HortResearch Publication - Humidity in Greenhouses
Humidity can be expressed in various ways: as relative humidity (RH in %), absolute humidity (in g/m3), moisture deficit or vapour deficit (also in g/m3), vapour pressure deficit (VPD in kPa or Pa), and dewpoint (in oC), as described previously in this series. For sophisticated greenhouse control we are interested in two aspects of humidity: RH and VPD. RH (or alternatively dewpoint) is used for disease control, while VPD is the best indicator for transpiration control. In fact it does not matter which form of humidity is measured, because it is easy to compute all the different forms of humidity from one humidity measurement, provided also the temperature is measured. So as long as humidity and temperature are measured very reliably and very accurately, it is not important which type of humidity sensor is used.
At present two types of sensors with different measuring principles are commonly used in greenhouses: number one is the dry and wet-bulb instrument or psychrometer, and second is the electronic humidity sensor or capacitive sensor. Other principles, like the hair-hygrometer, have been used in the greenhouse sector, whereas some new measuring principles are employed in other areas and are being tested for suitability in greenhouses. We will only consider the two current type of humidity meters.
Electronic sensors, mostly capacitive sensors, are usually relatively cheap, and easy to install and maintain. They produce a signal that is converted in the computer to RH or another humidity reading. They have a major disadvantage, that they easily become unreliable, in particular at high humidity levels. They are usually good when new, but they quickly deteriorate under the rough conditions in the greenhouse like changing humidity and spraying. In tests, most electronic devices, even the more expensive ones, became inaccurate within 3 to 9 months after purchase. If you choose to use an electronic sensor for humidity control, it should be calibrated every month, particularly under wet conditions. Mind that calibration under wet conditions with another electronic device is very dubious.
A simple and yet ingenious method to determine air humidity is with an instrument that contains two thermometers, also called bulbs: one measures the normal air temperature, the other is continuously wet and measures the wet-bulb temperature. The schematic drawing below shows the principle. (This is of an older type of Priva sensor, with thanks to Priva). In the drawing #1 and # 2 are the dry and wet-bulb thermometers, respectively. The wet-bulb (#2) is covered by a wick (black in the drawing) that is hanging in a small reservoir with clean water (#8). The wick sucks water and keeps the wet-bulb continuously wet. A built-in fan (#4) maintains a constant air flow along the two bulbs. The air flow absorbs water from the wick, which cools the wet-bulb. The dryer the air flow is, the more water will be absorbed from the wick, and the more the temperature of the wet-bulb will drop. Under rather dry conditions, the wet-bulb is a few degrees cooler than the dry-bulb. Under humid conditions, the dry and wet-bulb are almost the same. Under 100 % RH in the greenhouse air, the dry and wet bulb should be exactly equal.
The two signals, the one from the dry-bulb thermometer and the one from the wet-bulb thermometer are transferred to the greenhouse control computer. Here the signals can be converted to any form of air humidity, either relative air humidity, absolute air humidity, vapour pressure deficit or dew point of the air. Normally, however, only one form of humidity is displayed by the computer, usually RH.
Disadvantages of this sensor are the higher price and the need for frequent maintenance. First of all the bottle must be filled with clean water when empty. This can be needed once in a few days during dry weather, or once in a few weeks when it is humid. The need for filling also depends on the size of the reservoir, of course. The water must be absolutely clean, and the reservoir itself must be cleaned when needed. When the wick is not clean, either filthy by algae or fatty due to greasy fingers, it needs to be replaced carefully. Whenever the wick fails to absorb water, the wet-bulb becomes a dry-bulb, and the temperature difference between the two bulbs will be zero, thus the calculated RH will be 100%.
This is also the way to test this instrument: remove the wick, and make sure that both bulbs are clean and dry. Then the RH reading must be close to 100%, which means that the dry and wet bulb measure the same temperature. If that is not the case, you better ask your supplier for service or replacement.
As the dry and wet-bulb instrument is rather expensive, most growers will have only one instrument, or hopefully one per compartment. If this reading is used for humidity control, it is very important the sensor is placed at a good spot. In most greenhouses there is a huge variability in temperature and humidity, both in vertical and horizontal directions. Make sure that the measuring spot is characteristic for the whole greenhouse, and not too close to a heater, heating pipe, fan, wall etc. Also the bulbs must be shielded from sun shine. You can choose to hang it either at crop height, and also move it when the crop grows, or to have it always at the same height. Good positioning, as well as good maintenance of the sensors, can make an enormous difference in the costs and the effectiveness of the humidity control.