Assuming that no water is emulsified in the oil, some reasons for an increase in viscosity of a circulating oil might include:

• The oil may have oxidized.

• Pressure could have increased.

• Temperature might have decreased.

• Possible contamination with a higher viscosity fluid

• Evaporative losses of light oil fractions from high temperatures

• Glycol contamination

• Soot contamination

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Book Bits: Viscosity Measurement for On-site Analysis

From the "Lubrication Excellence 2006 Conference Proceedings":

An excerpt from the paper: What a Good On-Site Oil Analysis Lab Should Look Like.

There are two types of viscometry that should be considered.

The first is a viscosity comparator. Viscosity comparators compare the flow characteristics of a known reference oil against the flow characteristics of the test oil. They are generally quite accurate (in the area of 95 percent) as compared to laboratory-quality viscometers, but they can be negatively influenced by degraded oil and oils which have considerably different viscosity indexes from the reference oil.

It is important that these devices always be used at the same temperature for trending purposes, hence the desirability of having a climate-controlled environment for the lab. They should be used as a first line of defense, and if inconsistencies are noted or suspected then the comparator should be backed up by a proper viscosity reading.

The second type of viscometry uses a viscometer to determine the viscosity. These devices are more expensive than a comparator, but obviously more accurate too. Consider a viscometer as a part of a future expansion plan, or if you have a generous budget.

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Question: What is a typical flash point for light hydraulic oil?

Get the answer.

"I read once that if the base number (BN) falls to 50 percent of the value of new oil, that it would be time to change it. Is this accurate for automotive gas engines? If so, would a few ounces of fresh oil in a four-quart reservoir bring the BN back to acceptable levels? Note: Other parameters on the oil analysis are acceptable."

Base number testing measures the reserve alkalinity of the lubricant. It's the ability to neutralize acids created during the normal combustion process of an internal combustion engine.

Once the base number reaches 50 percent of the starting value for that lubricant (specifically that batch of lubricant vs manufacturer spec sheets), a cautionary alarm is generally broken. As the base number decreases over time, we can expect the rate of change to increase. This cautionary alarm allows us to time to prepare for that increased rate of degradation.

Once the base number reaches a value of 2 or less, it is time to completely replace the lubricant. Regarding adding a few ounces to a four-quart system ... this can be confirmed only with oil analysis. You are basically sweetening the sump with fresh oil.

I assume by "a few ounces" you are implying a sample bottle or two worth of oil (typical sample jar is 4-6 oz). This would not likely result in an increase of BN enough to remove the lubricant out of an alarm status. With a sweetening of the sump it is generally recommended to replace at least 25 percent of the lubricant. This depends on several factors with the severity of the lubricant being the primary factor.

Less than a 25 percent volume would not likely yield results worthy of the time put forth to do so. If you look at equal units of measure, you'd likely have a better understanding. You are thinking of adding a few ounces to a system that contains 128 ounces. You'd still be able to drive down the road, but your protection would be gone as well.

Matt Spurlock, Noria Corporation

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Resources

An Imperative Link in the Reliability Chain
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