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6.4 Sample Event Cycle
and Delivery of
Sample Volumes
6.4.1 Volumetric
Determination
6.4.2 Sample Delivery
The steps listed below describe a typical sequential sample event
in order to help you determine the correct sequence of events in
the cycle. Should you suspect a malfunction in the delivery of the
sample volumes, you can use this information to help determine
the point at which the problem occurs.
1. A sample event is initiated.
2. The distributor rotates to the position of the next sample
bottle to be filled.
3. The pump rotates in the reverse direction to air purge the
suction line inlet of any accumulated debris.
4. The pump direction changes, pumping in the forward
direction to fill the suction line.
5. After the suction line has been filled to the pump inlet, the
sample volume measuring portion of the sampling cycle
begins. The pump rotates in the forward direction until the
sample volume has been delivered.
6. The pump direction again changes, pumping in reverse to
air purge the suction line to avoid cross-contamination of
samples. The pump then shuts off.
7. The sampler waits until another sample event is initiated
and the cycle begins again at Step 1.
This process continues until the last bottle has been filled, at
which time the sampler automatically shuts off.
The following section briefly describes the method by which the
sample volume is determined. The 3700 uses a peristaltic pump
to transport the sample from the source to the sample bottle.
When compared with other suction-lift sample gathering
methods, the peristaltic pump has numerous advantages: sim-
p l i c i t y, r e l i a b i l i t y, n o r e q u i s i t e m e t e r i n g c h a m b e r, a n
easy-to-clean design, etc.
The 3700 Sampler determines the volumetric delivery of its peri-
staltic pump by electronically counting revolutions of the pump
rotor. Each revolution of the pump rotor corresponds to a fixed
number of "pump counts" and a certain volume of sample liquid.
However, the volume of liquid delivered by one revolution of the
pump rotor changes with the suction head and the type of suction
line. At each sample event, the 3700 liquid-detection system
automatically compensates for changes in suction head and
adjusts the volume delivered for each revolution of the pump.
Thus, for a given type of suction line, each revolution of the pump
rotor results in the delivery of a known amount of sample
volume.
There are three parts to the sample pumping portion of a sample
event: suction-line fill, liquid detection, and sample-volume
delivery. The sampler counts the number of pump revolutions as
the suction line fills. At liquid detection, the controller uses that
count to determine the number of pump revolutions required to
deliver the programmed sample volume.
3700 Portable Samplers
Section 6 Servicing
6-13
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