Analyzing Oil-Based Lubricants’ Moisture Content

The presence of moisture is a key problem for oil-based lubricants as it can negatively affect the efficiency of the lubricants, causing early wear to components and increasing maintenance costs.

Hence, monitoring moisture content in lubricants has become essential. Moreover, monitoring water removal systems is also imperative in order to ascertain their proper working condition.

Karl Fisher (KF) titrators are traditionally used for moisture content analysis. Although they yield accurate results, they require hazardous chemicals, constant maintenance, and fragile and expensive glassware, which limits the area in which these instruments can be staged effectively.

Arizona Instrument’s Answer

To handle these shortcomings, Arizona Instrument has developed a solvent free, moisture specific instrument capable of measuring water content in oils using RH sensor technology with the same accuracy as KF titrators.

The Computrac® Vapor Pro® line of instruments lowers analysis times and waste costs, and the rugged design of the Vapor Pro® instrument enables them to be used in more hazardous environments.

Experimental Procedure

This analysis involved the testing of virgin SAE 10W-30 motor oil, turbine oil, and used SAE 10W-30 motor oil for water content utilizing a Karl Fisher Titrator with the VA-110 vaporizer and the VA-110 vaporizer, and the Computrac® Vapor Pro® 3100L.

The turbine oil was kept in a clean Nalgene bottle, the used oil was stored in 0.5L Nalgene and the virgin oil was stored in its original package.

Karl Fisher Titration Method

The sample loading bubbler oven of the KF was loaded slightly above half with reagent grade toluene and set to a flow rate of 300mL/min and temperature of 90°C. The titration was started after a one minute delay subsequent to the addition of sample material to the bubbler. After starting the titration, there was at least one minute that the device titrated prior to test completion. This was to allow the lubricant to heat up and evolve water.

The ending criteria were set to complete the test when the amount of water being titrated was 0.1μg/sec over the baseline titration level. Testing would not begin if the baseline was beyond 0.3μg/sec. The titration vessel was loaded with 200mL of Coulomat A in the large, exterior container subsequent to the addition of toluene to the bubbler as required to boil it over into the titration vessel.

The interior cathode was loaded with Coulomat C until reaching the same levels as both Coulomat A and C. The titration cell was cleaned in between analyzing each material and checked for accuracy every day, before testing, utilizing a 1% methanol water standard. The sample size was 0.6mL for the motor oil and 3.0mL for the turbine oil.

Vapor Pro® 3100L

The following were the conditions used for performing the Computrac® Vapor Pro® testing:

Test temperature :   140°C
Bottle Purge :   40s
Ending criteria :   Time → Rate, minimum 3 minutes, 0.30μg/sec

For each test, a clean bottle with a new septum was employed and a 1mL syringe was used to load 0.6mL of fluid into the bottle for the motor oil samples. The sample size for the turbine oil was 1.5mL, due to its low moisture content.

After loading the sample, the vial was tightly sealed. At least nine tests were performed to determine the average ppm, standard deviation (SD), and relative standard deviation (RSD).

Experimental Results

The analysis results for the turbine oil, virgin motor oil, and used oil are summarized in Tables 1, 2, and 3, respectively. The results show a strong agreement between the KF Titrator and the CT 3100L for all three materials analyzed.

Table 1. Statistical Analysis for Turbine Oil.

Turbine Oil
CT 3100L   KF
30.67 PPM 23.33
4.82 SD 8.72
15.72 RSD 37.36

Table 2. Statistical Analysis for Virgin Motor Oil.

Clean Motor Oil
CT 3100L   KF
393.63 PPM 369.00
17.59 SD 24.48
4.47 RSD 6.64

Table 3. Statistical Analysis for Used Motor Oil.

Used Motor Oil
CT 3100L   KF
761.25 PPM 742.40
17.19 SD 63.56
2.26 RSD 8.56

Figures 1 and 2 show the graphs generated by the CT 3100L, providing real time data for a comprehensive analysis and enabling users to optimize testing criteria.

Rate graph for moisture measurement of virgin motor oil.

Figure 1. Rate graph for moisture measurement of virgin motor oil.

Total moisture curve for virgin motor oil.

Figure 2. Total moisture curve for virgin motor oil.


The results clearly demonstrate the advantage of using relative humidity sensor instruments like the Computrac® 3100L over Karl Fisher titration for testing lubricants. This solvent free method eliminates the use of harmful chemicals required for titration and also facilitates rapid throughput of sample analysis by considerably shortening maintenance and calibration times.

The results yielded by the CT 3100L showed good agreement with the KF Titration results, but with a reduced standard deviation and relative standard deviation measures.

About Arizona Instrument

Initially known as the Quintel Corporation, Arizona Instrument LLC was founded in 1981 by a group of engineers breaking away from The Motorola Corporation who were dedicated to the idea of providing precision moisture analysis instruments that were accurate, reliable, and easy to use.

The first instrument released was the MA Moisture Analyzer, but the company quickly expanded its Computrac® moisture analysis line and became an accepted leader in moisture analysis, setting a standard that has been adopted by many Fortune 500 companies. Today the Computrac® line is comprised of three technologies: rapid loss-on-drying, high temperature loss-on-ignition, and moisture specific analysis using polymer capacitance sensor, GREEN alternative to Karl Fischer.

Arizona Instrument

This information has been sourced, reviewed and adapted from materials provided by Arizona Instrument.

For more information on this source, please visit Arizona Instrument.


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