APPLIANCE MANUFACTURING WITH MULTIDROP
A consumer appliance manufacturer used the networking capability of the
HART protocol to measure level, flow, and pressure. HART multidrop provided substantial wiring and installation savings as well as digital accuracy with the elimination of the analog to digital (A/D) and digital to analog (D/A) conversions of the instrument and PLC I/O. Figure 22 shows pressure transmitters connected to a PLC via smart transmitter interface multiplexers.
Figure 22: Multidrop Network Example
The benefits of remote monitoring and rezeroing of smart transmitters using the HART protocol are dramatically illustrated in this example of two smart transmitters that control the fluid level in lauter tubs in a brewhouse application. Similar benefits would be realized in any application involving a closed vessel.
Two smart transmitters are installed on each lauter tub—one on the bottom of the tank and the other about nine inches from the bottom. The bottom transmitter is ranged ±40 inH
O; the upper transmitter is ranged
0–30 inH
O.
As the lauter tub is filled, the bottom transmitter senses level
based on pressure. When the level
reaches the upper transmitter, that point
is marked
as the new zero-level point, and the upper transmitter becomes the primary sensing instrument for the lauter-tub level. The nine-inch zero-level offset from the bottom of the tank is necessary to accommodate loose grain that settles in the bottom of the tank.
Transmitters that are coordinated and working together control fluid level in each lauter tub to within a few barrels. However, the upper transmitter requires periodic maintenance or replacement and rezeroing. An undetected false upper-transmitter level reading can cause a tank level error of up to 40 gallons.
The usual procedure for transmitter rezeroing takes about 95 minutes and has been required as frequently as twice a day. Rezeroing a transmitter using configuration software and PLC interface modules eliminates the need to locate and identify the problem at the site as well as the need for verification by control-room personnel and greatly reduces the chance for inadvertent errors. Estimated total time to rezero each transmitter is reduced to 15 minutes.
Through the configuration software’s instrument-status and diagnostic capabilities, a false level indication can be automatically detected while a lauter tub fill is in progress. The affected transmitter can then be automatically rezeroed by programming logic in the programmable controller to issue the appropriate command to the instrument.
