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Liquidew I.S. User Manual
1.3
Theory of Operation
Reliable and robust sensor design is fundamental to achieving accurate measurement of
moisture in liquids over a long period of time. Proprietary thick- and thin-film techniques
are applied in the Michell ceramic metal-oxide moisture sensor. Base metal layers, on
semi-conductor grade ceramic substrate, sense dissolved moisture within the sample
liquid flow. The inert materials of the sensor have a high resistance to aggressive media
while the inherent strength of the sensor and the thermal bonded connections to the
active device ensures reliable operation even in dense fluid samples.
Figure 2
The ceramic metal-oxide moisture sensor exhibits a direct response to partial pressure
of moisture vapor. Calibration is certified traceable to NPL (UK) and NIST (USA) through
the use of dew-point transfer standards.
With the Liquidew I.S. the amount of dissolved moisture dispersed throughout the
immiscible process liquid is measured on-line in real time (in units of ppm
content) using Henry's Law.
According to Henry's Law:
Where:
The impedance sensor measures P
The advanced firmware of the Liquidew I.S. provides moisture measurements in ppm
through the application of Henry's Law using pre-programmed saturation concentration
C
(
) values for the most common pure hydrocarbon liquid applications. A proportional
S
mixing setting can be used for mixtures of two solutes, i.e. propane and butane in LPG.
Two user-programmable entry tables enable
for complex fluid compositions where the user may wish to enter
•
their own sources
•
laboratory analysis of process samples
•
estimated values from proportional calculation based on the
each of the major components in the solute mix
Michell Instruments
Structure of the Michell Ceramic Metal-Oxide Moisture Sensor
C
= Moisture content
C
= Saturation moisture content at the analysis temperature
S
P
= Water vapor pressure from dew-point sensor measurement
W
P
= Saturation water vapor pressure at the analysis temperature
sat
C
P
K
/
=
(Henry constant)
sat
S
W.
Permeable Gold film
Hygroscopic Mono-layer
Base Electrode
Ceramic Substrate
C
___
S
C
x P
=
P
W
sat
C
C
. To calculate
, the
C
values to be entered for other solutes or
S
INTRODUCTION
moisture
W
must be known.
S
C
values from:
S
C
values for
S
W
3
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