Ludlum Measurements 3100, Руководство пользователя

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A-7
d. Momentarily open and reclose metering inlet valve.
e.
Momentarily open and reclose metering outlet valve, flushing the metering volume.
f.
Repeat steps “d” and “e” if required. The calibrator is now ready for use.
A.2.7 Closed Loop Calibration.
a. Connect the calibrator outlet hose to the outlet of the instrument and turn on its air pump.
b. If the cylinder pressure gauge shows any pressure reading, there is enough gas contained in the
regulator to proceed with the next step. If not, momentarily open and close the lecture bottle valve so
that this gauge reads upscale.
c.
Open metering inlet valve. Metering outlet valve should be closed.
d. Turn the outlet pressure adjust knob slowly clockwise, watching the reading on the outlet pressure
gauge.
e.
When the outlet pressure gauge reads the desired injection pressure (typically 20 or 30 psi), close the
metering inlet valve.
f.
Connect the inlet hose to the inlet connector on the instrument.
g.
Open the metering outlet valve for between two and four seconds, and then reclose it firmly. Longer or
shorter times will lead to incorrect results.
h.
In the above step, a known aliquot of gas was injected into the chamber. To inject a second aliquot,
repeat Steps “c” to “f” above. Any number of aliquots may be injected with the instrument reading
increasing proportionately.
In closed-loop calibration, the activity enters the chamber rapidly but is then redistributed throughout the volume of
the instrument and calibrator gas system. Since the bulk of the volume is in the tritium ion chamber, the scale
reading will rise (spike), then drop, and then level off to a constant value independent of time constants or pumping
speeds. The indicated scale reading depends solely upon the system volume and the pressure in the aliquot as
described in the next section.
A.2.8 Calculation of Response.
Using the technique described in the previous section, the 11 mL metering
volume is always kept filled with tritiated methane. When the outlet pressure gauge reads 20 psi and the metering
inlet valve is open, the actual methane pressure in the metering volume is 35 psia. With the metering inlet valve
closed and metering outlet valve opened momentarily, the methane pressure in the metering volume is now 15 psia.
This means the mass of methane injected can be calculated from the pressure differential (35-15 psia), which is
equal to the scale reading of the outlet pressure gauge.
It is very important to open the metering outlet valve for only 2 to 4 seconds. A longer opening time allows the
residual 15 psia of pure methane in the metering volume to mix with the air stream flowing into the calibrator
tubing, eventually distributing this extra gas throughout the system volume. The instrument scale reading will then
correspond to injection of 35 psi of gas using the formula below. Although this technique can be used, we do not
recommend it since it is slower and somewhat less reproducible.
A.2.8.1 Formula.
The response can be calculated using:
v = metering volume (11.0 mL in the CL-l)
V = total volume of instrument plus calibrator (in mL)
P = gauge pressure of gas in metering volume (psig) (i.e., reading of outlet pressure gauge)
A = original specific activity of gas in lecture bottle (µCi/liter at 25 °C and 14.7 psia)
n = number of aliquots injected
d = tritium decay since cylinder calibration (see below)