Temperature and humidity sensor (Part II)
- Mar 23, 2018 -
Method of measurement
Humidity sensitive resistor
Humidity sensor of several common measurement methods 
Humidity measurement technology comes from already a long time.With the development of electronic technology, modern measurement technology has a rapid development.From the principle of humidity measurement division 2, more than 30.Have absolute humidity on the representation method of humidity, relative humidity, dew point, moisture and the ratio of dry gas (weight or volume) and so on.But the humidity measurement has always been one of the world famous problem in the metrology area.A seemingly simple values, underneath it, involves quite a complicated physical and chemical theory analysis and calculation, ChuSheZhe may be ignored in humidity measurement must pay attention to many factors, which affected rational use.
Common humidity measurements are: dynamic method (dual pressure method, dual temperature method, the shunt method), the static method (saturated salt method, sulfuric acid method), dew point method, wet and dry bulb and a variety of electronic sensor method.
This dual pressure method, dual temperature method is based on the thermodynamics of P, V, T balance principle, balance time is longer, shunt method is based on the absolute humidity and precise mixture of absolute dry air.The adoption of modern measurement and control means, these devices can be quite precise, but due to the equipment complex, expensive, operation time consuming work, mainly for the use of standard measurement, the measurement precision can be up to + / - 2% RH - plus or minus 1.5% RH.
Saturated salt method, the static method is the most common method of humidity measurement, simple and feasible.But the saturated salt law of liquid and gas two-phase equilibrium is strict, higher requirements on the stability of the environment temperature.Requirements to use for a long time to balance, low humidity point for longer.Especially in the indoor humidity and humidity difference is bigger, the bottle open all need to balance 6 ~ 8 hours at a time.
Dew point method is to measure the temperature of the wet air reaches saturation, is a direct result of the thermodynamics, high accuracy, wide measurement range.Measurement with a precision dew-point meter accuracy can reach + / - 0.2 ℃ or higher.But with the modern optical - electrical principle of the cold mirror dew-point meter is expensive, and often standard humidity generator systems.
Wet and dry bulb method, this is the 18th century was invented by wet method.With a long history, the most commonly used.Dry wet ball method is an indirect method, which use wet and dry bulb equation conversion humidity value, and this equation is conditional, namely wind speed near the wet bulb must reach more than 2.5 m/s.Common use of dry wet bulb this condition is simplified, so its accuracy is only 5 ~ 7% RH, significantly lower than the electronic humidity sensor.Wet and dry bulb also obviously does not belong to a static method, don't simply think that as long as improving the measuring accuracy of two thermometer is to improve the measurement precision hygrometer.
This article would like to highlight two points: first, because the humidity is a function of temperature, temperature changes decisively influence the humidity measurements.By whatever method, measure and control temperature accurately is first.Notes even a constant temperature and humidity box, good heat insulation work indoor temperature also has certain gradient.So the space humidity is difficult to entirely uniform.
Second, due to the differences in principle and method, all kinds of measurement method is difficult to directly between the calibration and maintain, most can only use indirect way to compare.So between the two methods of moisture proofing all wet process (0 ~ 100% RH) relative humidity measurements, or want to be in all the temperature, the measurements at various points within the scope of the calibration is a very difficult thing.Such as ventilation psychrometer requires regulation wind speed the flow of air, and strict laws of saturated salt sealing, both can't compare.Best method or by the state on humidity measuring instruments calibration system (standard) and verification procedures to deliver way step by step.