A katharometer , or a thermal conductivity detector (abbreviated as DTP ), is a universal detector, very often used in gas chromatographs , which is based on the principle of changing the resistance of materials to temperature. According to GOST 17567, a “katharometer” is considered an unacceptable term; instead, a “ thermal conductivity detector ” is prescribed. [one]
Content
Principle of Operation
An incandescent filament of metal with a high temperature coefficient of resistance ( W , Pt , their alloys, Ni , etc.) is placed in the cavity of the metal block of the accident. As a result of passing through a DC thread, it heats up. In the case when the thread is washed with a pure carrier gas, it loses a constant amount of heat and its temperature remains constant. The impurity gas coming from the chromatographic column has other thermal conductivity indices; therefore, the temperature of the filament also changes . This leads to a change in the resistance of the thread, which is measured using the Winston bridge . The comparative carrier gas stream washes the R4 filament, and the gas coming from the chromatograph column washes the R3 filament. The bridge will be in equilibrium if both strands have the same temperature and, therefore, the same resistance. If you change the composition of the gas leaving the column of the chromatograph, then the resistance of the filaments of cells R3 and R4 changes, the equilibrium is violated and an output signal is generated. The detector responds to all components, with the exception of the carrier gas, and does not destroy them. [2]
Most accidents use two filaments (in cells R3 and R4, blown with gas). R1 and R2 are typically fixed or adjustable resistors . In some designs (for example, Agilent accident), a single-thread design is used, to which the flow from the column and the comparison flow are directed in turn.
Application
It is recommended to use helium or hydrogen as the carrier gas , since their thermal conductivity is very different from the thermal conductivity of most substances measured in gas chromatography. However, there are cases when it is necessary to measure precisely helium or hydrogen in a gas mixture, or to mask a component. For example, in a situation where it is necessary to determine the concentration of oxygen in the combustion products, argon is used as a carrier gas, since argon is present in rather significant amounts in the air used for combustion (0.916 mol.% In dry air [3] ) and, naturally, it remains unchanged in the products of combustion, and it is difficult to separate it chromatographically with oxygen. [2]
In terms of sensitivity, accidents are inferior to most specific detectors . Its main advantages are the versatility and non-destructive nature of the measurement. The maximum sensitivity is achieved by miniaturization of the detector components, which allows to achieve a lower detection limit of up to 1 ppm (0.0001 mol%) [4] [5]
Sources
- Chromatograph.ru (Retrieved May 19, 2009)
- Guide to practical work on gas chromatography. B.V. Stolyarov, I.M.Savinov, A.G. Vitenberg
Notes
- ↑ GOST 17567-81 “Gas chromatography. Terms and Definitions". - Paragraph 25.
- ↑ 1 2 ISO 14532: “Natural gas. Dictionary". Paragraph 2.4.9.
- ↑ GOST 31369-2008. Natural gas. Calculation of the calorific value, density, relative density and Wobbe number based on the component composition . Tab. B.2 - Molar composition of dry air.
- ↑ Agilent 490 Micro GC Portable Chromatograph (link not available) .
- ↑ V.V. Chupin., I.N. Zhiltsov. A comparative review of modern means of measuring the component composition of natural gas . “Gas industry”, No. 4, 2011 p. 13-16.