Presentation Title

Thermal Conductivity Meter2011-CH-2362011-CH-2442011-CH-2532011-CH-255By Group

Thermal conductivity analyzer measure the concentration of one component in a binary of gas or impurity of a sample Stream containing a composite mixture of impurities. It does this by comparing the difference in thermal conductivity of Sample stream with that of reference gas of fixed Composition. This is a simple, rugged, inexpensive, reliable, and easily maintained.Introduction

Each gas has the ability to conduct heat at a specific rate. This difference in thermal conductivities can be employed to determine quantitatively the composition of complex gas mixture. Heated metal filaments are exposed to the zero and sample gases. The amount of heat carried away by the gases changes the rate of cooling of the wire filament and, therefore its temperature. This temperature change causes a resistance change and Wheatstone bridge converted it to an electrical current that is available as an output signal.Principle:

Thermal conductivities of common gasesGASTHERMAL CONDUCTIVITYACETYLENE4.400AMMONIA5.135ARGON3.880CARBON DIOXIDE3.393CARBON MONOXIDE5.425CHLORINE1.829ETHANE4.303ETHYLENE4.020HELIUM33.60HYDROGEN39.60HYDROGEN SULPHIDE3.045METHANE7.200NEON10.87NITRIC OXIDE5.550NITROGEN5.680NITROUS OXIDE3.515

There are two type of measuring cell design

1.Two element thermal conductivity cell designs

2.Four element thermal conductivity cell designsMain Components1.Measuring cell

Measuring cellRegulated power supplyWheatstone bridgeCase temperature controlThe major components of a thermal conductivity analyzer are

Four element thermal conductivity cell designs

The measuring cell consists of a relatively large mass of brass metal to provide a stable heat sink. Through the metal block, ow passages one for the reference gas and one for sample gas are drilled. In typical thermal conductivity analyzer four platinum filaments are employed as heat-sensing element. Each of them is placed in a separate cavity in a brass block. The material used for the construction of filament should have high temperature coefficient.

Wheatstone bridge

Two filaments R1 and R2 connected in opposite arms of Wheatstone bridge act as the reference arms whereas the other two filaments S1 and S2 are connected in the gas stream and act as measuring arms. The use of four-cell arrangement serve to compensate for temperature and power supply variations

Two element thermal conductivity cell designsTCD block assembly with the measuring laments inserted into the both measuring and reference chambers. This is a ow-through design using a single (replaceable) measuring and a single reference lament congured into a classical Wheatstone bridge circuit Which have one Reference filament resistance and one sensing filament and two fixed resistors

Reference laments are used to provide compensation for temperature and barometric pressure variations. Because the reference laments are in opposite legs of the bridge relative to the measuring laments, small temperature variations in the cell should affect both laments equally and therefore cancel out. Reference ow is usually 40 to 100 cc/min.

The Reference Filament

Balanced precisely with the help of potentiometer

Working

WorkingWhen the gas stream passes through the measuring pair of filaments the wire are cooled and there is corresponding change in the resistance of the filament. The higher the thermal conductivity of the gas the lower would be the resistance of the wire and vice versa Consequently the greater the difference in the thermal conductivities of the reference and sample gas, the greater the unbalance of bridge.

Balanced precisely with the help of potentiometer

The unbalance current can be measured on an indicating meter on a strip chart recorder .

Only binary mixtures can be accurately measured by it.Design Pressure: Near atmospheric Sample Temperature: 35 to 110F (2 to 43C)Cell Materials of Construction: Brass, stainless steel, MonelInaccuracy: Inaccuracy is 1 to 2% of full scale for binary samples Cost: A portable leak detector with 105cc/sec sensitivity costs $1600; industrial analyzers for binary mixtures range from $2500 to $6000; analyzers with higher sensitivity and/or in corrosion-resistant materials cost from $6000 to $12,000. Characteristics

AdvantagesThe advantages of the thermal conductivity analyzer include its Low costSimplicityReliabilityReasonable speed of response It is used for leakage measurement, chromatography and the detection of hydrogen or helium

References http://commons.wikimedia.org/wiki/File:Thermal_conductivity_analyser.pngHandbook of Biomedical Instrumentation By KhandpurGuild, L., Design and performance of thermal conductivity detectors, Victoreen, 3.Hoffman, Computerized dry-air leak testing for process control, Med. Dev.Diagn. Ind., January 1996.Krigman, A., Process chromatography: difcult becomes routine, InTech,30(10), 3455, 1983.McNair, H. M., Process gas chromatography, Am. Lab., 19(1), 1720, 1987.Tye, R. P., The art of measuring thermal conductivity, InTech, March 1969.Villalobos, R., Process gas chromatography, Anal. Chem., 47(11),983A1004A, 1975.Weiss, M., Three keys to keeping process chromatographs on line, Control,May 1991.Yarborough, D. W., Thermal Conductivity, Kluwer Academic/Plenum, NewYork, 1988.Yaws, C. L., Handbook of Transport Property Data, Viscosity, ThermalConductivity, Gulf Professional Publishing, Houston, TX, 1995.