How Light Affects Oil Analysis Results for Varnish Potential

Bryan Johnson, Palo Verde Nuclear Generating Station
Tags: oil analysis

How Light Affects Oil Analysis Results for Varnish Potential

As oil or oil additives degrade, they develop varnish and begin to accumulate this material within the oil. The amount of varnish carried by or within the oil will progressively increase as the oil continues its service. The oil’s carrying capacity for varnish will also fluctuate based on temperature. As the oil becomes saturated, this material can settle from the oil and form harmful deposits. Fortunately, varnish can be extracted from oil and measured in a laboratory by its change in color.

The ASTM D7843 standard provides requirements for performing membrane patch colorimetry (MPC) testing. However, this test has been found to be biased by exposure to ultraviolet (UV) light. A recent experiment demonstrated that overhead lighting, such as fluorescent lights, can have a similar effect. Therefore, caution is needed to avoid contact with all but incidental light when handling oil samples that will require an MPC test.

Increases in MPC measurements of 50 to 100 percent have been demonstrated following several days of exposure to indoor lighting only. As such, seemingly benign sources like fluorescent overhead lighting can dramatically alter MPC varnish test data. For example, one oil sample increased from a measurement of 21 to 41 in a 16-day period due to exposure to fluorescent lighting.

ASTM’s Response

The research presented in this article was reviewed at a recent meeting of the ASTM Committee D02.C0.01 Turbine Oil Monitoring, Problems and Systems. We thank Bryan Johnson for bringing this matter to our attention. This is an example of a fluid user presenting research that helps to improve the value of our standards. With efforts like this, we make our standards better for all end users.

Sections 8.1 and 8.2 of D7843 discuss the requirement of protecting the sample from UV light, which we know can cause precision errors. However, these sections do not discuss specifically the potential harmful issues associated with fluorescent light. We already have an open work group item within our committee directed toward improvements of ASTM D7843 and have now added this research for potential modifications in the next standard revision.

Modifications of ASTM standards are not made lightly. They require full ASTM D02 ballot approval. Our objective is to recommend improved verbiage within the standard to reflect this user’s input and its impact on precision so that future users can gain from this experience.

As you use ASTM standards in your business, keep in mind that we are always looking to improve the precision and value of our standards. Become involved and be a part of this improvement effort! – Dave Wooton, D02.C0.01 Committee Chairman

Case Study

For a comparative study, a single oil sample drawn from a large turbine oil reservoir was processed for an MPC measurement. The sample was split and placed into light-blocking and translucent sample bottles at the time of sampling. All the bottles were made of high-density polyethylene (HDPE).

A series of tests was designed to determine the impact of time and temperature with the sample limited to light exposure within the laboratory. Duplicate oil samples were drawn on the same day by the same personnel. One sample was placed into the standard semi-translucent sample bottle. The second sample was drawn into a light-blocking brown bottle.

When the samples arrived at the laboratory for testing, a 90-milliliter subsample was immediately removed from the translucent container and placed directly into a dark drawer to match the standard practice. The sample was not reheated prior to storage. The 90-milliliter subsample was retained in the drawer for seven days from its date of sampling and then tested. An MPC of 21 was reported.

Both the light-blocking and translucent oil sample containers were staged on the laboratory counter surface for a 16-day period prior to additional MPC testing. In this amount of time, it would be expected that varnish would settle out of the oil. MPC measurements were made for both samples prior to heating to determine their as-found varnish levels. The translucent sample experienced a substantial increase from its initial 21 MPC measurements to a new level of 39. The light-blocking bottle increased its varnish load to an MPC of 26.

Oil Samples

Translucent Container MPC

Container MPC

As found, after 16 days (pre-heating)



Heated for 24 Hours at 60°C and Stored in a Dark Location


3 days after heating



7 days after heating



14 days after heating



Reheated Samples


7 days after second heating



Both samples were then heated, as specified by ASTM D7843, to return the varnish bodies into the oil and then retested after allowing storage periods of three, seven and 14 days from the reheating. The sample containers were stored in a dark location between each test to avoid additional stress to the oil from any light source.

The translucent sample was found to have an MPC of 38 at three days, 40 at seven days and 41 at 14 days. The sample from the light-blocking container had a test measurement similar to the original 90-milliliter control sample with a reported MPC of 22 after three days. It rose to 24 after seven days and continued to increase to 26 after 14 days from the time of heating.

Both the light-blocking and translucent oil sample containers were then reheated a second time and stored in a dark location for an additional seven days. Retesting the sample from the translucent container produced an MPC of 40, while the sample from the light-blocking container was measured at 27.


The following conclusions were drawn from this experiment. The comparison of test data from the translucent and light-blocking sample containers supports observations that light exposure can significantly impact MPC test results. In this case, the light exposure from the lab’s fluorescent lighting permanently doubled the test measurement within approximately two weeks.

Allowing the sample to remain at laboratory temperatures for a progressively longer time period resulted in moderate increases in test data. The variation in MPC test measurements for the light-blocking sample between a three- and seven-day period was less than 10 percent (22 to 24). Either time interval could be expected to produce acceptable trend data provided a single interval is used consistently. The results of the second reheating test also showed an increase for the light-blocking sample. This suggests an oil sample should be tested for MPC as soon as practical following its removal.

While light exposure can be managed by creating a subsample of the original sample taken in the field, which can then be separated and placed into a dark storage location, this alternative still affords the opportunity for the sample to be inadvertently left in a location with light exposure prior to being turned over to the lab for testing. In addition, the sample container may be exposed to laboratory light as it is processed, handled and tested. To avoid the possibility of poor handling practices for samples requiring an MPC varnish test, it is recommended that light-blocking sample bottles be used.