Two Pump Method (high
pressure mixing)
Each component is separately pumped through two variable
speed pumps:
The two-pump method is the most precise and accurate method for
gradient formation over a wide range. The two pump speeds can be
controlled to form multi-step gradients over varying ranges depending on
the choice of pumps and the gradient flow rate.
All pumps have a maximum speed at which they can operate. They also
have a minimum speed at which they can be accurately controlled. The ratio
of maximum speed to minimum speed is called the turn-down ration and is
expressed as a ratio such as 10:1.
Rotary lobe pumps have a small gap between the rotors and between the
rotors and the housing. Fluid flows backwards past the rotors through this
gap. As the flow rate decreases, or the pressure differential across the
pump increases, the amount of "blow-by" increases. These pumps
are controlled by feedback from flow meters to keep the flow rate
accurate. At flow rates below approximately 10% of the maximum rating of
rotary lobe pump, it is very difficult to control the output of the pump.
Diaphragm pumps have two means of control over the flow rate. The
stroke length may be adjusted as well as the speed of the pump. The stroke
length can typically be adjusted over a 10:1 range, from approximately 1.5
mm to 15.0 mm. The stroke frequency can also be adjusted over a range of
10:1, from 70 to 170 strokes per minute.
Diaphragm pumps are true positive displacement pumps. Their flow rate
does not vary significantly with varying pressure differential across the
pump. They are typically accurate to ± 0.5% over a 10:1 turndown of speed
alone and ± 2.5% over larger 100:1 turndown of both speed and stroke.
The two-pump method gradient accuracy therefore depends on the maximum
flow rate of the pumps used, and their turndown ratios:
If the pumps are forming a gradient at their maximum rated speed, and
they have a turn-down ratio of 10:1 they can form an accurate gradient
from 10-90%, but at gradient flow rates below their maximum speed, the
range of their gradient capability is reduced.
If two pumps with 10:1 turndowns are forming a gradient at ½ their
rate speed, a gradient may only be formed from 20 to 80% composition:
Below 20% of its rated maximum speed it cannot form a gradient at all.
The graph below shows a "Fish Curve" for a 50L/min pump with
a 10:1 turndown ratio. Gradients can be formed if they fall within the
curve. For example, the chart below shows that at a flow rate of 15L/min,
these pumps can form a gradient between 35 and 65%.
Forming a gradient over a wide flow range is different for a rotary
lobe pump. The diaphragm pump allows the user to set the stroke length to
the maximum speed of the gradient. Then the stroke frequency can be turned
down by 10:1 to form a gradient over the range of 10-90%. Therefore, the
fish curve looks like this:
| Advantages |
Disadvantages |
| High pressure mixing
can eliminate out-gassing, cavitation, and air entrapment
Can form multi-step gradients
Can perform step gradient
Can perform on-line mixing
Forms very accurate and repeatable gradients |
Increased cost due to
two pumps
Limited gradient range depending on pump type |
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