A preliminary analysis is made of the well-known instability, occurring in airlift pumps, when at sufficiently high rise the ability of the pump as a self-controlling system breaks down, and a rapidly increasing oscillation of the flow sets in. It is shown that the instability is due to the fact that a small time-dependent perturbation of the stationary flow satisfies a difference-differential equation with time lag, of a kind well known from control theory. The theoretically computed values of the critical rise and the time period of oscillation are in satisfactory agreement with observation.

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