The measurement specification for the range of MERLIN DX CP monitors was drawn up with input from pipeline operating companies and independent CP consultants. It includes all the measurements conventionally taken at test stations, cp posts and riser posts:
Monitoring channels are fully independent. This means that a 2-channel MERLIN can be configured to monitor either two pipe-to-soil inputs (the second input could be a sleeve or foreign pipe) or, if a coupon is fitted, one pipe-to-soil input and an instant OFF input. The MERLIN unit incorporates an internal relay, which is opened just before channel 2 is read.
All MERLIN DX CP monitors measure DC and AC voltages on each channel simultaneously. This has become extremely important as operators now recognise that induced AC on pipelines can have detrimental effects.
If a coupon is fitted at the test post, channel 3 can be used to calculate the DC and AC current by measuring the voltage across a shunt of known value (Abriox can supply suitable shunts and cable assemblies). Once the shunt value and coupon surface area have been entered in the CPSM software, the data can be displayed either as the current in mA or as the current density in A/m².
Another configuration enables the DC potential of an unprotected (“native”) coupon to be monitored. By comparing this to a protected coupon (one that is electrically connected to the pipeline and therefore receives cathodic protection) the effectiveless of the CP system can be verified.
High and low alarms can be set for each measurement channel (for DC and AC values). A MERLIN unit will alarm as soon as a measurement is detected outside the acceptable “window” set by the user.