低成本乾濕球濕度計之研製

A Study on Low Cost Dry/Wet-Bulb Psychrometer

李健(Chien L.) ； 鍾瑞永(J. Y. Chung) ； 楊清富(C. F. Yang)

《臺南區農業改良場研究彙報》 64期 (2014/12) Pp. 73-86

摘要

本研究提出一個低成本乾濕球濕度記錄器的設計，將空氣樣本藉由微型風扇抽進流道內，流道內放置一組校正後精度±0.1℃之乾濕球溫度計。溫度計採用低成本且小尺寸的半導體溫度計TMP37，球頭尺寸5×5×4mm^3。由於此濕球尺寸遠小於一般PT-100溫度感測探頭，具有反應速度快及水分蒸散量少的優點，亦可搭配0.35W低耗電量微型風扇。採用三階多項式飽和蒸氣壓近似公式，使計算相對濕度及蒸氣壓差的複雜度大幅降低，並且能夠在一般低成本微控制器中快速執行。根據Goff-Gratch 公式的誤差分析，以此方法得到相對濕度讀值準確性在15℃～50℃溫度範圍內能達到-0.7～+1.5。記錄器可將資料以指定格式傳送到micro-SD卡作儲存或顯示在文字LCD上，與兩個精確度為±2％之參考感測器相互比較，其相對濕度讀值差異可於約±2％以內，此外並執行溫室環境中的耐久測試。本研究證明低成本及低耗能之高可靠度乾濕球濕度計是可行的，且將可成為未來發展太陽能無線感測網路的基礎技術。

關鍵字

乾濕球濕度計 ； 濕球 ； 相對濕度 ； 飽和蒸氣壓 ； 蒸氣壓差

並列摘要

A low-cost architecture of psychrometer data logger is provided, where the air sample is pumped by a mini-fan into a channel with a pair of dry/wet-bulb temperature sensors arranged inside with accuracy of ±0.1℃, the wet bulb size is about 5×5×4mm^3. The bulb, smaller than the ordinary PT-100 sensor, provides the advantages of fast response and saving water for evaporation; besides, its corresponding mini-fan of 0.35W also provides a low power-hungry solution. Both dry/wet bulb temperatures are used to calculate, with high efficiency, the relative humidity and vapor pressure deficit in a low-cost microcontroller on basis of the 3rd-order polynomial approximation equation for saturation vapor pressure. The error analysis with the Goff-Gratch equations promises that the accuracy for relative humidity reading is within -0.7~+1.5 at 15℃~50℃ range. By programming in the microcontroller, the data can be displayed on a text LCD and stored in a micro-SD card with specified format and time interval. Through the comparison test along with calibrated humidity sensor of ±2% accuracy, the difference is within ±2% in relative humidity reading, beside that, a field test in greenhouse is also executed. It proves that this psychrometer, based on the off-the-shelf components, provides cost-effective and reliable performance.

並列關鍵字

Psychrometer ； Wet Bulb ； Relative humidity ； Saturation Vapor Pressure