Guide to Vacuum vs.
Peristaltic Fluid Samplers
All samplers are basically smart pumps. There
are two ways that the fluid can be moved from a source to a bottle (destination).
Fluid can be 'pulled' by creating suction (vacuum) in the tube.
The fluid will enter the tube to replace the air that was removed.
A fluid can be pulled horizontally a greater distance than it can be
lifted vertically. The pull method is physically limited in total
vertical height (top surface of source to maximum lift point) to
32 ft depending on altitude. Friction in the tube will cause the
fluid not to actually rise the full 32 ft.
The other method is to push fluid through the tube. This method
is only limited by the pressure the pump can generate and the tube can
hold. Approximately every 33 ft of vertical height will use 15
psi. There will also be losses for friction.
Both Vacuum and Peristaltic samplers use the pull method to collect
a sample. Pumps can be placed at the source to push water to the
sampler, assisting it in taking a sample.
VACUUM
A
vacuum sampler uses a vacuum compressor to create the vacuum and pressure
that move fluid. The fluid doesn't go through the compressor.
Vacuum samplers typically use a chamber into which the fluid is
drawn.
The compressor pushes air into the chamber, purging
liquid out of the intake hose. The compressor then pulls air out of
the chamber causing the fluid to flow into it. Once this volume
desired is in the chamber air is pumped into the chamber again. This
causes any fluid over the set volume (excess) to be pushed back out the
intake line. When the level is low enough that air is pushed out
instead of water, the remaining fluid is the sample volume. This
sample is then pushed into the sample destination (bottle).
Advantages
NOTE: These are based on data
relating to Manning vacuum samplers. Other vacuum samplers may or
may not meet all these advantages.
S Volume
repeatability
The sample is a true measured volume. Regardless of
head height, bubbles or other factors, the amount of fluid that
remains in the chamber is the same. See Myths below for
information about chamber cross contamination.
S Transport
velocity
The recommendation by the
EPA for transport velocity is 2-10 ft per second, faster being
better. Vacuum samplers can achieve transport velocities in excess
of 5.5 feet per second.
Long horizontal draws, in excess of a hundred
feet, aren't a difficulty. This allows you to have the sampler
farther away from hazardous sites (such as Class 1 Div 1, placing the
sampler outside the area, keeping employees out of the
area).
S No Consumable parts
The Vacuum sampler doesn't
have parts that are expended as part of the operation of the
unit.
S Solids
Solids sampled with a vacuum sampler,
don't go through a pump. Grains such as sand or larger rigid solids
won't harm the sampler.
S Strong Purge
The compressor in a Vacuum sampler
will generate up to a 45 psi purge. This is useful when dealing with
high fiber, rag or other material that can clog the intake tube.
The purge also uses a high CFM causing more liquid to be cleared from the
wetted parts.
S Greater Diameter Through Path
Vacuum samplers
lend themselves to variations in the diameter of the through path (minimum
id from source to bottle). 3/8" is the standard through path.
5/8 is also commonly used. Greater diameter through paths are
possible, but not commonly used.
PERISTALTIC
A peristaltic sampler uses a peristaltic pump to
move fluid. The peristaltic pump is named after peristalsis, the
squeezing of intestines to move material. The pump works with the
same principle. 2 or more rollers push against a flexible tube,
creating a pillow of fluid (or air). As the rollers move the pillow
is pushed toward one end of the tube. The pillow exits the pump and
the next pillow is pushed through.
Advantages
S Toxic
Applications
Anything listed as toxic (wetted parts glass, silicone,
stainless steel, or Teflon) is often suited to a peristaltic. The
tubing that the peristaltic uses is a often a silicone or approved
material. There are no modifications to the unit except the
intake tube.
S Setup
The peristaltic isn't very sensitive to
setup. There aren't many considerations besides getting no loops in
the intake line, and calibration.
S Vacuum
Peristaltic
can get very good vacuum (inches of mercury) when the tube is newer.
This can decline as the tube wears.
COMMON
MYTHS
S
Chamber Cross Contamination
Cross contamination in the chamber of a
Vacuum sampler or in the tubing of a Peristaltic are equivalent. The
chamber is a very small part of the surface area of the wetted parts
system. The Vacuum uses a higher pressure purge clearing more liquid
than the Peristaltic. The Peristaltic has slightly less surface
area, but doesn't have the higher Pressure purge. The USGS test in
Madison Wisconsin tested this with a distilled water blank in both Vacuum
and Peristaltic samplers. The conclusion was both units were equal
in cross contamination.
S Strong Purge Errors
It has been presented that
higher velocities and stronger purge will disturb the bedload more than a
slower lighter purge. The velocities used in samplers aren't going
to disturb the bedload unless the strainer is laying in it. In this
case either one will disturb the bedload. Air being blown into water
isn't going to travel any appreciable distance further at higher pressure,
and consequently won't disturb the bedload any more. The greater
volume of air is rising and the difference in affected bedload would be
insignificant at that point.
S Solids
Concentration
Strong Suction does not concentrate solids. The
physics in moving a less dense particulate (bio-solid) through a denser
matrix (water) require some force to act directly on the
particulate. A hose pulling in the fluid acts on the whole, and the
less dense material is carried along with the denser. It does not
travel independently through the water. Any Vacuum or Peristaltic
sampler creating stronger suction will give a equal or better
representation of the source liquid. This is supported by the EPA report on
samplers recommending higher transport velocity, i.e. stronger suction.
It is further support by the USGS test, which tested and found that
stronger suction did not concentrate solids.
Note: Suction in a
sampler is a combination of the vacuum (inches of Mercury) and the CFM
(Volume of air) generated. A sampler with high vacuum and low CFM
won't move much fluid, likewise high CFM and low vacuum won't move much
fluid. CFM relate more to transport velocity, and vacuum to the
height the fluid can be pulled.
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