6-5
" #!
To maintain a sample ready status, the 2700 SELECT self-calibrates. Calibrating establishes the
sensors’ response, in nanoamps of current, to a known concentration of substrate.
The sensors calibration response must be above 5 nA. A response below this value will result in
an error.
The 2700 SELECT self-calibrates when entering RUN Mode. While in RUN Mode, it calibrates
every 5 samples or 15 minutes. However, default calibration parameters can be altered to
tighten or loosen calibration specifications. In RUN Mode a manual calibration can be initiated
by pressing [CALIBRATE] or by remote control through an RS-232 interface.
A STABLE CALIBRATION IS IMPORTANT. The instrument re-establishes a calibration
reference point after every calibration. If a difference of more than 2% (default setting) between
the present and previous net calibration currents occurs, the instrument repeats calibration. The
sensors’ net current for a calibration is displayed and printed. An unstable calibration is
displayed and printed as a "cal shift". While establishing a stable calibration, the 2700 SELECT
will run 5 calibrations before the error
'unable to calibrate'
is displayed. The flexible parameter
selection allows the user to disable this error mode in menu setup.
In summary, by the default calibration settings, recalibration will occur after every 5 samples or
15 minutes, after a calibration shift of 2% or greater, or after a sample chamber temperature
drift of more than 1°C. After 5 attempts without successfully calibrating, the instrument
displays a calibration error message.
$
As discussed in Section 6.1, a sensor consists of an electrode and an enzyme membrane. As a
membrane ages, its response becomes non-linear (shown in Figure 6.4).
Figure 6.4
Aging Membrane Response
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