Anatel A643a - Anatel A643a Setup 63 of 220
RPS - June 2007 - Edition 12
Anatel Operator Manual
• Cycle Modes allows conductivity (or resistivity) and temperature readings to be reported
while the instrument is purging between analyses.
5.5.1.1 Sample Time
Default: 1 Minute
The Sample Time is the period during which water flows through the Anatel A643a’s
measurement cell prior to oxidation. This ensures that a discrete and representative water
sample is oxidized during each analysis. The interval is user-selectable from 00:00:00 to
17:59:59 and a proper setting helps assure accurate and timely analysis results.
During the Sample Time, Sample Valve 2 is open to allow the measurement cell to purge of
any residual contamination. An empirical method for determining an appropriate Sample Time
for any flow rate is to switch the instrument into the Purge Mode at the end of an oxidation cycle
and note the time it takes for the resistivity reading to stabilize. Double this time to obtain a
proper Sample Time interval. For systems where the water pressure varies, use the average
of several such readings or update the Sample Time settings when inlet conditions change.
The Analyzer must be rinsed completely in order to accurately measure sample resistivity.
Even minute amounts of residual oxidation products will increase the conductivity of a high-
purity water sample considerably. The Anatel A643a compensates the reported conductivity for
variations in temperature caused by using high intensity UV which, in conjunction with an
integral heater, warms the measurement cell. A substantial Sample Time reestablishes the
temperature balance as well as flushing the previous sample.
As an example, in 0.05 µS/cm water containing 100 ppb TOC, a rinse of 99% will lower the
measured TOC by only 1 ppb. Sample conductivity, however, will degrade by more than
0.4 µS/cm.
For analyses from sources where the water pressure exceeds 15 psig, a good rule-of-thumb is
to set the Sample Time to two minutes. The setting should be increased by one additional
minute for each two meters of 1/4” sample tubing, and another minute for each 10 °C
differential in temperature between the sample water and the room temperature.
Although a lengthy Sample Time guarantees the most accurate conductivity and temperature
readings, adequate TOC sampling can be accomplished in shorter intervals. In cases where
Fig 5-21 : Sample Time
Drain
Sample
Vessel
Check
Valve
Process
Water
Inlet
Sample
Valve 1
Needle
Valve
Check
Valve
15 Micron
Filter
Bypass
Check
Valve
Sample
Valve 2
Calibration
Valve
Pump
Analysis
Cell
Thermistor
Sample
Needle
Air
Filter
Air Inlet
Sample
Time
Note: Arrows indicate flow path.
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