The distinguishing feature is the addition of a sample delivery system designed to draw the sample through the particle sensor.. In this mode, a grab sampler can be used as a temporary r
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Grab samplers are covered in a separate chapter because they operate in a manner
substantially different from online systems The models currently in use are designed
for portability, although most of the time they are operated in the drinking water
plant laboratory
A EQUIPMENT FEATURES
For the most part, the components of the grab-sampling particle counter are
identical to those of the online system The distinguishing feature is the addition of
a sample delivery system designed to draw the sample through the particle sensor
The packaging and software must be suited to the different applications requirements
This section presents an overview of these additional features, as well as any
considerations regarding similar features used for a different purpose
One of the most useful features is the ability to function as an online unit In
this mode, a grab sampler can be used as a temporary replacement for an online
unit that must be taken out of service, or in a sample location where a permanent
online unit has not been installed
1 Sample Delivery System
a Portable Grab Samplers
As described in Part I, the grab sampler employs a pump to pull the sample
through the particle counter flow cell In the units currently available on the market,
a small gear pump is used This pump is small and lightweight, and produces a
virtually pulseless sample flow It draws very little power, allowing for battery
operation It is a fairly quiet running unit, compared with the peristaltic pumps
commonly found in laboratory applications
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These small gear pumps are not designed for continuous operation, and have an
operating life of only a few hundred hours This will provide several years of
operation with grab samples, but precludes long term online usage
The sample passes through the gear mechanism, and large particles or
concen-trations of particles can jam the pump This is not usually a problem when
low-concentration samples (low-concentrations not much higher than the coincidence limit
of the sensor) are used A small strainer should be placed between the sensor outlet
and the pump inlet to keep large particles from creating problems
As described in Part I, this type of sampler does not meter-out the sample by
volume, but runs for a fixed time period at a constant flow rate The sample flow
rate is controlled by the voltage applied to the pump In most cases, the flow rate
is set at a fixed level, and is not adjustable by the user The flow rate should be
verified periodically to ensure that it remains constant As long as the flow remains
constant, the data can be normalized to adjust for any deviation from the designed
flow rate
b Pressurized Batch Samplers
The larger pressurized batch samplers are rarely purchased for water treatment
applications now that the much less expensive portable units are available Several
of them are still in use in drinking water plants, so a brief description of the sample
delivery mechanism is in order
The pressurized batch sampler was designed to accommodate viscous oils and
lubricants, which require significantly higher pressures to achieve usable flow rates
than does water The sample beaker is placed in a sealed chamber, which is then
pressurized with compressed air that has been filtered with a sub-micron filter to
prevent the introduction of unwanted particles into the sample The pressure forces
the sample up into a stainless steel tube, which is connected to the inlet of the particle
sensor The sample passes through the particle sensor, and into a collection chamber
that measures out the volume of sample
The volume is metered using a graduated cylinder that is fitted with some type
of level-detecting switch Two switches may be employed, one to initiate the particle
count cycle, and one to end it when the desired volume has passed through the
particle sensor The first switch can be positioned to allow some of the sample to
flush out any residual particles in the collection tube or sensor flow cell before the
actual counts are initiated Once the run is complete, the sample is then flushed out
of the graduated cylinder
The pressurized batch sampler is not dependent upon a fixed flow rate, but only
a fixed volume The flow rate must remain within the allowable limits of the particle
sensor The flow rate is usually controlled with some type of needle valve adjustment
downstream of the sensor
These batch samplers are far more mechanically sophisticated than the portable
grab samplers, and are several times more expensive There is no reason for them
to be purchased for drinking water applications They are built for heavy-duty
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service, and many of the units purchased years ago will remain serviceable for years
to come
2 Packaging
As the grab samplers are designed for portability, most are packaged in some sort of carrying case, or equipped with a handle Some sort of instrument case is used for mounting the components, as well as providing space for extra sample tubing and other accessories
A primary consideration is that of keeping all the electronic components isolated from any water It is easy to spill water when performing grab samples, and residual liquid will be left in the tubing after the samples have been run Access to the sensor for cleaning is also important, as the flow cell windows are more likely to need cleaning with intermittent use If the window surfaces dry off, it is possible for water spots to form, affecting the optical clarity of the cell With many different types of samples being run through the same sensor, it becomes easier to contaminate the cell windows The tubing may need to be replaced more often As was the case for the online units, cell cleaning involves removing and reinstalling sample tubing to the sensor, which increases the likelihood of leaks
The pump, strainer, and battery are additional components that will require access for service and maintenance
3 Counting Electronics
The same types of counting electronics found in online units are used for grab samplers The different applications for the grab sampler should be taken into account when selecting the type and features of the counting electronics For example, user-selectable size ranges may be useful, as grab samplers lend themselves to a wider range of experimental applications Additional size channels may be important for the same reason
Some of the user-selectable features may require a computer to configure This
is not a problem since a computer should be used to download the data from the unit anyway An integral keypad and display will be of more value because data can
be collected from various sites without connecting the computer, for downloading
at a later time A paper printout may be useful for backup, but should not be the primary means of data storage
4 Computer Interface
a Hardware
Most data communications will be done via a standard RS-232 serial port, which
is directly connected to a standard PC Some units also provide an RS-485 interface, which allows the grab sampler to be connected to an online system
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b Software
Grab-sampling software may be used to upload configuration parameters as well
as to download the data collected by the grab sampler The most that can be expected from the software is that it can be used to organize the data in a usable manner, and provide a file structure that can be imported into commercially available spreadsheet and database programs