A retest failed to confirm the alarm, and therefore no other maintenance action was required.Ĭautionary limit values should be set at some level sufficiently within the boundaries of safe values that do not compromise functionality. In Figure 1, such an example can be seen in the data from Nov. Further oil analysis tests are often needed to investigate potential causes of a viscosity change so that they, and not the symptom, may be addressed. In the case of viscosity, an alarm might trigger a retest by a more precise testing method, or simply a confirming retest using the same viscometer. They should not, in most cases, be a cause for panic. Alternately, an alarm may trigger closer and more frequent parameter monitoring.Īlarms should stimulate further interest and initiate appropriate action. This may prompt further investigation and/or testing, perhaps resulting in a maintenance action. The purpose of alarms is to indicate that something unfavorable may have occurred. In this case, conditions that were intended to trigger an alarm-viscosity values elevated by 5 percent - do not, and therefore appropriate corrective action will likely not be taken, resulting in a false negative. This may cause an unnecessary alarm - a false positive.Īlternately, if a product is blended to be only 30 cSt when new, a viscosity increase of 8.67 percent would be required to trigger any further investigation. Remember that a product may exceed the midpoint by 10 percent and still be within grade. If alarms are based upon the nominal ISO VG viscosity of 32 cSt, a product at the upper end of the allowable viscosity range will tend to have upper alarms triggered too frequently, or even at delivery. Individual deliveries of this lubricant may have varying initial viscosities, which will make the actual alarm setting more or less than the intended 5 percent. An upper alarm is set at +5 percent, for a value of 32.6 cSt. For example, a particular ISO VG 32 lubricant is specified by the manufacturer to typically have a viscosity of 31 cSt, well within the limits of the viscosity grade (28.8 cSt to 35.2 cSt). The most common alarm variation in viscosity is within 5 percent of the new oil’s baseline viscosity. If specification viscosity values or ISO VG midpoint viscosities are used to established alarms, sensitivity is lost and false alarm rates may increase. To be effective, all alarm and limit values must be referenced to a starting condition known to provide acceptable performance. Published values on a specification sheet are typical values and are often not precise. Similarly, viscosity values for individual manufacturers’ products will vary by batch and even container. Actual viscosity values may vary from this midpoint by ☑0 percent and still be considered conforming (within grades). The ISO viscosity grade classification system (ISO VG 32, 46, 68, etc.) is merely a nominal value that establishes the midpoint of an acceptable viscosity range. The importance of establishing a baseline for viscosity monitoring cannot be over-emphasized. Therefore, it is a mistake to underestimate the importance of viscosity alarms and limits. Significant change in an oil’s viscosity is meaningful and should not be ignored. A fluid’s viscosity is one of its most important physical properties.
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