4. OPERATION
Page 53
© 2007 DH Instruments, a Fluke Company
The design life of the GFS reference gas cylinders is 15 years from the date of manufacture.
After 15 years the cylinders should be discarded and replaced. Hydrostatic testing should also be
performed on the cylinders every 5 years to ensure safe operation.
4.4
CONSIDERATIONS FOR MAKING RELIABLE
MEASUREMENTS
4.4.1
COOLING AND CONDENSATION AT HIGH FLOW
At flows above 2 slm, the fast discharge of gas pressure from the reference gas cylinder
causes a fairly rapid drop in the temperature of the cylinder and regulator assembly. This
cooling in itself does not create a large measurement problem for the GFS2102 system since
the air temperature surrounding the cylinder is measured to account for changes in air
buoyancy. This correction is a small contributor to uncertainty of GFS2102 flow
measurements. Also the design of the balance and AMH platform and the granite tabletop
keep the balance at a relatively constant temperature even under high flow conditions.
But a very significant problem can result if the temperature of any surface of the reference
gas cylinder assembly drops below the local dew point, causing water to condense on the
assembly. The added mass of the condensation makes reliable mass measurements
impossible. As a solution, an infrared (IR) probe is positioned to read the surface
temperature of the reference gas cylinder and used to predict when condensation is a risk
(See Section 3.3.13, 5.1.5.5). When the IR probe is detecting a temperature within 3°C of
the dew point, a “D” is displayed in the circular Ready/Not Ready indicator in the GFS Main
Run Screen (See
Table 15
in Section 5.1.5.2.1) and is also logged to the Status column in the
data file.
A secondary consideration occurs for gases that have a higher tendency to cool such as
CO2 or N2O. For these gases the first stage of the dual regulators attached to the cylinder
assembly will cool far faster than the cylinder itself. The surface area of this regulator is far
less than that of the cylinder, but if the temperature drops below the local dew point
condensation can occur at this localized point and it will make the mass measurements less
reliable. Since the IR probe is adjusted for the infrared emissivity of the cylinder, it cannot be
used to read the temperature of the first regulator.
To assist the system in overcoming these issues while maintaining automation, a “Pause
Flow” function is available in the options of a test setup. This function allows the test to
complete an accumulation and then pause for a user set predetermined amount of time,
usually 30 – 60 minutes, before proceeding with the next accumulation, thus allowing the
cylinder and regulator to warm back up and lower the risk of condensation (See Section
5.5.2.8).
4.4.2
STABILITY OF AMBIENT CONDITIONS
The automated nature of the GFS2102 system allows users to select the time to run test that
is best suited to making the best measurements. Two common examples of changing
conditions that can be avoided are a warm cylinder that has been recently filled and needs to
cool, or a laboratory area which experiences significant changes in temperature and humidity
throughout the day. Often when lab personnel leave the workplace, the laboratory
experiences a reduction in both temperature and humidity over the course of a few hours.
Outdoor conditions may also contribute to laboratory changes. Using a test start time delay
feature in GFS Tools software, the user can program a test to begin at a time when
environmental transients are minimized so that the GFS system, DUT and flow controller all
have the best chance to perform at their highest level. The possibility of unattended