In the vast majority of cases, hydrogen analysis is performed using the thermal conductivity method.
The measuring cell consists of two parts: in one of them, there is a chamber where the sample flows, and in the other there is a flowing or sealed chamber with a reference gas. Inside the chambers there are heating elements (two in each chamber), which are thin spirals of wire, through which current flows, as a result of which the spiral heats up. The resistance of thermocouples varies depending on their temperature. When gas flows through the chambers, the temperature of the thermocouples changes, which directly depends on the thermal conductivity of the gas. Thermocouples are connected by conductors to a bridge circuit in which the elements play the role of resistors. The bridge has power and measurement contacts. If the resistances on all shoulders of the bridge are equal, then between the measuring contacts of the bridge the potential difference is zero. Since in the chamber with a reference gas having a constant composition,
This scheme provides high measurement accuracy if the sample has a binary composition, that is, consists of two components, one of which is measurable, for example hydrogen and nitrogen. In this case, nitrogen or air having practically the same thermal conductivity is in the chamber with the reference gas. The thermal conductivity of hydrogen is about 8 times higher than the thermal conductivity of air, so the hydrogen concentration is easily calculated from the difference in the thermal conductivity of the sample and the reference gas. In cases where a high sensitivity of the detector is not required (when the concentration of the measured gas — hydrogen — is greater than 1% and less than 99% of the total sample volume), a sealed chamber with reference gas is used. However, when it is required to control the hydrogen content in the sample at a level of less than 1% by volume, or vice versa, to control the purity of hydrogen, a continuous supply of reference gas (nitrogen or air) is required. Besides,
In cases where the sample has a more complex composition, for example, when measuring hydrogen in a hydrogen-containing gas (HHG), the problem of an additional measurement error arises due to the fact that the composition of the sample is constantly changing and the thermal conductivity of the sample is changing not only as a result of a change in the content of hydrogen in the sample, but also as a result of a change in the concentration of other components . For example, WASH consists of hydrogen (50-100% vol.) And C1-C5 hydrocarbons in various concentrations. The reference gas must always have a known and constant thermal conductivity, therefore, in most cases, air is used as the reference gas, and the thermal conductivity of the other gases is taken as relative to the thermal conductivity of the air. If the SHG consisted only of hydrogen and methane, then the thermal conductivity of the SHG would depend only on the concentration of methane and the concentration of hydrogen, which could be easily calculated from the relative thermal conductivity of the WASH measured by the detector. In reality, when the SHG consists of hydrogen and C1-C5 hydrocarbons, the hydrogen concentration cannot be calculated with high accuracy from the relative thermal conductivity of the SHG, since it depends not only on the concentration of hydrogen, but also on the concentration of all other hydrocarbons. In this case, in order to achieve the required accuracy of measuring hydrogen in the WASH, a gas separation column is added in addition to the heat conduction detector, and a gas chromatograph is obtained. The sample at the inlet of the chromatograph is repeatedly diluted with a carrier gas, usually nitrogen, and separated into components in a column, as a result of which, through the detector, the sample components pass alternately in a mixture with the carrier gas, and the carrier gas also plays the role of the reference gas. The main disadvantage of this method of analysis is the time required to obtain a result associated with the passage of the components of the sample through the gas separation column. Often in the WASH contains aggressive substances such as hydrogen sulfide, which causes corrosion of thermocouples and, as a result, the failure of the detector in a short time. When a gas chromatograph is used, the corrosion of thermocouples occurs much more slowly due to the multiple dilution of the sample with carrier gas, and the period of their replacement is several years. When using on-line analyzers for measuring hydrogen in the WASH with a hydrogen sulfide content of up to 200 ppm, thermocouples are coated with gold dusting, which significantly reduces the level of corrosion.