Pure Water Conductivity Measurement: USP <645>, UPW Sensor and Verification
In purified water, water for injection and ultrapure water lines, conductivity is the primary measurement for acceptance or rejection; sensor selection and the verification method carry that decision.
Pure Water Conductivity: Five Points
- Limit: According to USP and Ph.Eur. monographs for PW and WFI, conductivity is at most 1.3 µS/cm at 25 °C.
- Method: USP <645> has three stages; the first stage is in-line measurement, and subsequent stages are conducted according to the conditions of the standard.
- Sensor: Low cell constant 2-pole UPW sensor (Hamilton Conducell UPW: cell constant smaller than 0.1/cm, 0.01–1,500 µS/cm).
- Temperature: Measurement and temperature compensation must be in the same sensor; in ultrapure water, compensation accuracy is decisive.
- Verification: Certified 1.3 / 5 µS/cm standards for the cell, and a traceable UPW Simulator for the electronics.
Why Is Pure Water Conductivity a Demanding Measurement?
In aqueous solutions, conductivity comes from the ions of dissolved acids, bases and salts. In ultrapure water where foreign ions are absent, only a small number of H₃O⁺ and OH⁻ ions are present and conductivity is at the lower limit of the scale: 18.2 MΩ·cm resistivity corresponds to approximately 0.055 µS/cm conductivity. At this level, a tiny contamination (CO2 dissolving from air, or a trace salt) shifts the reading noticeably.
For this reason, three factors become critical together in pure water measurement: a small cell constant (making the signal readable), accurate temperature compensation (referenced to 25 °C because conductivity changes markedly with temperature), and in-line measurement (without exposing the sample to air). Hamilton's catalog highlights these three points for the UPW sensor: superior temperature compensation, easy cleaning, and compliance with USP 645, EP and JP.
For definitions of PW, WFI and DI water grades, see our water grade selection guide. The typical generation train is RO + EDI; measurement points are typically the RO permeate outlet, EDI outlet and distribution loop.
USP 645 Three-Stage Test: What Must the Instrument Provide?
USP <645> "Water Conductivity" evaluates water conductivity through a three-stage test. Our engineering reference text specifies this three-stage structure and the in-line measurement limit of 1.3 µS/cm at 25 °C. The temperature table for the stages and the limits for subsequent stages are contained in the standard itself; on this page we do not reproduce tables in place of the official standard text.
On the instrument side, what matters is: (1) the sensor and temperature element must be verifiable, (2) the electronics of the conductivity meter must be checkable with a traceable reference (resistor), (3) the measurement must be in-line, and (4) records and calibration data must be retained. The Hamilton Conducell UPW Arc provides all four of these inside the sensor: Arc technology stores calibration, predictive diagnostics, automatic documentation and user/process assignment in the sensor head without requiring a transmitter. The Arc module is removable, and USP <645> compliance of the module can be verified in the field using the traceable resistor UPW Simulator (REF 243580).
Sensor Options for Low Conductivity
| Sensor | Range | Cell constant | Temperature and pressure | Connection | When to use |
|---|---|---|---|---|---|
| Hamilton Conducell UPW | Arc: 0.01–1,500 µS/cm; analog: 0.02–2,000 µS/cm | Smaller than 0.1/cm, 2-pole | Digital 0–130 °C, analog 0–110 °C; 10 bar (130 °C); autoclave, CIP, SIP | PG13.5 (VP6 REF 243640, Arc 243579); 1.5" Tri-Clamp (Arc 243578) | PW, WFI, ultrapure water; USP 645, EP, JP, FDA compliant |
| Walchem contacting, cell constant 0.01 | 0–0.3 mS (300 µS/cm) | 0.01 | PP 0–100 °C / 6.9 bar; SS 0–120 °C / 13.8 bar | 1/2" NPT, W100/W600 | RO permeate, condensate; pharmaceutical compliance not stated in brochure |
| Pyxis ST-724 | 0.02–1,000 µS/cm | 0.3, 2 Hastelloy electrodes | Sample 4–49 °C | GUARDIAN panel flow cell | Boiler feedwater |
| Hamilton Conducell 4UxF | 1 µS/cm – 500 mS/cm (analog) | 0.36/cm | -20…150 °C; 20 bar (135 °C) | PG13.5, Tri-Clamp | Not for pure water; CIP solutions and process water |
Flow Cell, Armature and Measurement Point
The Conducell UPW is installed either directly into the pipe via PG13.5 or Tri-Clamp, or in a flow cell. The Hamilton FlowCell (REF 242585) is a low dead-volume (approximately 8 mL), self-draining cell with a PEEK inner body and integral aseptic clamp pipe connection; in this cell, the Conducell UPW can only be used in TC-connected versions. The FlexiFlow SL 10 is also compatible with the Conducell UPW (REF 237340). The flow cell is rated for 0–16 bar(g) and -10…140 °C, with a standard FDA-approved EPDM seal.
When selecting the measurement point, the RO outlet (membrane performance), EDI outlet and distribution loop should be considered separately. If the line temperature is high (hot water sanitization), the sensor's 130 °C and 10 bar limits must be checked.
Low-Conductivity Standards and Verification
| Standard (25 °C) | Packaging | Matrix | Certified accuracy | Stability | REF |
|---|---|---|---|---|---|
| 1.3 µS/cm | 250 mL glass bottle | Glycerin | 1 percent | 12 months | 238973 |
| 5 µS/cm | 250 mL glass bottle | Glycerin | 1 percent | 36 months | 238926 |
| 15 µS/cm | 250 mL glass bottle | Glycerin | 1 percent | 36 months | 238927 |
| 84 µS/cm | 500 mL Calpack bottle | Water | 1 percent | 18 months | 238984 |
Conventional low-conductivity standards drift due to CO2 absorption once the bottle is opened; Hamilton's patented glycerin-matrix formulation is stated to maintain documented stability even if the bottle remains open for up to one hour. The 1.3 and 5 µS/cm standards are produced with 1 percent certified accuracy and independently verified by European metrology institutes. For details, see the Hamilton page.
If the sensor is calibrated in the field, the principle in the 4UxF manual also applies to the UPW: the cell constant depends on the mounting geometry; whenever possible, the sensor should be calibrated in an installation matching the process geometry, and O-rings are wear parts that should be replaced after one year at the latest.
Related Pages
Conductivity measurement (all sensor types) · Hamilton conductivity sensors · Purified water systems · Water for injection (WFI) systems · Ultrapure water systems · RO membrane selection · Toroidal conductivity sensor (not suitable for pure water)
Pure Water Conductivity Measurement: Frequently Asked Questions
Limits, UPW sensors, USP 645 verification, connections and calibration standards.
What is the conductivity limit for purified water and WFI?
According to USP and Ph.Eur. monographs, purified water (PW) and water for injection (WFI) conductivity must be at most 1.3 µS/cm at 25 °C; this value is given for in-line measurement. In the three-stage test of USP 645, the temperature-dependent limit table and the conditions for the subsequent stages are set out in the standard itself; therefore, your quality unit should determine the measuring instrument and verification method according to the current text of the standard.
Why is pure water conductivity measured in-line?
When water with very low conductivity comes into contact with air, it dissolves carbon dioxide and its conductivity rises rapidly. Hamilton's calibration standards catalog also notes that low-conductivity standards drift due to CO2 absorption after the bottle is opened. For this reason, continuous measurement with a sensor mounted in-line is preferred over taking a grab sample to the laboratory; the first stage of USP 645 is also an in-line measurement.
Why does a UPW sensor differ from a general-purpose conductivity sensor?
Pure water contains very few ions; the sensor cell constant must be small and temperature compensation must work very accurately. The Hamilton Conducell UPW is 2-pole, has a cell constant smaller than 0.1/cm, and measures 0.01–1,500 µS/cm in the Arc model. The wide-range 4-pole Conducell 4UxF starts from 1 µS/cm and is not designed for the lower end of the pure water range.
Are 18.2 MΩ·cm and 0.055 µS/cm the same thing?
Yes, they are two representations of the same quantity: resistivity is the reciprocal of conductivity. A resistivity of 18.2 MΩ·cm corresponds to approximately 0.055 µS/cm conductivity. The intrinsic conductivity of ultrapure water comes from H₃O⁺ and OH⁻ ions, which form the lower limit of the scale. The Pyxis SP-600 handheld meter displays conductivity, TDS and resistivity (10–30 MΩ) together.
How do I verify USP 645 compliance in the field?
The Arc module of the Hamilton Conducell UPW Arc is removable; USP 645 compliance of the module can be verified in the field using the UPW Simulator (REF 243580), which is a traceable resistor. For the sensor cell, certified 1.3 µS/cm and 5 µS/cm conductivity standards are used. Verification frequency and documentation format are defined by your quality system.
Which process connections are available for the Conducell UPW?
For PG13.5 (120 mm), models with VP6 (REF 243640) or Arc (REF 243579) output; for 1.5" Tri-Clamp (87 mm) connection, the Arc model (REF 243578). The wetted part is stainless steel DIN 1.4435, surface finish Ra is less than 0.4 µm (N5), and the O-ring is EPDM. The conventional (analog) model is ATEX and IECEx approved; the digital Arc is rated for 0–130 °C and 10 bar (130 °C).
Can I install the sensor in a flow cell?
Yes. Inside the Hamilton FlowCell (REF 242585), the Conducell UPW can only be used in TC-connected versions; the tube position is determined according to the manufacturer's compatibility table. The FlowCell has a low dead volume (approximately 8 mL), is self-draining, and features an integral aseptic clamp pipe connection. Other flow cell models such as the FlexiFlow SL 10 are also compatible with the Conducell UPW, whereas the Conducell 4UxF is not compatible with FlexiFlow.
Send the make and model of your current instrument to [email protected] — our engineers will propose the equivalent with the same connection and measuring range.
Planning a project? Send us your flow rate, feed water source and target water quality to [email protected] — our engineers will reply with a technical proposal.