What Are the Different Kinds of Online Oil Sensors?

Rafe sits down with Dr Guillermo Miro – Application Engineer at Atten2 to discuss all things regarding online/inline oil sampling and analysis. He explains that the current suite of deployable sensors technologies fall into four different categories, depending on the type of property they measure:

Rafe Britton: What kind of sensors are they? What exactly do they do they measure? Because I think most people’s conception of oil condition monitoring is going to be the raft of ASTM and DIN test methods that are performed in the lab. We’re obviously not miniaturizing a lab and putting it into the flow stream.

Guillermo Miro: So when I tried to explain the central family in the market, I focus on the issues that they are measuring. So I typically explain that there are four types of sensors.

Sensors measuring physical properties

Let’s say there is one sensor – let’s say physical property sensors – measuring viscosity, measuring density, heat modules or thermal properties. And as we know, viscosity alone cannot explain a lot of failure modes of the lubricant or issues on the equipment. But they measure [physical properties] really well.

Sensors measuring water

Based on the same technology that you can have in a weather station there are the moisture sensors that are specifically addressing water contamination in oil. They are conductive sensors that are able to measure the relative humidity of water in oil. So as soon as you have it submerged, you measure the amount of water that is dissolved in the oil. It can help you to understand how much water you have, for example.

And then the two more widely applied sensors; degradation sensors and the contamination sensors.

Contamination sensors

So contamination sensors are a sensor that they’re focusing on solid contamination in the fluid. Now we can measure wear particles, we can measure other types of contamination. Depending on the sensor, there are other natures of contamination or other types of contamination that can be present.

Typically there are two main types; the first measure electric & magnetic based properties. They are measuring changes in electrical eddy currents in magnetic fields to get information about the particle passing through; size, material.

And then we have the particle counters that focus on measuring the size and quantity of particle for a typical range of micron size.

Rafe Britton: So just to jump in there. Maybe the equivalent in the lab world that we’re all familiar with is, one is a little bit like an optical particle counter, measuring by ISO 4406. It’s shining a laser through and detecting when something passes through the beam versus the, the other type of sensor that you were talking about, which is maybe more analogous to the PQ index.

Guillermo Miro: Yes. 

Rafe Britton: Yes. Obviously not an exact comparison. 

Guillermo Miro: PQ index is catching every type of metallic particle for every size. And these sensors, these magnetic field sensors at the moment, they are, eh, they are measuring particles higher than typically several, eh, several thousands of microns. 70, 50, 100 microns, they are catching another type of issue. That is the wear progression. They are just catching very big particles that they typically form when equipment wear is severe.

So the particle content, the most similar thing to the lab in terms of correlation, a lot of optical sensors are offering an ISO 4406 measurement, so typical 4, 6, 14 microns cleanliness codes. But I would like to add that there are different technologies in the lab, and there are different sensor technologies in the market. It’s not the same to give an ISO 4406 code with uh, automatic particle counter, typically light extension particle counter that gives an ISO 4406 code based on microscope based particle counter. One of them is what you mentioned. You pass lights through a cell and you measure the, the size and the amount of obscurity means there are more particles. But the other type is a microscope – it is filtering the oil and getting the patch and just taking a picture and analyzing all of the data.

And in the sensor field, we have the same difference in technologies. Most are the sensor analyser, or light extension based sensors and other sensors are digitally imaging, so. It’s pretty similar to what is done with a microscope. But in general terms is we have the visible or optical particle counters and the magnetic particle counters.

Oil degradation sensors

And then last but not least, we have the degradation sensors that recently has appeared lab-like sensors, that mimicking what is done in the middle infrared with FTIR. But the vast majority of sensors at the moment, they are just measuring, let’s say optical or electrical properties and they are converting this with their own algorithms they are converting it into index. Percentages, numbers. So they are not giving you any lab-like results. They are just telling you the trend of your oil. If you have an algorithm for your oil, you see the trend going up, going down, going up, depending on the definition of value by the [sensor] manufacturer.

But in the end, you are just getting one of the parameters of the oil. For example, we are measuring some parts of the visible spectra and some near-infrared bands. There are other companies that are measuring the dielectric constant; this has been a very old way to be diagnose degradation and this is online now. That is a different approach I had in the lab where I was in the university; I had a very beautiful dielectric constant measurement tool. It was probably 30 or 40 years old, but now you can have real time insights ina small package.

So you can use different properties of the oil to monitor degradation, but there is no way to give lab results such as TAN, RULER, and or the viscosity with the degradation sensor.