PSI - Issue 13

A.M. Polyanskiy et al. / Procedia Structural Integrity 13 (2018) 1408–1413 Author name / Structural Integrity Procedia 00 (2018) 000–000

1410

3

U

1

2

u

4

Ar+H

3

Ar

2

Ar+H

Ar

2

Fig. 1. Schematic of the thermal conductivity cell.

of the gas mixed with hydrogen is equalized with the temperature of the carrier gas that passes through the same sorbent located in the nearby tube. After purification by the sorbent gases are fed to the detection thermal conductivity cell (see Fig. 1), which actually measures the potential di ff erence in the diagonal of the electrometric bridge. This di ff erence is linearly related to the di ff erence in the mean temperatures of platinum elements. The hydrogen analyzers record the dependence of the potential di ff erence on time. This function becomes an extraction curve after calibration. The area between the extraction curve and the background line is equal to the amount of released hydrogen. For calibration, either a standard sample is used or hydrogen is introduced from the calibrated volume through the porous membrane into the carrier gas. The hydrogen pressure in the calibrated volume is known in advance. One can estimate the average temperature di ff erence between the sensing elements, which occurs while realizing the standard measurement of the hydrogen concentration. The electric power in both sensitive elements of the thermal conductivity cell is dissipated into the gas that blows it and can be written as: q IAr = K Ar S Pt ( T PtAr − T Ar ) q I Σ H 2 = K Σ H 2 S Pt T Pt Σ H 2 − T Σ H 2 , (1) where lower subscript Ar refers to the characteristics of the flow of the pure carrier gas (argon), lower subscript Σ H 2 refers to the characteristics of the flow of the mixture of a carrier gas with hydrogen; q I is the electric power of the current passing through the helix of the thermal conductivity cell; K is the coe ffi cient of heat transfer; S Pt is the area of the blown surface of the platinum element; T Pt is the average temperature of the platinum element, T is the temperature of gas or gas mixture. In the case of a dynamic gas flow, the thermal conductivity cell measures the di ff erence in the heat transfer coe ffi cient based on the heat transfer from the platinum sensitive element heated by the electric current to the gas flow that blows it, and does not measure directly the thermal conductivity. Inside the thermal conductivity cell, the gas flow is laminar. The heat transfer coe ffi cient reads: K = 0 . 5 ( Gr · Pr ) 0 . 25 λ d , (2)

Fig. 2. Schematic of the extraction chamber of a hydrogen analyzer. 1 - metal specimen, 2 - crucible (usually graphite), 3 - quartz glass tube, 4 - grounded pedestal for the crucible, 5 - tube and pedestal sealsl.