KROHNE Summit 8800 Vol 2 - Manual (Page 22)

22 www.krohne.com 08/2013 - MA SUMMIT 8800 Vol2 R02 en
SUMMIT 8800 METERING PRINCIPLES 03
• A second lower frequency pulse, so a combination of HF with MF or LF. Off course the fre-
quency ratio or blade ratio between the two pulses must be given.
API has a classification on the quality actions taken on the pulses:
API level E is achieved solely by correctly applied
transmission systems, criteria and recommended
installed apparatus of good quality.
Basically a non-issue for flow computers
API level D system consists of manual error
monitoring at methods of comparison, as used in
Levels A through D.
This means: Only 1 pulse is needed on the flow
computer.
API level C consists of automatic error monitoring
for number, frequency, phase, and sequence and
error indication at specified intervals.
This means: two pulses must be installed: the
meter pulse and monitor pulse, which may be of
different frequency (see frequency ratio)
API level B consists of continuous monitoring, with
an error indication under all circumstances when
impaired pulses occur.
This means two pulses of the same frequency
must be installed: the meter and monitor pulse.
API level A: consists of continuous verification and
correction given by the comparator.
The major issue here is; the flow computer has
to correct when a wrong pulse occurs. This is
quite advanced and is fully implemented in the
SUMMIT.
Nowadays more and more electronics is incorporated into the meters, such as in ultrasonic
and Coriolis meters. These meters normally emulate two high frequency pulses, to make them
look the same as rotating meters from the installation standpoint. The flow is calculated and a
special pulse output is driven by the processor. Although the need for a second output pulse is
diminished, most meters still carry them. API Level A is not really required.
There are also meters with smart indexes. Here the indexes values itself can be read by the
flow computer. The advantage is that the totals on the meters index are identical to the flow
computer totals. Also, if the flow computer is replaced, the total will be automatically read. The
communication is then digital and can be read via the serial port.
3 .2 Ultrasonic meters
Ultrasonic meters are based on Transit Time Measurement of high frequency acoustic signals.
These signals are transmitted and received along a diagonal measuring path.
A sound wave going downstream with the flow travels faster than a sound wave going upstream
against the flow. The difference in transit time is directly proportional to the flow velocity of
the liquid or gas. This can be compared with the speed a canoe travels upstream compared to
downstream.
Figure 3 Ultrasonic measurement principle