Comparison of Measuring Methods

From Saltwiki

Author: Hans-Jürgen Schwarz
English Translation by Sandra Leithäuser


back to Air Humidity Measurement

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


Table 1: Comparison of moisture measuring methods (acceding 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
-
Microwave 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, visible
- 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,

as well as 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 taken into account. Since this amount of water is provided 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 when temperatures are lower. Other factors apply for sensors with different measuring principles, e.g., chilled mirror hygrometer.

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



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
Electrolytic < 30 min 0 / 100 ppm
Fiber < 10 min 10 / 90 % RH