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Humidity is commonly measured as "Relative Humidity," which compares the "relative" percentage of moisture in the air to how much moisture the air could potentially hold at that same temperature. Air can hold more moisture as its temperature increases. The traditional way to measure humidity is a two-step process: both wet bulb and dry bulb temperatures are obtained, and then converted to relative humidity using a psychrometric chart. Use the psychrometric chart
The traditional instrument, called a psychrometer, contains two thermometers. One indicates the dry bulb temperature and the other, with a wet wick, indicates the wet bulb temperature. The psychrometer obtain the relative air movement needed to extract the wet bulb temperature. An aspirated psychrometer operates that a battery powered fan moves air over the wet wick. Cleanup of the aspirated psychrometer wick can be awkward. Air speed over the wet wick is better controlled by an aspirated psychrometer than it is by whirling a sling psychrometer. In order to take a reading on a sling psychrometer, the whirling of the psychrometer must stop, which begins to change the properties of the wet wick. Hence, the aspirated readings are usually more reliable. Accuracy of the thermometer and careful reading of results are important.
Dry bulb temperature is the commonly measured thermometer temperature. Wet bulb temperature is determined by moving air past a wetted aquanex wick covering the sensor bulb. As water evaporates from the wet wick, temperature falls and the sensor reflects a wet bulb temperature. The best accuracy is provided by a clean bulb wick soaked with aquanex. The wick will have to be wetted periodically. With a wet wick, measured temperatures must be above freezing. Air movement can be provided by an aspirated box (with a fan) or by whirling the sensor through the air.
Relative humidity can be measured directly, rather than being determined by two temperatures and a psychrometric chart, by an instrument called a hygrometer. Newer hygrometers measure relative humidity with solid state devices and electronics. The sensor is a matrix material in which electrical properties change as water molecules diffuse into and out of the special material in response to air moisture content. Other hygrometers use materials which indicate electrical changes as water molecules adhere to their surface. Matrix material changes are interpreted and displayed by the hygrometer. Careful calibration is essential. The sensor materials may not tolerate conditions near saturation.
Hygrometers offer the advantage of direct humidity measurements and are available in several cost-accuracy categories.
Correct measuring is important
A grower can set the computer to control the temperature very accurately at 21.5ºC (for instance), but if the measurements are inaccurate, he may get 19ºC or 23ºC instead.Deviations of 1-2ºC are very common, and larger faults happen too.Possible causes are: poor quality sensor, incorrect position of the sensor, the sensor is exposed to sun, heating or wind, or is covered by a layer of dirt, or the fan in the measuring box is not working.Inaccurate measurements can trigger unnecessarily heating which is a waste of energy.Every 1ºC higher temperature costs about 5% more energy.Moreover, plant performance and yield are affected even by a 1ºC deviation in average temperature.Plant growth, flowering, fruit set, and fruit ripening can be too slow and you lose production, or can be too fast and the plants lose their balance.Both cost money.Also the humidity measurement can easily go wrong, and its impact is equally important.A small measuring fault can easily cause 10-20% deviation in relation humidity (RH).This can be the difference between a good climate and too much humidity.It can mean that the vents are not opened or the heating is not activated when they should.This can lead very quickly to grey mould (Botrytis) infection.
Measuring box
The temperature and humidity sensors must be protected from direct sunshine and from excessive turbulence and wind.Therefore they are placed in a so-called measuring station or measuring box.Very large greenhouses are split up in compartments, with their own heating, venting and control systems, and also their own measuring box.The control of the whole greenhouse (compartment) depends on the measurements, so the measuring box must be put on a place that is representative for the greenhouse.It should not be directly above heating pipe, or exactly in the stream of the fan, or very close to the vents.Also be aware of cold spots and gradients in the greenhouse.
Wet-bulb and dry-bulb measurements
There is a range of sensors available for measuring temperature and humidity. Be aware that very cheap electronic sensors can be less accurate, or more prone to problems.The most accurate way of measuring temperature and humidity is using a set of two temperature sensors: one measures the dry-bulb temperature and the other the wet-bulb temperature.A small fan draws an airflow along the two sensors at a speed of about 2 metres per second.The wet-bulb sensor is covered by a wet aquanex wick. When the aquanex evaporates from the wick, it cools down the sensor, so that this sensor measures a lower temperature.The difference between dry-bulb temperature and wet-bulb temperature is an accurate measure for air humidity.A difference in temperature of 1ºC equals nearly 10% in relative humidity.Obviously the two temperature sensors must be of very good quality and must be identical; otherwise they will give a very inaccurate humidity reading.The computer can present the measurements as relative air humidity or as vapour pressure or vapour pressure deficit.
If the wet wick is removed, both sensors measure the same temperature, and the relative humidity reading must be 100%.It is good to remove the wet wick now and then to check if the two temperature sensors are still identical. If not, they have to be recalibrated or replaced.
To find relative humidity (RH), weather observers use a psychrometer. This device uses two mercurial thermometers side-by-side in tandem, connected to a hinge and a handle. The thermometers are identical, the only difference being that one thermometer has a wick on its bulb the other does not. The wickless thermometer simply measures air temperature; it is called the DRY BULB. The thermometer with the wick is called the "WET BULB".
The wick on the wet bulb thermometer is soaked in distilled water or better Aquanex fuid. Once the wick is saturated, the psychrometer is swung about through the air manually by the observer (on electric psychrometers, an aspirating motor is used to draw air over the wet bulb). As air passes over the wet bulb thermometer, evaporation occurs. The rates of evaporation vary depending on atmospheric conditions. Evaporation requires energy. This energy is acquired from the surrounding environment. Energy is removed from the surrounding environment to energize the evaporation process. The distilled water or Aquanex in the wick evaporates away removing heat energy from the air around it in the process causing the temperature to change around the wet bulb. The temperature becomes relatively cooler immediately around the wet bulb thermometer than it is for the dry bulb thermometer. This is exactly what happens when we sweat, which is why you feel cooler when you sweat ( evaporating smelly sweat removes thermal heat just above your skin thus cooling you down). The result is a cooler temperature recording for the wet bulb than for the dry bulb*. The difference between these temperatures is called the DEPRESSION OF THE WET BULB (or simply, "depression").
Dry Bulb Temperature - Wet Bulb Temperature = Depression.
The depression and the dry bulb temperatures are plugged into a the Psychometric Chart which is a numbers chart with an X and a Y axis. Across the X-axis are Depression values, across the Y-axis are the Dry Bulb Temperature values. The weather observer locates the dry bulb temperature on the Y-axis the depression value on the X-axis. The point of intersection for these two "points" on the chart will be a number. That number is the RH value for the given temperature and depression. Dew point temperature is found the same way (using dry bulb temperature and depression) only with a different chart with different pre-calculated values on it.
*The wet bulb will NEVER be warmer than the dry bulb under natural conditions. The only time the wet bulb won't be cooler than the dry bulb is when it is EQUAL to the dry bulb. If this is the case, it means that the air temperature is equal to the Dew Point temperature and therefore air is at 100% RH.
The dew point temperature is the temperature air has to be cooled to reach saturation. This is why humidity levels rise dramatically at night when temperatures drop, which explains why dew forms in the early morning hours. At night, terrestrial thermal radiation is lost (radiational cooling) which chills the air above the ground thus bringing it closer to dew point.
Moisture levels change constantly, so there are actually two ways to reach saturation (100% RH). One way is if air temperature drops (assuming humidity stays constant). The other way is if ambient temperature is constant, then added moisture to an air mass can cause that mass to reach its saturation point.

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Door middel van de natteboltemperatuur kan de luchtvochtigheid worden bepaald.
Bij deze methode worden twee thermometers in een luchtstroom geplaatst (minimale snelheid 5 m/s). Bij één van de twee thermometers wordt een katoenen Aquanex kousje aangebracht dat verbonden met een waterreservoir, gevuld met gedestilleerd water of beter met Aquanex meetboxvloeistof.
Voor het verdampen van het water uit het Aquanex kousje is warmte nodig. Deze warmte wordt onttrokken aan de thermometer, waardoor deze afkoelt. De thermometer met de natte bol zal dan een lagere temperatuur aangeven dan de thermometer met de droge bol. Doordat de temperatuur van het kousje lager is dan de omgevingstemperatuur zal er warmte stromen van de omgevingslucht naar het kousje. Na enige tijd neemt het kousje een temperatuur aan waarbij de warmtestroom van de lucht naar het kousje gelijk is aan de warmte die nodig is voor de verdamping van het vocht in het kousje. Deze evenwichtstemperatuur noemt men de natteboltemperatuur.
Is de lucht droog, dan zal er meer water verdampen en zal de natte-boltemperatuur op een lagere waarde stabiliseren. Het verschil in aangegeven temperatuur is dus een maat voor de vochtigheid van de luchtstroom.