Comparison of the Measuring Method: Difference between revisions

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Revision as of 16:39, 9 November 2012

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 their application.


Table 1: Comparison of moisture measuring methods (according to [Weber:1995]Title: Technische Feuchtemessung
Author: Weber, Helmut
Link to Google Scholar
)
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[edit]

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: Response time of different measurement methods (according to [Weber:1995]Title: Technische Feuchtemessung
Author: Weber, Helmut
Link to Google Scholar
)
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


Literature[edit]

[Weber:1995] Weber, Helmut (1995): Technische Feuchtemessung, Vulkan Verlag, EssenLink to Google Scholar