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The conductivity of pharmaceutical grade water must be checked throughout the purification process to ensure that it is devoid of contaminants. There are multiple considerations for the selection, installation and calibration of pure water conductivity sensors.

Type of sensor

Purified water is low conductivity (typically 5 microSiemen/cm or less). These applications demand a two pole conductivity sensor with a cell constant of 0.10 or less. The Hamilton Conducell UPW Sensor is the best choice for these types of measurements.

Temperature compensation

Temperature directly impacts the conductivity of any liquid. The temperature coefficient for pure water can vary greatly depending on the actual temperature of the water. To avoid errors, USP <645> and European Pharmacopoeia standards mandate measuring only the raw conductivity value for purified water (Water For Injection, Water for Hemodialysis and pure steam condensate). The raw conductivity must be below the values listed in the table below.

Stage 1 – Temperature and Conductivity Requirements USP <645>

Temperature (accurate to 2 °C or better)
Raw Conductivity Requirement (µS/cm)
0
0.6
5
0.8
10
0.9
15
1
20
1.1
25
1.3
30
1.4
35
1.5
40
1.7
45
1.8
50
1.9
55
2.1
60
2.2
65
2.4
70
2.5
75
2.7
80
2.7
85
2.7
90
2.7
95
2.9
100
3.1

Temperature and Conductivity Requirements EP Aqua Purificata

Temperature (accurate to 2C or better)

Raw Conductivity Requirement (µS/cm)

0

2.4

10

3.6

20

4.3

25

5.1

30

5.4

40

6.5

50

7.1

60

8.1

70

9.1

75

9.7

80

9.7

90

9.7

100

10.2

Installation

The installation of the conductivity should be mounted so that no air bubbles or solids can accumulate on the surfaces. Examples of recommended pipeline installations are shown below.

Sensor mounted into a tee with a flow directed at the tip of the sensor then making a 90 degree turn

Sensor mounted horizontal in a vertical pipe with flow going upwards

Verification

USP and EP requirements state that the cell constant of the conductivity sensor must be verified within 2% using a certified liquid reference solution. In North America, ASTM D1125 Solution D (146.9 µS/cm) is commonly used while EP mandate a certified standard at 1500 µS/cm or less. Hamilton Conductivity standards are highly recommended for this type of verification and come with traceable documentation. The Conducell UPW sensor also provides a UPW simulator that allows ARC electronics to be verified according to USP <645> standards.