The microprocessor-based XMTC is a compact, rugged,
online thermal conductivity transmitter that measures the
concentration of binary gas mixtures including hydrogen,
carbon dioxide, methane, helium and many others. The analyzer
also combines computer enhanced signal measurement with
fast-response software, real-time error detection and digital
communication via an RS232 or RS485 interface.
Theory of operation
Two ultrastable, precision glass-coated thermistors are used—
one in contact with the sample gas and the other in contact
with the reference gas (such as air in a sealed chamber). The
thermistors are mounted so that they are in close proximity
to the stainless steel (or Hastelloy®) walls of the sample
chamber. The entire transmitter is temperature-controlled,
and the thermistors are heated to an elevated temperature
in a constant-current Wheatstone bridge. The thermistors
lose heat to the walls of the sample chamber at a rate that is
proportional to the thermal conductivity of the gas surrounding
them. Thus, each thermistor will reach a different equilibrium
temperature. The temperature difference between the two
thermistors is detected in the Wheatstone bridge, and the
resulting bridge voltage is amplified and converted to a linear 4
to 20 mA output proportional to the concentration of one of the
constituents of the binary or pseudo binary gas mixture.
