How to calculate wet bulb temperature and why?

The wet bulb temperature is the temperature value in contact with water in a liquid state. In other words, it is the value that takes into account the humidity in suspension (air humidity). There are two methods for obtaining this value precisely.

This data is fundamental in the case of prevention and defence against the risk of frost. Fruit and wine growers are particularly dependent on the risk of frost, especially in spring, from March to May. During this period, the buds start to form. A frost at this time of year risks burning the buds, which are not yet strong enough. The risk is major: losing all or part of the harvest!

Monitoring the wet temperature makes it possible to anticipate the minimum temperature that can be reached, and thus to act at the right time.

At Sencrop, we are fully aware of the need to accurately anticipate the onset of frost in order to reason and, above all, to activate your anti-frost protection (spraying, heating, air mixing) at the best possible moment. This is why we already integrate a frost sensor in your connected weather stations!

Two types of frost sensor

There are 2 methods to calculate the wet bulb temperature:

• The capillary measurement is done with a dedicated physical sensor (e.g. Comsag, Pessl, weenat...). In this sensor, the wet bulb temperature is calculated by a thermometer surrounded by a water-soaked lace.

• The measurement by algorithmic calculation is calculated from the temperature and hygrometry measurements (e.g. Sencrop, Davis, Meteus, Pessl...).

Sencrop has chosen this second method, by integrating an algorithm from the most recent scientific publications on the subject of wet bulb temperature.

With the Leafcrop, you benefit from a nomadic sensor to be placed in the heart of the orchard / vineyards, which allows you to get a precision data at the most risky place of your plots.

Comparison test: why opt for algorithmic measurement?

To understand the difference between the two calculation methods, Martin, quality engineer at Sencrop, carried out a comparative experiment: he compared the wet temperature readings, defining thresholds that simulate critical situations for farmers.

Test carried out on 15 days of night frost between the end of March and the beginning of April 2021 (5200 measurements), taken at the same height.

Results with a threshold at > 0°C

• Physical sensor measurement: 110 triggers
• Sencrop calculated measurement: 113 triggers.

Test conclusion:

For comparison, in more than 99% of the cases, the 2 solutions measure the same wet bulb temperature. The physical solution of the gel sensor therefore has no particular advantage compared to a solution by algorithmic calculation.

However, there are clear disadvantages to the physical sensor:

• A physical sensor is an additional investment for only 2 months per year. At Sencrop, the frost solution is already included in the weather station!
• The capillary solution needs very regular maintenance: during the risk of spring frost, it takes only 3 weeks for the water to evaporate. The calculation of the wet bulb temperature may therefore no longer be accurate. Almost daily maintenance of the sensor is then necessary, and the interest of the remote weather is lost.

• Some physical sensor models do not have a shelter and are exposed to the sun, with a significant impact on the life of the sensor. Indeed, it only takes a few months for the capillary lace to be baked by the sun! The station is then no longer usable in its current state for the following season. At Sencrop, measurements under cover avoid this exposure and extend the life of the sensor.

The argument of the physical sensor is finally a mainly marketing issue... which costs the farmer a lot of money 💸