Malvern Hydro 2000uP User Manual page 36

Chapter 4
Bubbles and pump speeds
Control the pump speed to prevent the introduction of bubbles while ensuring that
the sample is kept in suspension. This is true for all wet dispersion units but partic-
ularly important with the small dispersant volumes of the Hydro 2000µP. The
Mastersizer sees bubbles as particles and measures these. Bubbles will vary in size
but are typically in the region of 100 microns. In many cases bubbles can be clearly
seen as a separate peak when the measurement data is analysed.
Consider the following points when making measurements:
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When filling the system with dispersant, turn off the pump to prevent it intro-
ducing bubbles.
Fill the system with dispersant slowly to avoid trapping bubbles.
When the system is first filled, bring the pump speed up slowly to clear any
remaining bubbles, without breaking them into smaller ones. Ideally use the
Anaerobic fill sequence (described later) to remove air and bubbles from the
system.
Take care with the pump speed. The system is designed, and balanced, for
water. Other dispersants may tend to well up in the well or pump chamber as
the pump speed increases. Full speed (5000 rpm) is too much for most disper-
sants, so do not use 100% pump by default. See the table in the Anaerobic fill
section later for examples of settings.
Choose a pump speed high enough to keep the particles in suspension. Using
too high a pump speed will tend to draw air into the system.
Always fit the well lid during measurements.
Finally, on the last wash before filling the system to make a measurement, we
recommend that the pump is set to off whilst draining. This is because the
action of running the pump whilst draining can cause the formation of small
air bubbles in drops of dispersant that may remain in the system after draining.
If the system is then filled immediately in order to perform a measurement,
these air bubbles are circulated and sized in addition to the required sample.
Making a measurement
MAN 0389