Sensor four-probe type

Fig. 36.3. (a) Dependence of short-circuit current (/) of the amperometric proton-conductor sensor on H2 concentration, and (b) dependence of (inner potential difference) of the four-probe type sensor on Hj concentration in air at different relative humidity (25 (reprinted by permission of The Electrochemical Society, Inc,),... 

In the potentiometric sensing of H2, (ii) is useful for H2 in inert gases while (i) is especially suited for H2 in air. On the other hand, only (iii) is applicable to the amperometric sensor and the four-probe type sensor. 

Fig. 36.4. Structure of the four-probe type sensor. AAD, antimonic acid disc AAS, antimonic acid sheet (reprinted by permission of The Electrochemical Society, Inc.).

Specific resistivity p and critical temperature Tc were measured by the four-probe method. For the Tc value the point in the middle of the curve of transition to the superconducting state was taken. As a temperature sensor in the cryogenic zone a germanium thermometer of the VG-type was used. The values of the lower Hci and the upper Hc2 of the critical magnetic field at the temperature of 4.2 K were determined from differential curves of magnetization similarly to [8]. 

Conductivity sensors may be either the contacting tsrpe or the induction type. The contacting type may utilize either 2 or 4 electrodes. The four electrode cell permits measurement of higher ranges (up to 200 mS), and the correction of polarization effects due to deposits forming on the electrodes. Two electrode types are suitable for lower conductivity ranges and are easier to insert into a flow through cell but cannot compensate for polarization. Four electrode types are incorporated into the system in a similar manner to pH probes. 

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