28
SECTION 2,
continued
2.3.5
Removing Bubbles (Degassing)
Before measurement, removing air and other trapped gasses from the
sample is strongly recommended, even if bubbles are not visible. Four
degassing methods are commonly used:
1.
applying a partial vacuum
2.
adding a surfactant
3.
using an ultrasonic bath
4.
heating the sample
In some cases, more than one method may be necessary for effective
bubble removal. For example, use of both a surfactant and ultrasonic
bath may be necessary for some severe conditions. Use care with these
techniques. If misused, sample turbidity can be altered.
Removing air bubbles by letting the sample stand for a period of time is
not recommended. Particulates that cause turbidity may settle and the
sample temperature may change. Both conditions may alter sample
turbidity, resulting in measurements not representative of the original
turbidity.
2.3.5.1
Application of vacuum
Apply a vacuum with any convenient, clean, oil-free vacuum source.
The vacuum lowers the atmospheric pressure, allowing trapped bubbles
to escape into the air above the sample. Vacuum works well with non-
viscous samples (such as water) that don’t contain volatile components.
Applying vacuum to viscous, volatile-containing samples (paint resins)
may cause the volatile components to come out of solution and
aggravate the bubble problem.
To apply a vacuum, use a sample degassing kit equivalent to Cat No.
43975-00 (Degassing Kit) or 43975-10 (Degassing and Filtration Kit).
These kits contain a syringe and rubber stopper for vacuum degassing.
An electric or hand-operated pump equivalent to Cat No. 14283-00 or
14697-00, respectively, may also be used.
