Campbell Scientific PVS4100/4120/4150 Portable Automatic Samplers - Manual (Page 33)

Section 3. Operation
3-3
The volume control tube should always be located below the
liquid sensing rod.
Hold the bottom nut while loosening / tightening the top nut, or it
may become loosened from the metering chamber cover and
create an imperceptible leak in the vacuum system.
3.2.2 Liquid Sensing Rod
This probe, also called the “level control rod”, is used to stop the sample
intake. Always ensure that its lower end is located above the volume control
tube. Approximately 1” difference is sufficient. If the fluid intake is turbulent
within the metering chamber, more than 1” may be required to ensure
splashing of fluid does not trigger probe.
In applications with substantial oil or grease, the rods can become coated and
lose their conductivity. This is prevented by cleaning the rods regularly. In
extreme cases, extra SS wire can be wrapped around the liquid sensing rod to
increase its surface area.
IMPERATIVE: THE LIQUID SENSING ROD AND VOLUME CONTROL
TUBES MUST BE KEPT CLEAN TO ENSURE CONDUCTIVITY
NECESSARY TO DETECT THE PRESENCE OF THE FLUID.
Most PVS Samplers incorporate a Barrier Valve in the metering chamber
cover, where the tubing from the pump enters. It consists of a cage containing a
ball that will float if the sample should rise to the top of the chamber without
detection. Should rod conductivity fail, the fluid brings the float into contact
with an O-ring surrounding the pressure / vacuum port, sealing the entry to the
tubing and the pump (where the fluid may cause serious damage). This O-ring
Barrier Valve should be inspected regularly and replaced as necessary.
Due to the restriction of Wetted Materials (i.e. stainless steel, glass and
fluorocarbons etc.), some models of the sampler do not contain this barrier
valve. In these units, a secondary liquid-sensing circuit may be added as a
precaution. This circuit is connected to the pump tubing fitting on the Metering
Chamber cover.
IMPORTANT
NOTE