Short answer: An incubator display is only as good as its sensors, and sensors drift, especially in Ghana's heat and harmattan dust. Compare the thermometer and hygrometer against a reference you trust, once a month and after every power cut, then write down the difference and correct for it. A machine that reads true lets you use the temperature and weight-loss numbers in the chart, instead of guessing.
Key takeaways
- Check the thermometer against a separate reference at egg height, not just at the probe on the wall.
- Use a saturated salt test to check humidity; it gives you a reference point at about 75 per cent.
- Write the offset down and apply it, so the display and the real temperature are never confused.
- Recheck after power cuts, dust storms and any knock to the machine.
- Keep one reference thermometer for calibration only, and store it safely between checks.
A thermometer that lies for three weeks
The display on a machine is the most trusted number in the hatchery, and it is often the least checked. The digital readout looks precise to a tenth of a degree, so growers assume it is accurate to the same level. It usually is not. Sensors drift with age, heat and dust, and a probe that sits a few centimetres from a heater reads differently from one at egg height.
At our hatchery in Kasoa we have found machines reading half a degree or more away from the eggs' real environment. The machine did not break. Its sensor sat in the wrong place, or its calibration had slipped over a year of use. Nobody noticed because the display looked steady.
A lying thermometer does not announce itself. It lets the embryos develop a little too fast or a little too slow for three weeks, and then you open the machine and see a poor hatch. The fix is a routine check, not a new machine.
What a degree or two does to an embryo
A small error in temperature does not kill an embryo outright in most cases. It does shift the timing of development and the balance of the hatch. A machine that runs warm can bring chicks out early, sometimes weak, sometimes with the yolk poorly absorbed. A machine that runs cool can stretch the hatch and leave late chicks that never quite catch up.
Humidity works the same way. Too dry and the eggs lose weight too fast, the air cell grows too large, and the chick can dry down before it hatches. Too wet and the chick cannot lose enough water, and it struggles to pip through the membrane. A hygrometer that reads 10 per cent off gives you a wrong picture of how much water the egg has lost, which is one of the main checks in the 21-day chart.
Our notes on incubation temperature and humidity give the settings you aim for, and how an egg incubator works shows where the sensors sit inside the machine. Calibration tells you whether the numbers you are reading are the numbers the eggs are actually getting.
Two thermometers, one reference, one day
The most reliable check is simple. Buy or borrow a second thermometer that you trust, ideally a calibrated digital probe with a recent certificate or one you have compared against a lab-checked unit. Keep it in a box when it is not in use, away from dust and knocks. Use it for calibration and nothing else.
Run the test for a full day at your normal setting, so the machine has time to settle and you catch the swings. Place the reference at egg height, in the middle of a tray, beside the machine's own probe if you can. Do not put it on the water pan or against the wall. Take a reading every two hours, write both values in the log, and note the time.
- Let the machine run at its setting for at least four hours before you start.
- Place the reference and the machine's probe side by side at egg height.
- Take readings every two hours for a full day, including the night.
- Subtract the machine reading from the reference reading each time.
- Average the differences to get your offset, and note whether the machine runs high or low.
If the offset stays within about 0.3 degrees, the machine is good enough for normal work. If it is more than about 0.5 degrees, correct for it or get the sensor checked before you set eggs.
Ice water as a low-end sanity check
A quick sanity check uses crushed ice and clean water. Fill a cup with crushed ice, add just enough cold water to make a slush, stir, and wait a few minutes. Put the thermometer probe in the middle of the slush without touching the cup. The correct reading is close to zero degrees Celsius.
This check does not tell you much about the setting temperature of 37.5 degrees, which is what matters for the eggs. It tells you whether the sensor is broadly honest across its range. A probe that reads three or four degrees off at zero has a problem worth fixing. A probe that reads half a degree off at zero is probably fine for incubation, but you still need the two-thermometer test to know its behaviour at the setting.
Do not use ice water with a probe that is not rated for it. Dry the probe completely before you put it back in the machine. If the probe is part of a machine you clean often, our routine in cleaning and disinfecting an incubator shows where to keep moisture away from the sensor.
Where to put the probe, and where not to
Placement causes more false readings than the sensor does. A probe that is close to the heater reads high, and one that sits in the draught from the fan reads low. A probe under the water pan picks up the cool of the water and reads low. The reading that matters is the one the eggs feel.
- Put the probe at egg height, in the middle of the tray, not at the edge.
- Keep it away from the heater element, the wall and the water pan.
- Do not let it touch an egg or a tray wire.
- Check that the fan moves air across the probe rather than blowing it straight at the sensor.
- Mark the spot so the probe is put back in the same place after every clean.
The position you choose for calibration should match the position you use every day. If you check the machine at one spot and then move the probe for a clean, the offset you calculated will not fit the next run.
Checking the hygrometer with a salt jar
Humidity is harder to check than temperature, but there is a reliable reference. A saturated solution of common salt in a sealed space holds the relative humidity at close to 75 per cent, at normal room temperatures. You can make one at home with table salt and clean water, and a clear jar with a lid.
Put a little water in the bottom of the jar, add salt until it stops dissolving and a small amount stays visible at the bottom, and leave a shallow layer of the mixture on a dish so it does not touch the hygrometer. Place the hygrometer inside the jar, close the lid, and leave it for six to eight hours or overnight in a cool, steady room. Read it without opening the jar more than you need to.
A good hygrometer should read close to 75 per cent. If it reads 70 or 80, it is out by that much, and you need to correct for it. If it reads very differently, check the batteries and the sensor before you trust it again. Our article on fixing incubator humidity problems covers the practical side of what to do when the numbers are wrong.
When you have no salt: the wet and dry bulb method
If you cannot make a salt test, a wet and dry bulb pair will give you a reading you can check against a printed psychrometric table. Wrap a thermometer bulb in clean cotton that is kept wet with distilled or clean water, and let it hang in the air beside a dry thermometer. The wet bulb reads lower because the water evaporates and cools it. The gap between the two readings tells you how dry or humid the air is.
This method takes more care than a salt jar. The cotton must stay wet, the air must move past the bulb, and the readings must be taken once they have settled. It works well for checking a hygrometer against a known value, and it is a useful habit if you run a machine in a room where you cannot easily control the air.
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Recording the offset and using it
Calibration is only useful if you write the result down and act on it. The offset is the difference between the reference and the machine, and you apply it every time you read the machine. The table below shows an example for the thermometer.
| Date | Machine reads | Reference reads | Offset (reference minus machine) | What you set |
|---|---|---|---|---|
| Week 1 | 37.5 | 37.9 | +0.4 | Set the display to 37.1 to get 37.5 at the egg |
| Week 5 | 37.5 | 37.6 | +0.1 | Within tolerance; keep 37.5 and note it |
| Week 9 | 37.5 | 38.1 | +0.6 | Too far out; service the sensor before setting |
In the first row, the machine reads 37.5, but the egg is at 37.9. To bring the egg to 37.5, you set the controller to 37.1, so that the real temperature comes down by the offset. Write the number you set beside the number you wanted, so the next person who uses the machine does not reset it to the wrong value.
Do the same for humidity. If the hygrometer reads 5 per cent low against the salt test, the real humidity is 5 points higher than the display, so set the display 5 points below your target.
Ghana's conditions: harmattan dust, heat and power cuts
Calibration in Ghana has three enemies that growers in cooler countries do not face in the same way. Harmattan dust settles on sensors, shifts their readings and sometimes blocks the fan. Heat in the room can push a machine's internal temperature up, so the display drifts from the eggs. Power cuts stop the machine, reset the controller and sometimes leave the display showing a value that was never set.
In December, wipe the probe and the fan grille at least weekly, and check the offset more often than the monthly routine. In the hot months, check that the machine is not standing in direct sun or next to a hot wall. After every power cut, check the display setting first, then take a reference reading before you assume the machine is back to where it was.
Our article on incubating through power cuts in Ghana covers the backup side. Good backup power is wasted if the controller comes back on the wrong setting.
How often to check, and when to replace
A simple routine works well for most small hatcheries. Do a full two-thermometer check once a month, and a quick reference reading every time you set eggs. Check the salt test for humidity at the same time as the monthly thermometer check, or whenever a hygrometer looks wrong.
Replace a sensor or a whole unit when the offset keeps growing, when the display freezes or jumps without reason, or when a calibration cannot be brought back within tolerance. Keep the replaced unit out of the hatchery so nobody uses it by mistake. Our poultry record keeping guide shows how to fit calibration into a log you already keep.
Common mistakes and a checklist for the monthly check
Most calibration problems come from a few habits. Growers trust the first reading after a power cut, forget that the probe moved during a clean, or treat a single reading as the offset. Here is a checklist we give growers who ask us to run through their machines.
- Confirm the set point on the controller before you take any reading.
- Place the reference at egg height, in the same spot as the normal probe.
- Take readings over a full day, including the night.
- Work out the offset from the average, not one reading.
- Apply the offset to the setting and write the number in the log.
- Check the humidity with a salt test or a wet and dry bulb pair.
- Clean the probe and fan grille, and dry them before you put them back.
- Store the reference thermometer in its box until the next check.
If you are new to machines altogether, our egg incubator guide for beginners is a better place to start. If you run a hatchery and want to compare calibration notes, or if you are buying day-old chicks and want to know how our machines are checked, you can message us on WhatsApp at +233 24 915 1503. You can also read more about our hatchery or place an order through the order form.
Questions farmers ask
How accurate does an incubator thermometer need to be?
For setting eggs, aim for a machine that agrees with a trusted reference to within about 0.3 degrees Celsius at egg height. Up to about 0.5 degrees is tolerable if you correct for it in your settings. Anything beyond that should be fixed before you set eggs, because small errors add up over 21 days.
What is the salt test and how do I do it?
Put a little water in a sealed jar, add salt until a small amount stays undissolved, and place the hygrometer inside with the lid closed. Leave it for several hours in a steady room. A good hygrometer reads close to 75 per cent. If it does not, correct for the difference in your humidity settings.
Can I use a kitchen thermometer as my reference?
Only if you have checked it against a known standard first. Many kitchen probes are built for food, not for steady readings at 37 degrees over days, and they can drift. A reference you trust is one that has been checked recently against a calibrated instrument.
How often should I calibrate my machine?
Do a full thermometer check once a month and a quick reference reading each time you set eggs. Recheck after any power cut, cleaning, dust storm or knock to the machine, because each of those can shift the sensor or the display.
Why does my machine read the right temperature but the hatch is still poor?
The display may be correct at the probe but wrong at the eggs. Probe placement, airflow and humidity can all change what the eggs feel. Check the reading at egg height with a reference, and then look at humidity and turning before you blame the sensor. Our guide to <a href="/blog/why-eggs-dont-hatch-incubation-problems/">why eggs don't hatch</a> lists the other causes to rule out.
Do I need to calibrate a hatcher differently from a setter?
The same method works for both, but the settings differ. Check each machine at its own working temperature and humidity, and record the offset for each one separately. Do not assume that a setter offset applies to a hatcher.
