Info Centre >> ANALEXrs Sensors FAQ >> Oil Condition Sensor: Frequently Asked Questions
1. Why extended CAN?
The standard length CAN address does not allow enough character space to assign sensors with a unique identification (ID). Extended CAN allows multiple sensors of the same type to be used on the same CAN network, and the sensor reporting a reading of interest can be identified through its ID.
2. Why does the sensor output not compare directly with lab results?
The sensor does not monitor a single parameter in the oil, such as water content. Additionally the sensor works on an index scale – it is not absolute. Contaminants such as Soot, Oxidation products, Water, Glycol, along with changes in TBN, alter the di-electric properties of an oil. The Oil condition sensor analyses a component part of the di-electric properties of the oil, known as the Tan-delta, which has been shown to change in proportion to the amount of contaminants, as above.
This measurement method is found to be sensitive to changes in contamination, but tolerant of variations in most widely available additive packages. It increases with the introduction of polar molecules such as Esters, Aldehydes and Ketones. Individual contaminants cannot be discretely identified.
However, as a trending tool, the user monitors the output of the sensor for change and rate of change. This is a consequence of the change and rate of change in tan delta, and hence, oil condition. These changes in oil properties are be reported to the end user via the electronics and signal processing within the sensor head itself.
In some situations where there is a single likely contaminant; water ingress in a paper mill for example, the output of the sensor can be approximated to lab results. To do this the sensor output needs to be recorded at the time of the taking of an oil sample for laboratory analysis. The sensor is intended to support maintenance of machinery, and not necessarily replace laboratory oil analysis programmes.
3. Why does 4-20mA display and RS232 sometimes not match exactly?
The analogue output value is controlled by the microprocessor, with the digital value interpreted and converted to a mA current value.
For example, with the output scaled to 160 OilQ, the mA per OilQ value is calculated thus:
Through this digital to analogue translation it can happen that there is a small discrepancy between outputted and theoretical analogue value, because the analogue circuit can be affected by temperature.
4. How often should the sensor be set to monitor the oil quality?
This is to be determined by the maintenance engineer. It depends on the criticality of the machine, operating environment and maintenance schedules. If data is logged every 10 seconds, 8640 data points/24 hours will be generated. The data rate needs to be appropriate to the machinery being monitored.
5. Does the sensor head need to be completely filled with oil?
Yes. The electrical properties of air and oil are largely different. Air bubbles in the sensor will affect the sensor output.
6. How long does the sensor take to respond?
The sensor averages the reading over a 5 second interval to give output stability, it may be necessary to allow for several updates – this is especially for calibration and rapid oil sample change.
7. Can the sensor be used for bench top testing of oils?
Yes, however this works best if the sensor is installed in a pipe and supplied with sample oil, rather than putting it into a pot of oil. This is because the residual oil from the last reading needs to be removed from the sensor and replaced with the new sample oil. The hydraulic lock effect of the oil in the sensor head may affect this happening. In a machine this problem does not occur as new oil is forced into the sensor head, and old oil forced out of the sensor head.
The sensor averages the reading over a 5 second interval to give output stability, it may be necessary to allow for several updates when set-up for measuring oil samples. In the interests of accuracy, measurements taken in this manner should be conducted with oils at the same temperature – ideally in a temperature controlled oven.
8. What is the best way of removing oil from the sensor head?
Compressed air, below 8 Bar will remove the oil. Eye protection and appropriate clothing must be worn, as oil is ejected from the sensor head at speed. Do not use chemicals or solvents to clean the head, as this can have an adverse effect on future readings from the sensor.
9. Why does the output sometimes go negative?
Refer to the paragraph: ‘Oil Quality output with temperature’. Typically the sensor output will have been recorded when the oil was at a lower temperature. If in doubt, check the temperature of the oil by pressing the button once.
Best practice is to take oil quality measurements when the lube oil is at approximately the same temperatures. If the sensor has recently been installed, consider re-calibration of the sensor. Additionally if the oil in the machinery requires topping up (addition of new oil) and the sensor output was 0, it is possible the quality of the oil has got better. This would lead to reducing of the oil quality value – hence negative readings.
10. What causes the output to be unstable (fluctuate)?
This is common where a thermostat closes supply to an oil cooler, especially cold ambient conditions. Consider re-location of the sensor to a higher and more temperature stable mounting position. Check the temperature of the oil as reported by the sensor.
The system monitored is under high load. This may be more prominent in hot ambient conditions. If the problem occurs often, consider re-location of the sensor to a lower temperature part of the oil circuit - after an oil cooler and more temperature stable mounting position.
Alternatively the oil mixture may not be homogeneous – this is common when the sensor is first installed or just after an oil change.
11. Sensor gets clogged or contaminated?
Check the sensor is not positioned in the bottom of a sump or where debris will gather. Best results are observed with the sensor horizontal or pointing down into an oil flow.
The sensor may be poorly located – consider re-location of the sensor where there may be greater oil flow. Check the viscosity of the oil is not too high and restricting flow.
12. How far does the sensor need to be into an oil flow?
The sensor is supplied in two lengths, so long as the last 10mm of the sensor head is in the oil, and the oil is homogenous (well mixed).