Protection
Accuracy
2 - 2
SKF Multilog O
n-Line System IMx-M
User Manual
Accuracy
Facts to Consider Regarding Accuracy
•
Protection modules (PRM) and condition monitoring modules (CMM) can show
different values due to the following reasons.
–
They are two separate individual measurement devices.
–
They have two different purposes. PRM is to optimize the quick response time
as defined in API 670 whereas CMM is to optimize the early fault detection.
–
They use two different measurement algorithms.
–
They have two different filter setups.
•
API 670 specifies ±1% and ±0,5 dB filters in passband which leads to ±6% in total.
•
Calibration of reference instruments does not mean it is exact. It only means that it
is within the specification.
•
Note that most reference instruments are not accurate for low frequencies below
50 Hz.
•
The following diagram shows an example of same or different readings for PRM
and CMM depending on the input signal frequency in conjunction with filters.
Figure 2 - 1.
Comparison of PRM and CMM Readings.