Fertraso Thermo-hygrometer + sensor

Reads temperature indoors and out via a 1.5 metre sensor, with humidity alongside and a memory for the extremes.

The Fertraso Thermo-Hygrometer shows two values together along the bottom (indoor temperature and humidity) with the sensor reading beside them. Wall mounted or on a stand.

VPD: why temperature and humidity only mean something together

Most growers treat temperature and humidity as two separate settings. They are not, and that is why a schedule of "22 degrees and 60 percent" works fine in one room and not in another.

What your crop actually experiences is the difference between how much moisture the air could hold and how much is in it. In horticulture that is called vapour pressure deficit, usually shortened to VPD. That difference is the force pulling water out of the leaf, and that evaporation is the engine driving water and nutrient uptake from the roots.

Why you cannot read it from one number: the same percentage means something quite different at another temperature.

• 70% at 20 °C: the air cannot hold much moisture to begin with, and is already full. Evaporation is close to stalled.
• 70% at 28 °C: warm air can hold far more, so there is room left. The plant evaporates considerably harder here, despite the identical percentage.

Which gives the practical lesson: change your temperature and you change your evaporation, even though your hygrometer still shows the same percentage. A grower who dims the lights and adjusts nothing else has quietly reduced evaporation, and with it uptake.

You do not need to memorise a VPD table. What you do need is to read both values together, and when you change one, look at what happens to the other. A meter showing them side by side is exactly right for that.

Technical Specifications:

• Measures: indoor temperature, outdoor temperature via sensor, and relative humidity
• Sensor cable: 1.5 metres
• Display: dual readout along the bottom: indoor temperature and humidity
• Memory: minimum and maximum temperature
• Placement: wall mounted or on a stand

Frequently asked questions

Can I use the sensor indoors?
Yes. It is called an outdoor sensor, but it is simply a second measuring point. Among your canopy, for instance, while the display hangs by the zip.

Does it calculate VPD?
No, it shows temperature and humidity. Those are the two values VPD follows from; for a calculated figure you need a climate computer.

Does the sensor measure humidity too?
No, temperature only. Humidity is measured at the unit.

Why does it only store temperature?
The memory covers temperature extremes. To capture humidity peaks as well, look at a model with min/max on both values.