API MPMS 5.5:2005(R2015) pdf download.Manual of Petroleum Measurement Standards Chapter 5—Metering
5.5.6.2 LEVEL D
A Level D system consists of manual error monitoring at specified intervals by methods of comparison. This level of security is intended to give protection against functional errors and failures and is a method of verification by manual action. The readout can be visually checked against an inde- pendent totalizing system. Figure 2 illustrates a simple system with means of making a periodic manual assessment of security. The secondary readout may be permanent or temporary, local or remote. Manual comparison made during a periodic check will moni- tor the integrity of the transmission and totalizer elements. It may be less convenient than provisions of Level C, as the sys- tem may have to be stopped for readings to be taken. Overall security is mainly inferred from the performance during the error monitoring period
5.5.6.3 LEVEL C
A Level C system consists of automatic error monitoring and error indication at specified intervals by methods of com- parison. This level of security is intended to give protection against functional errors and failures and may be achieved by design methods. The time intervals for error monitoring may be subject to revision in the light of experience gained. Figure 3 illustrates a dual transmission system with a dual pulse comparator of simple design. If the pulses delivered become numerically out of a step, warning will be given by the com- parator (differential counter). Level C security will be defeated by other disturbances dealt with by higher level security systems. For example, simultaneous interference superimposed on both channels will not be detected because a numerical difference between channels is not caused. It is intended that this form of error monitoring be carried out periodically; the monitoring equipment may thus be shared with other metering systems. Level C security is inferred from the results obtained dur- ing the monitoring period.
5.5.6.5 LEVEL A
Level A consists of continuous verification and limited correction by methods of comparison. This level of security is intended to give protection against transients and other spuri- ous common mode influences, in addition to functional errors and failures. Figure 5 illustrates a dual transmission system protected against both dynamic faults arising from monitor- ing the duplicated pulses and by static testing the electrical integrity of the transmission circuits. The system should still operate as a Level E system if one of the transmission chan- nels fails. An incidental advantage of Level A is its ability to detect some mechanical faults in the transducer. Simulta- neous pulses caused by symmetrical interference are auto- matically rejected and do not influence the system. Other than a complete failure of one of the transmission channels, no attempt is made to automatically compensate for lost or gained pulses on either transmission channel. Alarm will be given in all circumstances when impaired pulses are received by the comparator. It may be desirable to provide redundancy in one or all of the elements shown.