Psychrometer

Psychrometers are test instruments for determining air humidity. In the true sense of the word, this only includes the air humidity meters that determine the ambient humidity from the temperature difference between a dry temperature sensor and a temperature sensor enclosed by moist material. The sensor enclosed by moist material usually shows lower temperatures because it is cooled down by the evaporation of the water. Psychros comes from the Greek language and means frosty. In the human body, cooling also works through evaporation (i. e. sweat or damp clothing.) How well this cooling works depends above all on the humidity of the environment. The velocity of the air also plays a role because when the air is exchanged, new air is brought in which can again absorb water vapour. The temperature that appears on the wet bulb thermometer - in contrast to the air temperature - is called the humid temperature or wet-bulb temperature. Hygrometers that do not determine the air humidity via two thermometers are often called psychrometers. A more general term that includes all devices for measuring air humidity is hygrometer.

The German Meteorological Service (DWD) uses traditional psychrometers, e.g. a fan-ventilated Assmann psychrometer . In this device, both thermometers, dry- and wet-bulb, are protected from the effects of radiation and are ventilated at a speed of about 2.5 m/s for three to five minutes. The more moisture evaporates during this process, the more the wet-bulb thermometer cools down. This is because energy is required for the change from liquid water to water vapour. However, the possibility of evaporation only exists as long as water vapour can be absorbed by the air. This can be tested on your own body in very humid air, for example in a tropical house. At very high humidity, sweat and damp clothing hardly dry off and therefore do not have a cooling effect.

Types of digital psychrometers
Digital psychrometers offer the advantage that the humidity value is already determined in the device and displayed digitally. The humidity must therefore no longer be calculated by the user with the help of the two temperature values via a formula or taken from charts. Only a few digital psychrometers still work with evaporative cooling. In these instrument models, for example, a wick kept moist by a water tank ensures reliable cooling of the wet bulb thermometer. Such psychrometers allow long-term measurements with comparatively high accuracy.

However, the majority of digital psychrometers do not determine the humidity from the wet and dry temperature but via a capacitive or resistance sensor. With this humidity sensor, the humidity in the environment is determined by the changing resistance or capacitance values. An additional temperature sensor, which may be located on the same component, measures the temperature. Many of these psychrometers can determine and display the wet bulb temperature and dew point from the measured values for temperature and humidity via internal procedures.

Selection criteria for digital psychrometers
Before actually looking for a device, it should be determined which functionalities the psychrometer must have. The following section briefly discusses some key differences to look out for when making a selection.

Psychrometers for stationary or mobile use
Depending on whether the measurements are to take place at a fixed location or at various locations, a model for fixed installlation or a mobile device can be chosen. For continuous monitoring in greenhouses, wine cellars, archives and storage rooms or at fixed measuring points for air-conditioning technology, stationary models that can record the measured values or transmit them to other devices are suitable. In contrast, mobile devices with battery operation should be used for measurements at different locations. Portable psychrometers are usually equipped with a display for direct indication of the measured values. However, there are both mobile and stationary device variants that do not display the measured values on site but save them or transmit them wirelessly.

The sinter cap as additional equipment for some psychrometers To prevent the sensors from being affected by deposits of water or solid particles when measuring in very humid or dusty environments, special sinter caps are available for some devices. These do not only protect the meter from liquids and dirt but also protect the sensor from mechanical stress. The sintered cap acts like a filter. The small pores allow gaseous molecules to pass through and retain the liquid and solid particles. Sintered caps can be unscrewed and cleaned separately.

Measured and calculated values of a psychrometer
In addition to temperature and humidity, many digital psychrometers can also determine the wet bulb temperature and the dew point. The wet bulb temperature is the lowest temperature that can be achieved by evaporative cooling. It occurs when the water vapour release of a moistened surface is in equilibrium with the water vapour content of the ambient air. The cooling effect results from the energy required for the transition from water to water vapour. If the relative humidity of the ambient air is 100 per cent, i.e. the air is saturated with moisture, no more water can evaporate, so the air temperature and the wet bulb temperature are the same. In this situation, slight temperature drops or additional water vapour cause the excess water vapour to turn into liquid water. This water is called condensation. The dew point indicates which temperature a surface should exceed in order to prevent condensation from forming on it.

The additional values that can be calculated by some devices include absolute humidity. Psychrometers equipped with additional sensors can also measure air pressure or flow velocity, for example. The measured velocity can also be used to calculate the volumetric flow for ventilation ducts or other pipes for gases if the cross-section can be determined. Some devices are equipped with an extreme value function that allows the minimum and maximum values of the last measurement series to be displayed. To ensure that critical values are noticed in good time even without constant monitoring, there are also models with an alarm function. These warn visually, acoustically or even by email when previously set limit values are reached.

Calibration of a digital psychrometer
Digital psychrometers are supplied calibrated or factory-adjusted. To ensure that the specified accuracy is achieved even after prolonged use under possibly critical conditions, they should be calibrated at regular intervals. For this purpose, the instruments can be sent to a specialised calibration laboratory. There, the accuracy of the measurements for humidity and temperature is checked according to specified procedures and a certificate is issued. If the device-specific accuracy is not achieved during the test, the device can be readjusted or repaired if necessary. Some hygrometers can also be calibrated by the user. The temperature, for example, can be checked by comparison measurements with a thermometer of higher accuracy and for the humidity, there are calibration kits with different humidity standards.