Proper oil sampling does not have to be a difficult or unpleasant experience. In fact, when the best sampling methods (and hardware) are employed the experience should be just the opposite. There are many important lessons to learn when it comes to pulling a sample. Many run contrary to intuition. Most of them have to do with insuring that what enters the bottle is both rich in information and remains undisturbed by the sampling process itself. It is this later concern that is addressed by this article.
Modern oil analysis programs typically include some tests that can be influenced by environmental contaminants entering the oil (bottle) during the sampling process. Tests of greatest risk include particle counting, elemental spectroscopy, and total acid number. In situations where there is considerable dust in the environment at the time the sample is pulled, a concerted effort needs to be made to insure that this dust does not contaminate the oil. Examples of high-risk situations/environments include mine sites, construction sites, primary metals industries, foundries, windy outdoor conditions, and sample points close to the ground.
Experiments on the influence of environmental dust on particle counts have shown a marked effect. It is not unusual for an oil’s ISO code to increase 2-3 range numbers when a bottle is left open just a few minutes. For instance, the actual oil sampled might be a rather clean ISO 13/10 but after exposing the sample bottle to atmosphere it can show an ISO 16/13. The amount of dirt needed to accomplish an ISO 16/13 is only about 1 ppm.
Recently, a new method called “clean oil sampling” has emerged that greatly simplifies the process and minimizes the risk of dirt entering the bottle. It involves the use of common zip-lock sandwich bags and sampling hardware such as vacuum pumps and probe devices. Below is an outline description of this procedure:
Obtaining a good oil sample begins with a bottle of the correct size and cleanliness. The topic of bottle cleanliness will be discussed in greater detail in a future issue of Practicing Oil Analysis. However, it is understandable that the bottle must be at a known level of cleanliness and that this level should be sufficiently high so as not to interfere with expected particle counts.
Some people relate this to a signal-to-noise ratio, i.e., the target cleanliness level of the oil (signal) should be several times the expected particle contamination of the bottle (noise). For more information on bottle cleanliness refer to ISO 3722.
Before going out into the plant with the sample bottles place the capped bottles into very thin zip-lock sandwich bags; one per bag. Zip each of the bags such that air is sealed into the bag along with the bottles. This should be done in a clean-air indoor environment in order to avoid the risk of particles entering the bags along with the bottles. After all of the bottles have been bagged, put these small bags (with the bottles) into a large zip-lock bag for transporting them to the plant or field. Sampling hardware such as vacuum pumps and probe devices should be placed in the large bag as well.
After the sampling port or valve has been properly flushed (including the sampling pump or probe if used) remove one of the bags holding a single sample bottle. Without opening the bag, twist the bottle cap off and let the cap fall to the side within the bag. Then move the mouth of the bottle so that it is away from the zip-lock seal. Do not unzip the bag.
Thread the bottle into the cavity of the sampling device (vacuum pump or probe). The plastic tube will puncture the bag during this process, however, try to avoid other tears or damage to the bag (turn the bottle, not the probe or pump, while tightening). If a probe device is used, it is advisable to break a small hole in the bag below the vent hole with a pocket knife. This permits air to escape during sampling.
The sample is then obtained in the usual fashion until the correct quantity of oil has entered the bottle. Next, by gripping the bottle, unscrew it from the cavity of the pump or probe device. With the bottle free and still in the bag, fish the cap from the bottom of the bag onto the mouth of the bottle and tighten.
With the bottle capped it is safe to unzip the bag and remove the bottle. Confirm that the bottle is capped tightly. The bottle label should be attached and the bottle placed in the appropriate container for transport to the lab. Do not reuse the zip-lock bags.
Benefits of Clean Oil Sampling
This simple procedure effectively permits samples to be obtained without exposing the fluid or the bottle to the atmosphere or surface contamination. A clean sample can even be obtained with dirty hands. There are no expensive materials to purchase and the technique can be applied to a large number of sampling situations. For best results practice a few times with a spare bottle until the technique is perfected.