments in place. This lets the temperature and humidity settle and reveals a drifting thermostat or a room that swings warm before eggs are at risk. Confirm it holds 99.5–100°F (forced-air) or 100.5–102°F at egg-top (still-air) before you set.
How often should I recalibrate?: Re-verify before every set. Thermometers can drift and hygrometers especially lose accuracy over time and with humidity exposure. A quick ice-point check and salt test between hatches keeps you honest and takes an evening.

Sources

Important incubation factors — Mississippi State University Extension Service (University extension)
Incubation temperature requirements — Mississippi State University Extension Service (University extension)
Hatching Quality Chicks (incubator setup & instruments) — Mississippi State University Extension Service (University extension)