A method for determining the velocity of water flow by means of random salt injection and by applying a correlation technique is described. The salt solution is injected into the flow as a tracer at one point of the stream under a random sequence and detected by the electrodes mounted at the other point downstream in the flow. The correlation function between the injecting signal and the detected variation of conductivity gives a means of determining the velocity of flow. The time delay by which the correlation attains its maximum informs the time taken for the travel of a particle from the point of injection to the detecting electrodes.
The injection of salt solution is performed by a binary pseudo-random (m-sequence) signal, which is photoelectrically obtained from a pattern printed on a circular transparent film disk. The device which calculates the correlation and automatically seeks the peak of the correlation consists of one differential gear, two film disks and transister circuits. The disks with the same printed pattern are mounted on the two axes of the differential gear. This mechanism is capable of rotating these disks in the same uniform speed and at the same time of changing their relative angular position under their rotating state by turning the third axis with a servo. The signal obtained from one of the disks is used for instructing the salt injection and the one obtained from the other disk is used as a delayed signal for computing the correlation.
Automatic seeking of the peak of the correlation function is carried out by controlling the relative angular phase of the two disks with a servo, so that the signal of the derivative of the correlation function will be minimum. The derivative signal is obtained by differentiating one of the two signals to be correlated before applying them to the inputs of the correlator.
The device is tested on a water flow system and the degree of precision with which the measurements are consistent was of about 0.95%, in comparison with the measurements obtained with orifice flow metering.
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