
Manual CWD 2005
Measurement principle
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Manual CWD2005.doc
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9.2.1. Specific Gravity measuring cell
The CWD 2005 (The W 2005 is supplied without the Specific Gravity cell) is fitted
with a specific gravity measurement cell, which is mounted in the enclosure. The
specific gravity cell determines the specific gravity of the process gas in a specially
designed sample chamber in the presence of a modulated acoustical field.
9.2.2. Operational overview of the measuring cell
The process gas flows through the measuring chamber of the measuring cell. The
oscillations of a transmitter membrane with a constant amplitude and frequency are
transferred to a pressure transducer by the gas.
The amplitude of the signals on the pressure transducer is directly proportional to the
density of the process gas.
This inherent, oscillating signal is amplified and further processed, in such a way that
a continuous output signal in volts is generated.
The expansion of the sound waves in the measurement chamber is not only
dependent on gas density but also other parameters such as viscosity and heat
conductivity. The selectivity of the measuring cell to gas density is a question of
geometric shape, dimensioning and a few other proprietary factors.
The key to stability is a reference chamber that compares the measurement with
atmospheric air at ambient conditions. The signal received from the chamber with the
reference measurement controls the amplitude of the oscillating transmitter
membrane and the density measurement is insensitive to small particles on the
oscillating membrane. A small heater is used to maintain thermal stability.
Warning
:
The gas pressure in the measuring cell may not deviate considerably
from the ambient pressure in the reference air cell. Any spikes in
sample pressure may result in apparent density deviations that have
nothing to do with a change in sample gas density.
The sensitivity of the measuring cell to external influences, such as mechanical
vibration, impact or pressures are greatly reduced due to its mounting system,
relatively high mass, and electronic filters.