Response times, mass-flow sensors

Usually in the operation of biosensors the flow conditions are adjusted to provide a mass transfer rate from the solution to the membrane system which is fast as compared with the internal mass transfer (exception implanted sensors). On the other hand, variations of the diffusion resistance of the semipermeable membrane are being used to optimize the sensor performance. A semipermeable membrane with a molecular cutoff of 10 000 and a thickness of 10 pm only slightly influences the response time and sensitivity. In contrast, thicker membranes, e.g. of polyurethane or charged material, significantly enhance the measuring time, but may also lead to an extension of the linear measuring range. 

To test the time response of the sensors, a 10% H2/90% Nj ambient was switched into the chamber through a mass flow controller for periods of 10,20 or 30 s and then switched back to pure Nj, Figure 5.9 shows the time dependence of forward current at a fixed bias of 2V under these conditions. The response of the sensor is rapid (< 1 s), with saturation taking close to 30 s. On switching off the hydrogen-containing ambient, the forward current decays exponentially back to its initial value. This time constant is determined by the transport properties of the test chamber and is not limited by the response of the diode itself. 

In conclusion, Pt/4H-SiC diode rectifiers of the type used for high-power electronic applications are also shown to be effective in gas sensor Hj detection. The time response of the diodes is limited by the gas mass flow transport characteristics, with the intrinsic response due to changes in the interfacial OH-dipole layer being very rapid. 

Flow sensitivity is measured as the change in detector output for unit change in flow rate through the sensor cell. The response of the FID is virtually unaffected by flow rate changes and, in fact, only responds to the mass of solute passing through it per unit time. In contrast, the katherometer is very sensitive to changes in flow rate and requires to be operated with a reference cell to compensate for any fluctuations in column rate. 

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