SRS Labs QMS 100 Series, Руководство пользователя

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1-10 Capillary Design
SRS QMS Gas Analyzer
Capillary Design
The inlet uses a bypass configuration that results in a fast response time. A large flow is drawn through
the capillary tube, which drops the pressure 3 decades. The typical capillary used at atmospheric
pressure has a bore diameter of 0.125 mm and a length of 0.7 m. Any number of combinations of
length and bore diameter can achieve the same flowrate and pressure drop. Capillaries are available in
several materials. The factors affecting the choice of capillary are:
•
inlet pressure
•
required response time
•
distance to sample point
•
material restrictions
•
cost
The possibilities for capillary choice are numerous, and SRS offers only a few types. The standard
capillary shipped with the QMS is mainly provided to test the system and provide a reference. The
following sections contains some guidelines to designing a capillary suited to the users application.
Length & Bore
To choose the dimensions of the capillary, three parameters must be fixed: the inlet pressure, exit
pressure, and flowrate. These three values allow the conductivity to be determined (equation 1). A
typical design point is an exit pressure of 1-2 mbar and a corresponding flow rate of 1.5 to 3.5 milliliter
min
-1
(see figure 4). The inlet pressure is determined by the users application. The capillary conductivity
is a function of the geometry, pressure, temperature, and gas properties. A common formula for air
flowing through a tube at 20
°
C is
C
d
L
P P
=
+
135
2
4
1 2
, (7)
where C is the conductivity in
l
s
-1
, d and L are in cm, and the pressures are in mbar. More general
formulas including temperature and viscosity are contained in the texts listed in the references. This
formula also assumes laminar flow.
There are many approximations used in equation 7 and users may be concerned about its accuracy.
The formula shows that the conductivity is a strong function of diameter. As a practical consequence,
this strong dependence means the standard manufacturing tolerances on bore diameters will cause more
uncertainty than the formula itself. A typical 0.005 inch bore capillary might have a ±10% tolerance.
While it is reasonable that the bore could vary from 0.0045 to 0.0055 inch, this uncertainty causes the
conductivity to vary by about ±40%. Use the standard formulas as a guide, but cut the capillary long to
begin with. Measure the actual performance and trim as necessary.