An oil sample indicates the condition of the bulk operating oil and the equipment at the time that it was taken. Since operating conditions change, the longer an oil sample sits before being analyzed, the less pertinent the data will be. An equally important timing decision is determining when individual systems are to be sampled, and it is this aspect of timing that provides the most debate between operations and oil analysis specialists.

There are three fundamental techniques to determine oil analysis sample intervals: 1) sampling when a problem is suspected; 2) analysis on a "spot check" basis; and 3) trendline analysis. Each technique has its supporters, though not surprisingly, most decisions are ultimately made on cost.

Perhaps the most widely used technique is to sample only when a problem is suspected. The operator may detect excessive noise or vibration, experience a decrease in system performance, or notice changes in the color or odor of the oil. Some experienced equipment operators contend that this type of monitoring catches most serious problems, but most admit that it is not foolproof. Even though it is the least expensive of the three, it does require an experienced operator, and one that has learned how to detect minor variations in system performance.

Sampling only problem systems is popular because it requires little investment to get started. Some companies maintain a small inventory of sample bottles for these emergency situations, though most use whatever container is handy. Unfortunately this often includes empty paint cans or glass jars.

In many cases their suspicions are confirmed. The equipment does indeed have a problem, and one that needs to be acted on immediately. In most instances the equipment is repaired, put back into service and the suspected cause of failure noted. More often than not management concludes that the equipment would have failed anyway, even with more frequent oil analysis. Unfortunately by this time the point is simply academic. The net result is that the maintenance budget is kept at a lower level, and the machine failure is recorded as just another operating cost.

Some operators prefer to analyze their oil on a "spot-check" basis, perhaps once a year. In teir opinion this is better than waiting for problems to become serious, and yet it is not as expensive as monthly or quarterly analyses. To be sure, they will identify some equipment that is in the early stages of a problem, and this can reduce the seriousness of the problem. It may also allow scheduling of downtime to the most convenient time.

However, "spot-check" analysis can be even more frustrating than not sampling at all. There is always the risk that equipment will fail between the annual oil analysis. Most lubrication or hydraulic problems take between 60 and 90 days to fully develop from initial contamination to component failure. Any oil sample and analysis that is more than 90 days old is therefore much less significant as an indicator of equipment condition.

some operators have actually found that the additional annual sampling costs are not justified on the basis of the problems that they have found. In those cases they have either gone back to sampling problem equipment, or increased their frequency to monthly or quarterly.

A growing number of equipment operators are using periodic oil sampling called "trendline analysis" for their critical or severe duty applications. The sample interval for this technique is determined according to the type of equipment and the kind of application it is used in, and the period is rarely longer than three months.

For instance, one lab recommends sampling diesel engines in over-the-road service every 10,000 miles or 250 hours, whichever comes first. Diesel engines in off-highway applications may require a sample interval only half as long, depending on the levels of silicon and wear metals in the oil. They also recommend sampling gasoline engines, particularly those used in stop-and-go service, every 5,000 miles or 200 hours whichever comes first. Industrial applications such as hydraulic systems or gearboxes should be sampled every two months or 1000 hours if they are in continual use.

Another lab recommends sampling diesels in long distance over-the-road service every 10,000 miles or monthly. Diesels in short-haul service should be sampled every 5,000 miles. They recommend sampling construction equipment every 200 to 300 hours or on a monthly basis. And they advise sampling gasoline engines every few months depending on the normal maintenance schedule. Compressor sample intervals are every 600 to 800 hours, and hydraulic systems should be checked once every 3-6 months.

While the individual intervals may vary from lab to lab, there are several distinct aspects that are common to all. Each has developed their initial sample interval based on equipment manufacturers' recommendations. These average periods are then further refined with actual experience over a period of many years. Each lab also recognizes that the operating environment and type of service will affect oil and equipment condition, thus shorter intervals are recommended for severe duty or highly contaminated service. And each lab recommends more frequent sampling at the beginning of a program to develop an accurate baseline. Once the baseline has been developed, sample frequency can often be reduced to more easily conform to operating requirements and budget limitations.

There also seems to be agreement that the sample interval should, if at all possible, coincide with other routine equipment checks. These procedures could be the normal fluid level or safety checks that are performed on a monthly basis, or routine maintenance operations such as oil drains and chassis lubrication. Adopting an existing schedule not only insures that the equipment will be available to pull a sample, but that the maintenance personnel will not have to change too many habits to accommodate the new procedure. changing worker habits is seen as one of the critical features of starting any new oil analysis program.

It is also important to know what equipment needs to be sampled when, especially for large operations with dozens or even hundreds of systems. To make the sampling more reliable, some labs have set up complete sample schedules that list all of the equipment and the sample intervals. On a regular basis the operator receives notification that specific systems should be sampled routinely, and what samples are late. The format may be a simple reminder letter that a sample is due, or a more detailed computer printout. At least one company offers a program that indicates the sampling history during the previous month, and a schedule for what equipment needs to be sampled during the next 45-90 days. This service can be extremely valuable for larger operations, especially construction companies or operators with a lot of mobile equipment.

And, interestingly enough, there can be problems when a program has been extremely successful. In its ultimate form, oil analysis is simply another type of insurance, providing early warning of developing equipment problems and reducing serious equipment failures. However, as equipment downtime is reduced, there is sometimes the tendency to cut back on the fixed expense of the oil analysis program. To maintain continued support for trendline analysis, maintenance personnel should continually account for oil analysis expenditures in terms of reduced downtime and replacement part costs.

One final note. In addition to its role as a preventative maintenance tool, periodic oil analysis is increasingly being used to extend lubricant drain intervals. This should benefit an operator by saving oil changes and labor costs. Once an operator exceeds the normal equipment drain intervals, however, it becomes even more important to maintain adequate sample intervals. Extended oil drains do not suggest extended sample intervals.