Appendix C
Procedure to Calculate System Scan (Polling) Rates
This procedure allows the user to calculate the expected system scan rate for a given application. The system scan
rate is controlled by several user programmable parameters. The programmable parameters that affect system scan
rates are described below.
Tank Multiplex Assignment
The Level Plus monitor polls each DDA tank gauge in a sequential order based on the tank numbers that are enabled
on the tank multiplex assignment programming menu. Level data is sequentially polled from each active tank gauge
in the scan loop. The level scan loop is repeated five times and then temperature data (if enabled) is polled from
the first tank gauge in the multiplex assignment. The level polling loop is repeated again (five times) and then tem-
perature (if enabled) is polled from the second active tank gauge in the multiplex assignment. This polling sequence
is continued until temperature data has been polled from all active tank gauges in the multiplex assignment. The
polling process then repeats continuously in the defined order.
Temperature Display (Tank parameter)
The temperature display programmable parameter determines which tank gauges should be polled for temperature
data. Only the tank gauges that have the temperature display function enabled are polled for temperature data. If
the temperature function is disabled then the overall system polling rate will be faster. The temperature function can
be enabled or disabled on a gauge by gauge basis. In addition the temperature display programmable parameter
determines which temperature command is sent to the DDA tank gauge. The higher the temperature resolution set-
ting (low, medium or high) the longer it will take for the DDA tank gauge to acquire and transmit the temperature
data. Consequently, the overall system polling rates will be slower. The typical per gauge time delays are based on
equations from the DDA tank gauge operation and installation manual.
Low resolution (1 degree Fahrenheit) = 1.0 seconds + 0.9 seconds per RTD
Medium resolution (0.2 degrees Fahrenheit) = 1.7 seconds + 1.6 seconds per RTD
High resolution (0.02 degrees Fahrenheit) = 2.9 seconds + 2.7 seconds per RTD
Fast temp (1 degree Fahrenheit) = 0.5 seconds + 0.5 seconds per RTD
Level Resolution (Tank parameter)
The level resolution programmable parameter determines which level command is sent to the DDA tank gauge. The
higher the level resolution setting (0.1, 0.01 or 0.001 inches) the longer it will take for the DDA tank gauge to acquire
and transmit the level data. Consequently, the overall system polling rates will be slower. The typical per gauge time
delays are based on equations from the DDA gauge operation and installation manual. Note that the level data time
delays for Dx model DDA tank gauges are less than for LD models. Delay times for single float DDA tank gauges
are listed below in seconds.
‘Dx’ ‘LD’
tank gauge tank gauge
models models
Low resolution (0.1 inches) = 0.30 0.45
Medium resolution (0.01 inches) = 0.46 0.73
High resolution (0.001 inches) = 1.32 2.20
The actual time delays for level data may be slightly less than the values listed above since the level time delay is
also a function of tank gauge length. The shorter the gauge length, the slightly less the time delay will be. The above
listed values are based on the worst case time delays assuming maximum gauge lengths per model type (Dx and
LD).
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