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4 Micro Motion 3098 Gas Specific Gravity Meter
Introduction
If the sample gas pressure rises above that of the reference chamber pressure, the pressure control
valve opens to vent the excess gas via an outlet orifice in the enclosure side, so that the sample gas
pressure is reduced to equal the reference gas pressure. For gas to flow it is necessary that the supply
pressure is greater than the reference pressure, which in turn must be greater than the vent pressure.
(Typically the line pressure must be between 15% and 25% above that of the reference chamber
pressure).
Note: The principles of operation that describe this operation are given in Appendix B.
A pressure gauge is fitted in order to monitor the pressure within the gas density meter. This is
desirable when charging the reference chamber and also for general maintenance.
Electrical connections to the 3098 specific gravity meter are taken through the cable gland in the
enclosure side and then into the density meter’s electronics housing.
When the enclosure is sealed, the complete instrument is insulated so that rapid changes in ambient
temperature will not upset the temperature equilibrium of the unit and produce thermal shock errors.
Note: The 3098 specific gravity meter may have been supplied without an enclosure – see Safety
guidelines on page 1.
1.3.1 Meter sensing element
The gas density meter consists of a thin metal cylinder which is activated so that it vibrates in a hoop
mode at its natural frequency. The gas is passed over the inner and outer surfaces of the cylinder and
is thus in contact with the vibrating walls. The mass of gas which vibrates with the cylinder depends
upon the gas density and, since increasing the vibrating mass decreases the natural frequency of
vibration, the gas density for any particular frequency of vibration can be determined.
A solid state amplifier, magnetically coupled to the sensing element, maintains the conditions of
vibration and also provides the output signal.
1.3.2 Installation
The 3098 specific gravity meter has been designed to be installed mounted to a wall (wall mounted), a
typical installation set-up being given in Figure 1-3 below.