INSTRUMENTS AND EQUIPMENT.

An analog voltage recorder with a chart speed of at least 10mm/sec, capable of showing a trace during a one second period. It must have a range of 0 to 4 volts DC and a chart at least 100mm wide. It must have at least 4 channels, capable of working concurrently, leaving identifiable traces.

Sufficient synchronized current interrupters to switch all sources of CP.

Cu/CuSO4 electrode.

Two lengths of flexible cable, each fitted with a voltmeter connector and a crocodile clip.

Field notes

Report forms


METHOD

5.1.1 Set up the chart recorder and carry out procedures 2,3 4 and 5.

5.1.2. Mark the chart with the location of the test, the time and date. (unless this is done automatically by the recorder.)

5.1.3. Install the synchronized current interrupters at all the CP installations that effect the test location.(see procedure 5) The cycle should be set to switch the CP off for 3 seconds and back on for 12 seconds.

5.2.1. Connect the pipeline test point to the negative pole of channel 4 of the recorder.

5.2.2. Connect a Cu/CuSO4 electrode to the positive pole of channel 4 of the recorder.

5.2.3. Place the electrode firmly in the standard position at that test point.

5.2.4. Check that the chart is running at sufficient speed to show a 'kick' in the trace which might occur during the first second 'off'.

5.2.5. Mark the start of the procedure on the chart paper



5.2.6. Run the recorder for at least two minutes, to secure eight 'on/off' cycle traces.

5.2.7. Decrease the chart speed to 1mm/sec and leave running until a consistent trace pattern is obtained on all channels.


NOTE. This is to allow analysis of the traces on channels 1,2 and 3 which are used to correct errors caused by surges in potentials.


INTERPRETATION

The value known as the 'polarized potential' is the voltage at the first deviation from the vertical part of the recorded trace. Unfortunately, the mechanics of the recorder leave no vertical trace, but the first deviation, from the downward movement of the chart pen, is normally discernible.

If the current is interrupted by switching off the AC supply to the transformer, there can sometimes be a momentary reversal of polarity in the CP system as a whole. This is sometimes seen as a distinct ledge or even a small peak in the otherwise downward trace.

Any negative value below -0.850v is then considered to be unprotected.


NOTES

'Off potentials' sometimes show alarmingly low readings, from which it is then assumed that there is no protection at all. However, when the pipe is excavated, it is often found to be in pristine condition, even at coating faults where the pipe metal is exposed to corrosive electrolyte.

In many such instances, other steel objects, buried in the same backfill, have been found to be corroded to destruction. It was then clear that the CP was effective and the measuring technique was not giving the correct indications.

This could be due to the difficulty in obtaining true polarized potentials, which sometimes have to be read within the first second of the impressed current being switched off.

One of the major difficulties is in actually seeing the polarized potential on a digital meter, as the decay can start within the first second, and be quite rapid, so we are looking for a kick which may be masked by the meter itself.

A digital meter has to change its display sufficiently slowly to allow the human eye to register the readings, and the important 'kick' can be masked in the continuous change of voltage readings shown from the moment that the CP is switched off.

Even data-logging systems are prone to this disadvantage, so an analog chart recorder is the preferred instrument for this procedure.