Gases & Instrumentation - January/February 2012

NATURAL CONVECTION
RADIATION
FORCED
CONVECTION
To+T
MOUNTING LOSSES
d
To

Figure 1. Illustration of thermal conductivity losses: A illustrates basic principle, B outlines exterior loss mechanisms at work

A) THERMAL CONDUCTIVITY LOSSES B) INCLUDING OTHER LOSSES

Figure 2: Typical reference cells

The change in the thermal conductivity with the gas composition for binary or pseudo-binary mixtures is a useful means to determine the concentration of the gas components, provided that the components have different thermal conductivities.

However, the simple relationship shown
in Equation 1 has omitted the other heat
loss mechanisms present. These are
radiation, thermal conduction down the
electrodes or mounting mechanism and
other losses to the gas itself via convection
(Figure 1B). There are two types of convec-
tion: natural convection, which is due to
thermal gradients and forced convection
due to gas flow. Natural convection occurs
when temperature differences within the
gas affect density and hence buoyancy. In
both cases, the heat losses to the gas are
related to the energy required to heat the
gas, which in turn is related to the gaseous
heat capacity and hence gas composition
and pressure. Note that gaseous thermal
conductivity is inherently independent of
pressure, whilst the thermal mass (heat
capacity) is proportionally influenced by
the pressure.

Traditional Reference Cells

The traditional reference cell uses an identical element to the measure element. This element is mounted within the reference cell in the same arrangement as the measure element within the measure cell, differing only in that it is held within a sealed or continuously flowing defined gas mixture, typically the zero or span calibration gas. A reference gas, flowing at the same flow rate a the gas in the measure cell, better matches at calibration the thermal losses being experienced by the measure element. However, the use of such a continuously flowing gas stream requires precise gas composition, pressure and flow control. In addition, the flowing reference gas leads to increased cost of ownership for katharometer users, while regular replacement of the reference gas bottle may require analyzer recalibration (see Figures 2A and 2B for generic examples).