The invention relates to the metering of fluids at low flow rates. More particularly, the invention provides a flow regulator for flows as low as about ½ ml per hour, provided with locking means for any set value within its range. The flow regulator comprises: an inner non-revolving core element, an outer housing, having a hollow base section and being revolvably supported co-axially with the core element, first and second chambers disposed between the core element and the outer housing, each chamber having an inlet and an outlet, the outlet of the first chamber and the inlet of the second chamber being connected by a flow restriction path, a diaphragm separating the two chambers and sealing an inlet to a discharge tube when pressure in the first chamber is higher than pressure in the second chamber, and a circular spring placed in the secondary chamber below the diaphragm and around the discharge tube. The major improvement being the addition of a graduated flow rate locking ring housing a disk-like body disposed between the outlet of the discharge tube and the base portion of the outer housing.