This paper discusses calibration uncertainty and linearity issues of the typical back-pressure air gauge. In this sort of air gauge, the correlation between the measured dimension (represented by the slot width) and the air pressure in the measuring chamber is used in a proportional range. However, when high linearity is required (e.g., nonlinearity less than 1%), the measuring range should be shortened. In the proposed method, based on knowledge of the static characteristics of air gauges, the measuring range is kept unchanged but the nonlinearity is decreased. The static characteristics may be separated into two sections, each of them approximated with a different linear function. As a result, the nonlinearity is reduced from 5% down to 1% and even below.

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In the article, the authors analyze and discuss several models used to the calculation of air gauge characteristics. The model based on the actual mass flow (which is smaller than the theoretical one) was proposed, too. Calculations have been performed with a dedicated software with the second critical parameters included. The air gauge static characteristics calculated with 6 different models were compared with the experimental data. It appeared that the second critical parameters model (SCP) provided the characteristics close to the experimental ones, with an error of ca. 3% within the air gauge measuring range.

This article describes the measurement system designed in order to determine the metrological properties of air gauges. The said scientific study makes it possible to determine the static p k = f(s) and flow q v = f(s) characteristics. It consists of three modules: a mechanical module, a control and register data module and a special software module. Apart from the possibility of determining the static and flow characteristics, the presented study makes it possible to measure the temperature in the duct that supplies the compressed air to the transducer as well as in the measuring chamber. The above-mentioned measurement system makes it possible to determine the pressure applied on the surface measured by an air stream coming from the nozzle. Apart from a detailed description of a test station and the software, the article also contains sample results of tests performed on air gauges.