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3.13.2 Stabilization Issues
3.13.3 Probe Signal Stability and Amplitude
3.13.4 Probe Temperature Considerations
16116Operation.fm
The meter will not accept calibration unless the probe signals are stable over
time and in amplitude. It is also important that the probe and calibration water
are in equilibrium. Following the procedural guidelines in this section will help
ensure fast and accurate calibrations.
One of the critical factors in a successful calibration is the stability of the
reading. The instrument will not calibrate if the displayed reading is seen to be
varying by more than approximately 0.02 ppm (or 0.02 mg/L) per 60 seconds.
In this case, you will have to increase the instrument damping until the reading
has the required stability (see
In addition to monitoring the stability of the probe signal, the instrument also
checks the amplitude of the probe signal level to insure a proper calibration.
During zero calibration, the probe signal must be below the level recorded
during the last span calibration. Likewise during span calibration, the probe
signal must be above zero by at least 0.01 ppm. When the probe signal is
stable and the signal level is correct, the instrument will continue with the
calibration. If these two conditions are not met, the calibration timer will expire
(see
section 4.3 on page
54). Do not bypass the stability verification unless
directed.
Due to the diffusion rate of chlorine through the probe membrane, higher
temperatures increase the probe output signal, and lower temperatures
decrease it. If left uncorrected, drift errors would appear in displayed
concentration readings. Membrane temperature is monitored by the
instrument to compensate for the displayed chlorine readings. For this
purpose, a thermistor is mounted inside the probe body close to the
membrane. The thermistor resistance is converted into an amplification factor
which the software applies to the probe output signal. The instrument
software compensates by increasing the signal at lower temperatures, and
decreasing the signal at higher temperatures.
Because the thermistor is mounted inside the probe body and surrounded by
electrolyte, it may be delayed in reaching the temperature of the sample
stream. If the probe has been left in a container of deionized water for zero
calibration, the thermistor may have stabilized at the temperature of the
deionized water. When installing the probe into the sample stream for span
calibration, allow time for the thermistor to equilibrate to the temperature of
the stream.
Example:
When the probe is zeroed in a solution of deionized water prepared in a room
where the air temperature is 77 °F (25 °C). It is then desired to span the probe
in a flow cell where the water temperature is 59 °F (15 °C). If the chlorine
concentration is 0.20 ppm, and the probe is taken from the zero solution
directly into the flow cell, it will take nearly 30 minutes for the probe thermistor
reach its final temperature. If enough time is not permitted, the instrument
may be spanned to a temperature compensated output that is lower than its
final value. The effect will be that the displayed concentration readings will
tend to drift higher, until the thermistor is at its final temperature.
In general, the speed of calibration and accuracy may be improved when the
zero and span calibrations are done at the same temperature. See
section 3.14.2 on page
36.
section 3.22 on page
43).
Considerations Before You Calibrate
Section 3
Page 35
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