Instruction manual

NGA 2000 MLT Hardware
MEASURING PRINCIPLE
2 - 15
Emerson Process Management GmbH & Co. OHG
Instruction Manual
90002929
07/2006
2.4 Thermal Conductivity
To measure gases like Hydrogen (H
2
), Argon (Ar) or Helium (He), the measurement method of
thermal conductivity (TC) will be used.
2.4.1 Sensor Design
A complete in glass encapulated temperature resistor is the basis of the measuring.
Four of this resisitors are arranged in a Wheatstone Bridge which is mounted into a block. The block
itself is thermostatted to supress influence of external temperature change.
2.4.2 Analysis Cell
Both the cell volume and the mass of its measurement resistor have been minimized in order to
obtain short response time.
The block contains two gas paths for sample and reference gas. Always two sensors are
located in the sample and the reference gas path.
Depending on application the reference side either is closed gastight and optional filled with
reference gas or it is opened and can be purged with reference gas (corresponding to the
respective measuring application).
Materials in contact with sample gas are aluminum, Viton, glass and gold in the standard type.
Measuring cells stand for corrosive applications, block and tubing are of SiO
2
coated stainless steel
or from Hastelloy according to the measuring application.
Fig. 2-8: Sensor block (thermal conductivity detector TCD)
THERMAL CONDUCTIVITY