Measuring Very Slow Flow Rates of Air

Abstract

The contamination of air is defined as the pollution component within the volume under test. Physical-chemical instruments gauge for it. The analysis becomes all the more precise the slower the rate of flow is arranged. This demands a precision flow meter: We deal with the new “Gradient Method”. It uses a bridge with a heated sensor being subjected to the flow Q. The sensor is fed with a DC-current being superimposed by an AC-component. The bridge voltage U approaches the square root of Q. Using a tracking sampler the gradient of U(Q) is detected. It is high for small flow providing an error less then 0.3 %, using a platinum sensor down to 3 l/h of flow. This is compared with the merits of the same sensor bridge, but employing the conventional “Deflection Method”. It is characterized by a small error for high flow rates, being of interest up to 200 l/h for the breath analysis. –Both Methods have been combined within a PC. It processes the input signals of the flow sensor and temperature sensor of the same type. The instrument changes the methods as soon as the point of equal error is approached either upwards or downwards.