A home weather station consists of various meteorological measuring instruments. This captures the so-called weather elements:
Temperature with thermometer
The dependence of electrical resistance in wires to the ambient temperature is used in modern measurement technology for accurate temperature measurements. At the same time, the choice of wire material (a pure metal, often platinum), as well as the diameter of the measuring element, must be taken into account. Ideally, measured resistance and temperature are linear. One thermometer of this measurement is the cold-wire resistance sensor. A clinical thermometer is an example of this category.
Another temperature measurement via the electrical voltage is possible with a thermocouple. For this purpose, two different metal wires are connected to each other at the desired measuring point and provided with electrical voltage. At a reference point, the measuring point temperature can be calculated by the voltage difference between the two metals and the known reference temperature.
Humidity with hygrometer
The measuring principle of a hygrometer for determining the relative humidity is based on the electrical capacity of modern devices. The measuring sensor (a so-called substrate: usually ceramic, glass or plastic) is arranged as a polymer layer between two electrodes. Depending on the humidity, this substrate absorbs water vapor from the air or releases it. As the humidity changes, the sensor measures a change in electrical capacity and converts it.
The best-known measuring device is the hair hygrometer. This mostly human hair is used, which expands depending on the humidity. In addition to animal hair, one often uses synthetic fibers with a known coefficient of expansion. To illustrate the current moisture content of the air, the hair is coupled with a lever pointer and a scale.
Barometric pressure with a barometer
A well-known gauge for the air pressure is the doses barometer. A can with negative pressure, usually consisting of special alloys to reduce the temperature sensitivity, is compressed to varying degrees by the air pressure. In this case, an expansion and with increasing pressure, a compression of the can is stimulated with decreasing pressure. Via a coupling, the change is then transferred to a pointer and visualized on a scale.
Official air pressure measurements are based on an electrical capacitance measurement. In this case, a sensor registers the change in the size of a bending membrane depending on the change in air pressure. As the membrane flexes at high pressure, a vacuum is compressed at the ends of which electrodes are located. The distance between the electrodes thus also decreases and results in a physically measurable change in capacitance. From this value, the air pressure can be finally calculated.
Rainfall with rain seesaw
Many weather stations use the rain waiter or precipitation scoop to determine the amount of rainfall or altitude. It fills a bowl with precipitation. When a certain weight is reached, it tilts down, is emptied and a second chamber catches the precipitate again up to the triggering weight. By the number of tilting movements together with known contents of the shells, the precipitation can be summed up. Latest equipment directly measures the mass of water in a large bowl and calculates the precipitation totals.
Often the Hellmann rain gauge is used. Here, the rain falls through a standard opening of 200 cm² in a metal housing with collecting can. To measure the amount of precipitation, the contained water is simply added to a graduated cylinder.
Wind with anemometer
A cup anemometer with a separate wind vane uses three semicircular trays, each with the same distance to measure the wind speed. In addition, the wind vane is intended to indicate the wind direction. The flag itself always points in the direction of the wind. A disadvantage of the cup star is its inertia so that it continues to rotate after a wind pulse when there is no wind.
A propeller anemometer with integrated wind vane measures both speed and direction of the wind in one component. As with the separate version, the wind vane always points in the direction of the wind. Thus, the propeller is always directed against the wind and can record the maximum speed of the air flow. The speed of the wind is proportional to the propeller revolutions and can thus be calculated. As with the cup star, the inertia of the propeller is measurement uncertainty.
The ultrasonic anemometer is the most widely used variant for wind measurement. The measuring principle is the time required for two sound pulses to travel between the probes. With the known distance of the probes and the transit time of the pulses, the wind speed can be calculated. If you bring the sensors in several spatial axes in juxtaposition, so you can calculate the wind direction. The big advantage of ultrasonic measurement is the short reaction time.
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