KROHNE Summit 8800 Vol 2 - Manual (Page 24)

24 www.krohne.com 08/2013 - MA SUMMIT 8800 Vol2 R02 en
SUMMIT 8800 METERING PRINCIPLES 03
Figure 4 DP measurement principles
The shape of the restriction is determines the type of meter: orifice, V-cone venture or nozzle
(see later paragraphs). For each type there are several parameters that will be required to suc-
cessfully calculate the flow rate.
A single dP transmitter can used, but the problem is that a transmitter typically only has a 1:3
turndown ratio, so the accuracy for low flow is very limited. For that reason in custody transfer
applications multiple dP transmitters with different ranges are used for one meter and the flow
computer switches between them over depending on the flow.
The SUMMIT can handle 1 to 3 ranges:
Figure 5 Up to 3 dP ranges
dP 1 will always measure the high range. In case of multiple ranges, an automatic switch-over
to dP 2 will occur to medium range if the flow decreases to the dP measurement range, optimiz-
ing the accuracy. If 3 ranges are available, dP 3 will kick in when the flow gets within its meas-
urement range.
In the SUMMIT the switch-up and switch-down values for the dP may be given. They will be
normally be different to have some hysteresis to prevent continues switch-up and –down when
at the threshold.
In high end applications, where the accuracy is crucial, multiple dP transmitters per range can
be used for the following reasons: