Varec 7532, Installation And Operation Manual

Page: 100 / 106

Appendix Radar Tank Gauge
100 Installation and Operations Manual
11.4.3 Custody transfer mode
7500 RTG is a weight and measure approved level transmitter. Either the innage or the
ullage can be selected as the custody transfer variable.
The selected variable is the basis for the subsequent calculation of the current amount
of product in a tank, along with other measured variables such as (average) temperature
and pressure.
This opens up numerous application options in custody transfer:
• Quantity calculation of mineral oils
• Quantity calculation of alcohols
11.4.4 Weight and measure approval, Standards Authorities ap-
proval, Compulsory reapproval
The
type approvals for custody transfer
issued by the PTB and NMi, a copy of which is
enclosed with every device, prove the fundamental suitability of the various types for
custody transfer.
In addition to this, the
accuracy
of every single device is documented using a calibration
certificate, which is issued in the factory after the device has been tested on a reference
test rig.
On request, a separate
initial verification
of the devices can be carried out with a
National Standards Authorities inspector present, who issues a preliminary test certifi-
cate
for every device. In the initial verification, the device is tested to ensure that it com-
plies with the limit of error in legal metrology , which lies at +-2mm for radar measuring
devices in Germany.
Essentially, this proves that the devices are
weight and measure approved
. The devices
must not, however, be used in custody transfer mode straight away.
The measuring device is not approved until after the approval after installation by the
Standards Authorities. For this, the device's level measurement is compared with the
tank gauging by a National Standards Authorities inspector using manual dips (also "Ini-
tial verification"). As a rule, a quiescent tank gauging is dipped by hand three times in a
row and then compared with the value displayed by the level radar. Depending on
national regulations, the transfer error limit, calculated as the arithmetic mean of the
absolute deviations of all three measurements, must not exceed double the limit of error
in legal metrology (compare, for example, the German "Eichordnung" or the American
"API 3.1B", in which the necessary procedures are also defined).
Depending on national regulations, this test is repeated with various tank gaugings.
Using linearization tables to compensate any non-linearities that occur in measurement
is permitted. For this, the 7500 RTG level radar offers a special dip table, compare
“Mounting calibration with VU 331” on page 57.
After the measurement has been approved by an inspector, he seals the level radar at
the stamp position and thereby also secures the programming status of the device.
Those operating an approved level transmitter are obligated to obtain reapproval in
accordance with the applicable national regulations from the Standards Authorities.