Comparison of the Measuring Method

Author: Hans-Jürgen Schwarz

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The table lists the different measuring principles, evaluates them and shows the different criteria for their application.

Table 1: Comparison of moisture measuring methods (according to Weber 1995)

Name	Principle			Properties	Measuring range	Temperature range
Chilled mirror hygrometer	condensation on a chilled mirror	a	g	+ very accurate - expensive; prone to damage by pollution	-40 - +50 °C dew point	0 - 40 °C
Capacitive (Al-Oxide)	change in capacitance	a	g,l	+ small, low cost, fast - drift, chemically sensitive, calibration	-100 - +60 °C dew point	0 - 70 °C
LiCl	LiCl equilibrium temperature	a	g	+ robust, grease resistant - continuous operation required	12 - 100% RH	-10 - 50 °C
Infrared	absorption of infrared light	a	g,l,s	+ solid, contact free - installation, expensive	0 - 30% water content	-
Microwaves	absorption of microwaves	a	g,l,s	+ solids, contact free - installation, expensive	1 - 1000 g/m²	-
Conductivity	measuring the electrical conductivity	a	s	+ solids (wood, concrete), hand held - accuracy	6 - 100% water content	-
Psychrometer	temperature difference wet/dry	r	g	+ robust - needs maintenance	0 - 100% RH	0 - 50 °C
Hygrometer	change in length of hair or synthetic fiber	r	g	+ direct display, rinsible - inaccurate, sensitive to dust	0 - 100% RH	-30 - 110 °C
Capacitive	polymers change in capacitance	r	g	+low price, small, quick - drift, chemically sensitive, calibration	0 - 100% RH	-40 - 180 °C
Electrolytic	current flow through phosphorous oxide	a	g	+ no calibration, ppm range - flow rate, measuring range	5 - 1000 ppm	0 - 40 °C
Resistive	resistance of a hygroscopic layer	r	g	+ low cost, good measuring effect, hysteresis - condensation, temperature influence	30 - 90% RH	10 - 40 °C

a - absolute, r - relative, g - in gas, l - in liquids, s - in solids

Time response

Criteria for the usefulness of sensors are:

• the accuracy of the measurement,
• the transition function,

but also the

• time response.

The time response for most sensors is mainly determined by the amount of water needed for equilibrium. However, the time the water takes to diffuse onto the sensor needs to be considered, too. Since this amount of water can be provided or discharged by the ambient air, the amount of air that flows onto a sensor plays an important role.

Because the diffusion rate is a function of temperature, a sensor will always have a slower reaction the lower the temperature. Other factors apply for sensors with different measuring principles, e.g. chilled mirror hygrometer.

In addition, filters complicate the exchange of gases. When using filter sizes of below 5 - 8 µm no more direct gas exchange takes place, but only gas diffusion.

Table 2: Time response of different measurement methods (according to Weber 1995)

Type	T50-time	Interval skip???????
Resistive	< 60 s	30 / 80 % RH
Capacitive	< 5 min	30 / 80 % R
LiCl	< 5 min	-1 / 10 °C TP
Chilled mirror hygrometer	< 60 s	-20 / 0 °C TP
Elektrolytic	< 30 min	0 / 100 ppm
Fiber	< 10 min	10 / 90 % RH

Capacitance is a measure of the amount of electrical energy stored (or separated) for a given electric potential: C=Q/V [Farad = F] In a parallel plate capacitor, capacitance is directly proportional to the surface area of the conductor plates and inversely proportional to the separation distance and the material between the plates (dielectric).

an official verification of measurement accuracy of instruments.

relative humidity