Three Hygrometers, Twenty Points Apart: Placement Mistakes

Three sensors in one 60 x 45 x 45, read at 4 p.m.: 91, 88 and 64 percent. Same tank, same hour, same model of sensor. The keeper had been running the tank on the 91 and could not understand why his frogs were spending the afternoon in the water bowl.

The 64 was the honest one. It was 10 cm above the substrate, in the middle of the tank, shielded from the spray. The 91 was stuck to the back glass 2 cm above the substrate, in a corner where the misting landed. The 88 was behind a cork panel, in dead air.

Mistake one: glass-mounted, low and near the spray

A sensor on the glass just above the substrate sits in the wettest, stillest air in the tank. Water runs down the glass past it. Droplets land on it. It reads the humidity of the film on its own housing, not of the air the animal is breathing.

This placement typically reads 15 to 25 points high after a misting and stays high for hours because the water is right there. It is the most common mistake and the one that hides a drying substrate for weeks.

Mistake two: in the misting path

A probe hung from the lid, 20 cm down, directly under a nozzle, gets a burst of fine droplets every cycle. The reading spikes to 95 for an hour and then falls. Averaged over a day, it looks acceptable. What it does not show is the 55 percent the tank hits at 3 a.m. on a cold night.

Mistake three: behind structure, in dead air

Behind a cork panel or in a corner filled with plants, air exchange is close to zero. Humidity there tracks the substrate, not the tank, and it stays high. A keeper using that number thinks the tank is stable while the open middle is drying out.

Where to put one

• Height: 10 to 15 cm above the substrate, above the mid-point of the tank’s air volume.
• Position: centre, or a third of the way along, away from glass and out of any nozzle’s cone.
• Shield: a small cover above the sensor, open at the sides, so drips cannot land on it.
• Quantity: two per tank if it matters, one at each end, and read the lower one.
• Calibration: once a year, salt test in a sealed bag, 75 percent at 20 C.

The salt test is worth doing. Put a tablespoon of damp table salt in a small cup inside a sealed bag with the sensor, leave it six hours, and the reading should settle at 75 percent plus or minus 3. A sensor that reads 68 in that test has been telling you the tank is drier than it is, and you have been over-misting to compensate.

The counter-intuitive finding

The sensor that read highest was also the newest and most expensive. Price does not correct placement. In the three-sensor comparison, a cheap sensor in the right position gave a more useful number than a calibrated one in the wrong one.

The second surprise: the tank with the lowest average reading had the healthiest frogs. It was a well-vented tank at 72 percent with a wet corner, and the animals moved between zones. A single high number encourages a keeper to seal a tank, and a sealed tank is where respiratory problems and mould start.

Reading a gradient instead of a number

Take a reading at substrate level, at mid-height, and at the top, at the same time of day, for three days. If the three values differ by more than 15 points, the tank has poor mixing and the animal is living in a gradient it cannot escape. If they are within 5, the tank is well mixed and the number means something.

Placement is also the reason published figures for humidity measurement with polymer substrate vary so much between keepers. Two people can describe the same tank as 90 and 65 percent and both be telling the truth.

Move one sensor tonight to 10 cm above the substrate, centre, away from glass. Log it for three days before you change anything else. The number will be lower, and it will be the one worth acting on.