An outlet is a device at the head of a watercourse (field channel or cultivator’s channel) which connects it with a distributing channel.
The common structures in practice today as outlets are, Pipe or barrel, Open flume, Orifice semi module, Pipe-cum-open flume and Pipe-cum O.S.M. etc.
But an outlet with the following features is supposed to be the best:-
An outlet that is strong and does not have moving parts liable to derangement or requiring periodic attention.
Interference by the cultivator is difficult and, if made, is readily detectable.
The outlet draws its fair share of the silt carried by the parent channel, but is not liable to derangement by silt or weeds.
It is possible for the outlet to work efficiently with small working head. The larger the working head, the higher the water level required in the parent channel and the higher the cost of the distirbutary system.
Out of the three classes of outlets; the Module outlets, the semi-modular outlets and non-modular outlets, A.P.M. Blocks fall in the 2nd category. It, therefore, has all the possible features of semi-modular outlet.
The A.P.M. Block is an outlet whose discharge, although depending on the water level in the parent channel, is independent of the water levels in the watercourse, so long as the minimum working head required for the working of the semi-module is available. Therefore, the A.P.M. Block is efficient in working even with a small working head. The minimum working head required of an A.P.M. Block is 0.5 ft.
The discharge through an A.P.M. Block can be varied according to the needs of the crop. And the discharge once set, the discharge co-efficient remains constant.
A.P.M. is the abbreviated form of Adjustable Proportional Module but he present day A.P.M. Block is an improvised form of it and in working is an adjustable orifice semi-module. Structurally, the A.P.M. may be regarded as a long throated flume with a roof block capable of vertical adjustments, introduced into the upstream end of the parallel throat. With the roof block removed, the A.P.M. differs from the flume module only in regard to the length of the throat. It belongs, however, to the orifice type of flexible module.