viscosity issues too?

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hello we are running Mobil DTE Extra Heavy in our large industrial air compressors. these compressors are used to fill bottle stations which are used to supply wind tunnels here at NASA. Now i know the normal viscosity is around 150. Lately there has been a trend of loosing viscosity down to as far as 129. i know that oil looses some viscosity as it heats up but it returns back to normal when it cools correct? when the facility just put 300 gallons of new Mobil DTE extra heavy in for an oil change 2 months ago why would my viscosity drop from 150 to 129. Samples were taken fresh from the drums as we filter pressed the oil into the compressors for the filling. so i guess im trying to figure out what i already think i know. by taking oil samples when the machine is hot and running my viscosity is lower than 150, but by the time it gets to the lab 3 days later it should be back at 150 right?

6/5/127:25 AM

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jmorganMember

Start from the basics. First, is the viscosity your using what is called for? Second, since we know heat causes viscosiy drop, is there anything to do that will drop oil temperatures. I tried to attach a study but the file is to big for the forum. PM me your e-mail I will send you information.

6/5/1211:35 AM

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rttechMember

Hi, was a viscosity test completed on the new oil before it was put into use??
Remember all new batches of lubricating fluids should be checked for additive levels, viscosity and cleaness before being applied to plant items as with the small difficulty at present the new lubricant may have been delivered light,
The temperature of the lubricant should not influence the viscosity analysis at the lab as the lubricvasnt would be tested @ 40°C when tested.
If the lubricant has reduced viscosity in 3 months of service then somehow the lubricant is being stressed so the heavier ends of the base fluid are polymerising/gelling out and if so we would expect the lubricant to appear and smell heat damaged.
Rob S

7/13/121:24 AM

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robertcMember

You could also run a ferrogram, it may show polymers, which are an indication of heavy stressing.

An FTIR of new and used oil may be useful too, but tough to interpret without ExxonMobils help.

If you have multiple machines and only 1 is a problems, FTIRs may show a different curve shape, which might indicate a condition to narrow in on.

But first, compare new to used. Everything is not always what it seems.

7/27/1212:37 PM

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renewaMember

I agree with the above comments. You should get a baseline on the new oil out of a new drum. This should be tested for viscosity, additives and ISO cleanliness. The other question I have is what are the temps and pressure on the compressor? The only time I have come across viscosity drop in oil is when it is being "thermally cracked" under temp and pressure. It means the breaking of the carbon to carbon backbone chain. The smaller the carbon chain, the lower the molecular weight which may translate to a reduction in the viscosity. A check on the flashpoint of the oil may help determine if this is happening - new oil flash point versus used oil flash point - you may wish to check both the closed and open cup flash point.

8/3/121:25 AM

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