LI-COR LI-6400, Manual

Page: 473 / 846

Calibration Issues
Flow meter
Using the LI-6400 18-17
Flow meter
Flow in the LI-6400 is measured with an electronic mass flow meter in the
console.
Factory Calibration
The flow meter is calibrated at three temperatures using a series of mass flow
controllers. (The flow controllers are, in turn, periodically calibrated with a
precision device that operates on a volumetric displacement principle.) The
flow data for all temperatures is fit with a straight line, whose slope becomes
the
a
f
term in Equation (14-13) on page 14-7. This value is shown on the
LI-6400 calibration sheet, and is stored in the instrument, as the object of the
CalFlow= configuration command.
Zeroing the Flow meter
The flow meter zero program is run at the userÕs discretion. The program is
fully automatic. When you select
"Flow meter zero"
in the Calib Menu, the
flow will be shut off, and the system will begin a 10 second count down
(Figure 18-9). After ten seconds, the flow meter signal should read within 1
mV of zero. Press
OK
.
FLOWMETER ZERO
7 secs to Zero Flowmeter
Flowmeter signal: 0.7 mV
NoAuto Cancel
FLOWMETER ZERO
Done
Flowmeter signal: 0.7 mV
Adjst
-
Adjst
¯
OK
When entered, the zero rou-
tine turns off the pump, waits
10 seconds, and automati-
cally zeros the flow meter.
Skip the Automatic Zero
Afterwards, you can monitor
the flow meter signal, and
manually adjust the analog
output channel that is used
to zero it. If you do, the value
of the channel is displayed
instead of the word ÒDoneÓ.
Figure 18-9. Zeroing the flow meter is usually automatic, but manual adjustment is also possible. The flow
meter zero is controlled electronically by an analog output channel, whose value is displayed in the Zero
column of the View, Store Zero & Span program (page 18-18).