Hach SIGMA 900, User Manual

Page: 12 / 94

Page 12
Principle Operation
8837int.fm
Section 1
sample source, the time required for liquid to reach the sensor will increase.
The microprocessor automatically compensates for this change by allowing
the peristaltic pump to deliver sample liquid for a corresponding longer period
of time. Conversely, if suction lift decreases due to an increase in level at the
sample source, the time required for liquid to the sensor will decrease. Again,
the microprocessor automatically compensates for this change by decreasing
the sample delivery time.
1.2.2 Intake Tube Pre-Rinse
The liquid sensor also allows the sampler to rinse the intake tubing with the
liquid from the sample source before taking each sample.
Upon sample initiation, the pump purges the intake line. The pump then
reverses, pulling liquid through the tubing, until it reaches the liquid sensor.
When the sensor detects liquid, the pump purges back to the source, and
then draws a sample. After the desired sample is collected, the pump purges
the intake line and awaits the next sample cycle. The sampler can be
programmed for 1 to 3 rinses before each sample.
1.2.3 Sample Retry
The liquid sensing system permits the sampler to repeat a collection cycle if a
sample is not obtained during the regular cycle.
The intake line length is user-programmed into the sampler memory. For a
line length of 3 to 99 feet, the sampler has a built-in “look up” table that
detects the maximum time required for liquid to reach the sensor. If liquid
does not reach the sensor within the defined time period, the pump will
automatically purge the intake line and initiate another sample cycle. The
sampler may be programmed for up to three repeated attempts. If a sample is
not obtained, the sampler retains in memory the time, date and reason for the
missed sample.