43
DIRECT/USP MEASUREMENT
In this measure mode the user can check for ultra pure water using the United States Pharmacopeia standard
(USP <645>).
This USP standard consists of three stages (one in-line and two off-line tests) as following:
Stage 1
- this is an in-line test.
Test steps:
• Measure the temperature of the water and the uncompensating conductivity readings. The measurement
may be performed in a suitable container or as in-line measurement.
• The temperature will be round down to the nearest 5 °C and look up the corresponding conductivity value
in the below table.
• If the measured conductivity is lower than the conductivity in the table, then the water meets the USP
requirements.
• Otherwise, proceed to Stage 2 testing.
Stage 2
- this is an off-line test.
Test steps:
• Store the water sample in an enclosed clean container that has been rinsed previously with water of the
same quality.
• Adjust the sample’s temperature to 25 °C and agitate the sample to ensure that it has equilibrated with
ambient CO
2
.
• If the measured conductivity is less than 2.1
µ
S/cm, then the sample has met the USP requirements.
• Otherwise, proceed to Stage 3 testing.
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