IOM006GVAE1103
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4120
Section 2 - Operation
The functional blocks of the Varec Model 4120 Multi-Element Temperature Transmitter
are described below.
Temperature
Configurations
The METT handles three temperature system configurations as follows.
API Averaging Bulb. API standard 2543 is used. An API averaging bulb has elements at
3', 5', 7', 10', 14', 20', 26', 32', and 40' for a 50' tank. For levels below 3', a spot probe
is expected at the 2'. The standard specifies that non-merchantable product should be
kept at least 1' below spot probe. The METT handles the bottom spot and a top spot for
vapor space temperature measurement.
Non-API Averaging Bulb. This is a multiple-spot system which consists of up to 10
averaging RTD elements. The elements may be of non-standard lengths. The length of
each element is configurable. Top and bottom spot temperatures are supported as
above.
Multiple Spot. A multiple-spot system can consist of up to 12 3-wire RTDs. The height
of each RTD is configurable. The first 11 can be averaged. The last spot is used as the
top spot for vapor space temperature measurement.
Temperature Averaging
Averaging of the temperature depends on the type of temperature configuration being
used. For averaging bulb configuration, the METT reads the longest immersed element
and reports its temperature. The top probe is never used for averaging.
For multiple spot configuration, the METT reads all immersed elements and averages all
readings. For simplicity, spot temperatures are averaged given equal weight to all
immersed probes. The top probe is never used for averaging.
A host computer can read all temperature inputs via the HART protocol. Assuming the
host knows the level and has the temperature configuration in its database, it can then
calculate the average temperature. A host computer is also able to periodically write the
current level and read back the average temperature. This method is preferred since the
METT contains all the configuration information.
HART Communications
Interface
The METT communicates to a host system via the HART communication protocol. The
HART protocol allows the average temperature, temperature, and resistance to be read.
It also allows the current level and configuration information to be sent to the METT.
