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USP Purified Water and WFI Conductivity Limits by Temperature (Stage 1)

USP chapter 645 sets the conductivity test for purified water and water for injection, and its first stage uses a table of limits that depends on the temperature of the sample. This reference gives that table next to the other water limits QC and utilities teams look up daily.

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Temperature or parameter	Limit	Unit	Note
0 deg C	0.6	uS/cm	Stage 1 maximum, non temperature compensated
5 deg C	0.8	uS/cm	Stage 1 maximum, non temperature compensated
10 deg C	0.9	uS/cm	Stage 1 maximum, non temperature compensated
15 deg C	1.0	uS/cm	Stage 1 maximum, non temperature compensated
20 deg C	1.1	uS/cm	Stage 1 maximum, non temperature compensated
25 deg C	1.3	uS/cm	Stage 1 maximum, non temperature compensated
30 deg C	1.4	uS/cm	Stage 1 maximum, non temperature compensated
35 deg C	1.5	uS/cm	Stage 1 maximum, non temperature compensated
40 deg C	1.7	uS/cm	Stage 1 maximum, non temperature compensated
45 deg C	1.8	uS/cm	Stage 1 maximum, non temperature compensated
50 deg C	1.9	uS/cm	Stage 1 maximum, non temperature compensated
55 deg C	2.1	uS/cm	Stage 1 maximum, non temperature compensated
60 deg C	2.2	uS/cm	Stage 1 maximum, non temperature compensated
65 deg C	2.4	uS/cm	Stage 1 maximum, non temperature compensated
70 deg C	2.5	uS/cm	Stage 1 maximum, non temperature compensated
75 deg C	2.7	uS/cm	Stage 1 maximum, non temperature compensated
80 deg C	2.7	uS/cm	Stage 1 maximum, non temperature compensated
85 deg C	2.7	uS/cm	Stage 1 maximum, non temperature compensated
90 deg C	2.7	uS/cm	Stage 1 maximum, non temperature compensated
95 deg C	2.9	uS/cm	Stage 1 maximum, non temperature compensated
100 deg C	3.1	uS/cm	Stage 1 maximum, non temperature compensated
Total organic carbon, Purified Water	500	ppb as C	Maximum, USP chapter 643 or an oxidisable substances test
Total organic carbon, WFI	500	ppb as C	Maximum, USP chapter 643
Bacterial endotoxin, WFI	0.25	EU/mL	Maximum
Microbial action level, Purified Water	100	cfu/mL	Guidance level from USP general information chapter on water systems
Microbial action level, WFI	10	cfu/100 mL	Guidance level from USP general information chapter on water systems

Stage 1 applies to in-line or off-line measurement without temperature compensation. If the measured value is at or below the limit for the measured temperature, the water meets the requirement. Stage 2 and Stage 3 use different tests. Check the current USP-NF for official wording and your pharmacopoeia if you ship to other markets.

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An optional review of reading historian and LIMS data on-prem to alert on drift before limits are crossed.

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How to use the table

Read the conductivity and the temperature from the same measurement. Find the closest temperature row that does not exceed your reading and compare the measured value with the limit shown. For example, water at 25 deg C with a reading of 1.2 uS/cm passes Stage 1 because the limit is 1.3. If the temperature is between rows, use the next lower row from the table for a conservative result.

Where the numbers come from

Pure water is not perfectly non-conductive because it self-ionises, and the amount rises with temperature. The Stage 1 table sets a limit for each temperature that allows for this effect plus a small allowance for dissolved gases and trace ions. That is why a limit of 1.3 uS/cm at 25 deg C is higher than 0.6 uS/cm at 0 deg C, and why the limit is flat between 75 deg C and 90 deg C.

Using it in operations

Sensors in a loop usually report conductivity at the actual temperature, and the control system compares it with this table to raise an alert before the batch or sample is affected. Record the sensor calibration, the temperature, the reading and the limit on the log. When a reading fails Stage 1, do not discard the water immediately but proceed to the next stage defined in the chapter.

Calibrate conductivity and temperature sensors on a schedule.
Log alert and action levels separately from the USP limit.
Document failures and the stage used in the deviation.

Common mistakes

A common error is applying a temperature compensated reading, which makes the table invalid. Another is using the 25 deg C limit for every sample regardless of temperature, which can let hot samples fail unnecessarily or allow cold ones to pass wrongly. Finally, microbial levels in this table are guidance, not compendial limits, and sites set their own action levels.

Frequently Asked Questions

What is the conductivity limit for purified water at 25 deg C?

Under Stage 1 of USP chapter 645, the maximum is 1.3 uS/cm at 25 deg C, measured without temperature compensation. If the reading is at or below the limit for the sample temperature, the water meets Stage 1.

Does the same table apply to WFI?

Yes, the Stage 1 conductivity limits are used for both purified water and water for injection. The two grades differ in other requirements, for example WFI has a bacterial endotoxin limit of 0.25 EU/mL and tighter microbial guidance levels.

What happens if Stage 1 fails?

The chapter allows a Stage 2 test, which uses a pH dependent limit after equilibration with air, followed by Stage 3 if needed. Many sites treat a Stage 1 fail as an alert and investigate the cause before testing further.

Are the microbial values compendial limits?

No. The 100 cfu/mL for purified water and 10 cfu/100 mL for WFI are widely used action level guidance. Sites set alert and action levels from their own data, and your specification and validation define what is mandatory.

Netray can help build a private on-prem alerting layer that reads loop sensor data and historian tags, compares them with limits like these and shows operators what the rule was.

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