SADP_Operating_ Instructions_0114
8.0 The Sensor
The Shaw High Capacitance Sensor has been undergoing continuous development since it was
invented by Mr. J. L. Shaw in the late 1940’s. Protected by world patents the sensor is manufactured by
a unique automated process, which results in a long life device with fast response, high accuracy and
good stability.
Construction of the sensor starts with an ultra-high purity aluminium wire, which is coated with a
hygroscopic layer and finally covered by a film of porous gold. The gold film and the aluminium core
form the plates of the capacitor. The capacitance value, and the change in capacitance over the
measuring range of each sensor is many times greater than any other device, resulting in a system
which can operate at low frequency [50 or 60Hz] without any risk of interference or pick-up from
external cables or other sources.
Some of the water vapour molecules in the atmosphere surrounding the sensor will enter the dielectric
layer where, due to the extremely small size of the pores, their Brownian motion will be limited, their
energy will consequently be reduced and they will condense into liquid water. Due to the very high
dielectric constant of water [about 80] compared with the other vapours which may be present, this
produces a marked change in the dielectric value of the sensor which is then measured by the
analyser. A dynamic equilibrium will exist between the water vapour outside the sensor and the
condensed water within the pores. This equilibrium is maintained, and the response time of the sensor
can generally be considered to be at least as quick as the system into which it is installed.
Molecules larger than water vapour [one of the smallest gas molecules] cannot enter the pores, making
the sensor resistant to many contaminants and specific to water vapour pressure regardless of the
carrier gas. Even molecules such as Hydrogen which may enter the pores, have such a low dielectric
constant, and will not condense, and so cannot interfere with the reading.
This is a feature unique to the Shaw sensor. All other sensors of the capacitance type are affected by
molecules such methanol which cause great inaccuracy.
8.1 Gaining Access to the Sensor
The sensor is mounted inside the test chamber (head) of the instrument, and is accessible by
removing the test chamber from the instrument as follows:-
1.
Undo the three screws in the base flange of the head assembly and lift the complete head
off the instrument. Pull out the plug from the sensor.
2.
Using a 22mm A/F wrench, unscrew the sensor from the head assembly. Do not leave the
head without a sensor fitted for more than a few minutes, as the desiccant will absorb
moisture from the room air.
3.
When refitting the sensor, push the coaxial plug in firmly and don't forget to fit the brass
spacer.
