A simple cylindrical choke-type instantaneous flow meter, responding to the change in flow rate in high frequency up to 400 Hz, has been proposed and developed by the authors. The validity of the unsteady flow meter has been confirmed experimentally through comparisons with a laser Doppler flow rate sensor and gear motor-type flow meter. However, to apply the flow meter for practical use, there are problems still to be solved how the measurement dynamic accuracy is influenced due to the upstream flow condition.
In this research, particularly, the influence on measurement accuracy due to the existence of coaxial dynamic jet-flow velocity profile in front of the chokes is investigated experimentally.
In addition, a simple mathematical model of the pressure drop of the choke under the influence of the dynamic jet is proposed. The theoretical results are in good agreement with the experimental results. It is also confirmed that the equipment of newly-devised rectifiers upstream the chokes give sufficient effect to avoid any measurement error due to the unsteady jet flow.

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Hydraulic restrictors are being widely used in hydraulic systems and hydraulic components, with many cavitation studies on various restrictors being performed. However, the effect of the downstream duct behind the restrictor for the cavitation characteristic was not studied.
At the beginning of this paper, the cavitation tests in single-stage restrictors, when the diameter of the downstream duct is small, are investigated. As a result, the following facts are revealed.
When the diameter of the downstream duct is smaller, the critical outlet pressure for cavitation on the restrictor is higher. The relationship between the pressure difference, which is defined by the pressures measured at the corner tap, and the flow rate does not change independently of the diameter of the downstream duct. The measured pressure recovery in the downstream duct agrees with the theoretically predicted value.
In the latter part of this paper, the cavitation and the flow characteristics in the two-stage restrictor, in which an upstream orifice and a downstream round edged choke are connected with a small diameter duct, are investigated.
When the diameter of the duct between the upstream and downstream restrictor is smaller, the cavitation on the upstream restrictor occurs at higher outlet pressure of upstream restrictor, furthermore the cavitation on the downstream restrictor occurs at higher outlet pressure of downstream restrictor. The influence of the distance between the upstream and downstream restrictor on the pressure-flow characteristic is different according to the diameter of the duct between the both restrictors.

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Recently high response solenoid valves have been developed capable of switching the airflow up to 200 cycles per second. If these vales are used as switching devices of the digital pneumatic pressure servo system, it enables response as quick as milliseconds for the output flow change.
In this paper, the design and performance of digital pneumatic pressure servo system using two high response valves are described. By the results of the tests on the flow characteristics and responses for output flow change, the performance of new system is shown as follows. The output of the servo system can yield a higher accuracy mean pressure than the conventional diaphragm-type pressure regulator and responses within 15 millisecond. The output pressure contains ripple noise within 1 or 2 percent of its level.

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