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UGA Series Universal Gas Analyzers
Calibration and Input Design
5 – 1
As an example, consider a capillary for atmospheric pressure and
an capillary exit pressure of mbar. From the speed curve for the
diaphragm pump, the throughput, Q, is 2.5 × 0
-2
mbar liter s
-
(Q
= P x S) and the required conductivity is 2.5 × 0
-5
liter s
-
. The
horizontal dotted line in Fig. 5-5 shows that for each capillary diam
-
eter there is an appropriate length. The application dictates which
bore diameter is appropriate. If the system being measured was far
away from the UGA, .8 meters of the 7/000 capillary would be
best. On the other hand, for something close 0.9 meters of 5/000
would be less expensive. The gas velocity through the capillary
quickly reaches the speed of sound. Thereby, until the capillary be-
comes
very
long, it is not an important contributor to the response
time of the UGA.
Users will find vendors of gas chromatography supplies a good
source for capillaries and fittings. Capillaries are available in many
materials. No material is ideal for all applications. The following
table list features of several materials
material
min. bore
diameter
advantages
disadvantages
stainless steel
0.005 in
• rugged
• high temper-
ature
• durable con-
nections
• difficult to
cut
without
clogging the
bore
•
marginal
flexibility
PEEK
0.005 in
• highly flex
-
ible
• can be cut by
user
• weaker con-
nections
glass
lined
plastic
0.002 in
•
smallest
bore
• cost
• not flexible
• low temper-
ature
Three conventional methods are available for making connections
to the capillaries: metal compression fittings, graphite seals, and O-
ring seals. Metal compression fittings are suitable for stainless steel
tube. The steel is capable of deforming to make the seal. The out-
5.3.6 Materials and Fittings
