This document describes the experimental process of developing a non-mechanical hot plate wind and rain sensor. It aims to discuss practical aspects of the necessary hardware to fulfil such a function and different software approaches to controlling such a system. Specifically, it discusses an experiment based on 30W hot plates controlled by Labview in an attempt to measure changing wind and rain conditions. It also looks at the data gathered in this instance and calibration of the sensors. Finally it looks at ways of improving the functionality of such a system, through improved hardware design and better software control.
The measurement of wind speeds and rain rates are important in terms of atmospheric monitoring and meteorological measurements. This project is to develop a combined wind-rain sensor using hot metal plates. For wind speed measurements, the moving air carries heat away from the hot plates at a rate dependent on wind speed. Providing power to maintain the hot-plate temperatures at their set point values can provide the quantification of wind speed. Similarly, raindrops landing on one of the hotplates will evaporate causing a further power drain on this plate. This project is to design, build and calibrate this sensor using Labview.
By keeping the two heated plates at a constant set point temperature it is possible to measure the power necessary to maintain this temperature. The change in power over time needed to maintain this state can be equated with wind removing heat from the plates. We can also evaporate water off one of the plates and measure the work done by the system in evaporation. The further energy required by this plate for water evaporation when measured will constitute a rain sensor. These plates will from now on be referred to as the rain plate or top plate and the wind plate or bottom plate.
If the bottom plate is kept dry while being exposed to the wind it can effectively act as a reference plate to evaluate how much extra work is being done to the plate exposed to the rain also. The plates can be seen as a point source in terms of wind exposure since the heated area is small, being ~35mm*65mm. By measuring the work done on the bottom plate from some initial reference point an estimate of the cooling effect of the wind on the plate can be made, e.g. if it takes 10W/hr to maintain a temperature of 25°C in a dry wind free environment of 20°C and it takes 2W/hr to maintain a temp of 21°C in the same environment then in this case if there is an additional 2W/hr used in the system we can say that a wind chill factor of –1 is present. By using a chart of wind chill factors and an equation to calculate the wind chill velocities we can estimate the wind speed. The current standard equation describing wind chill is:
Ideally this equation can be solved with a temperature reading derived from the wind plate and compared to a chart of wind chill factors to find a velocity value. The problem with this approach for laboratory purposes is that the equations results becomes unreliable for temperatures above about 10°C and since all experimental readings were conducted at room temperature i.e. ~23°C it was difficult to use this method satisfactorily.
For the evaporation of rain the latent heat of evaporation or enthalpy of evaporation of water DHvap is tabulated and has been measured as
Assuming that at 90° the value is 41kJ/mol and that the molecular mass of water is 18.01508 gram/mol then using this value we can say that:
If we also take into account the temperature of the Aluminium tray and the heat it passes to the water then we should get an estimate for how long it will take the system to evaporate 1ml of water. For the Al tray the specific heat is taken as 938J/kg/°K and the temperature drops by about 8°C when 1ml of water is applied. Using
References: 1. Marsh, K N., Ed., Recommended Reference Materials for the Realization of Physicochemical Properties, Blackwell, Oxford, 1987.
5. Temperature-Electromotive Force Reference Functions and Tables for the Letter-Designated Thermocouple Types Based on the ITS-90. Natl. Inst. Stand. Technol. Monograph 175; 1993. 630 p.
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