K is the meter constant used in fluid mechanics. The document discusses differential pressure meters which can measure pressure differences in industrial processes. These meters can be fitted with additional instrumentation to output the pressure readings electronically or pneumatically.

1. Introduction

25. K is the meter constant. A full explanation of these meters is
covered in the tutorials on fluid mechanics. The picture shows
an industrial d.p.meter. Extra instrumentation heads can be
fitted to produce an electrical output (4 – 20 mA) or a
pneumatic output (0.2 – 1 bar).