Theory Of Operation; Figure 2 Structure Of The Michell Ceramic Moisture Sensor - Michell Instruments Liquidew I.S. User Manual

Liquidew I.S. User's 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-fi lm techniques
are applied in the Michell Ceramic Moisture Sensor. Base metal layers, on semi-conductor
grade ceramic substrate, sense dissolved moisture within the sample liquid fl ow. 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 fl uid samples.
Figure 2
The ceramic moisture sensor exhibits a direct response to partial pressure of moisture
vapor. Calibration is certifi ed 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 fi rmware 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 fl uid 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 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
. To calculate , the
C
values to be entered for other solutes or
S
INTRODUCTION
moisture
W
C
must be known.
S
C
values from:
S
C
values for
S
W
3