I'm not familiar with industrial (as opposed to aerospace) hydraulic fluids, but I'll throw out what I'm thinking.

Is there a specification for Mobil DTE 25?  Does it give in-service limits for the fluid?  How about for new fluid?  You are probably right about the fluid in the equipment being cleaner than new fluid, as 3 micron filtering would get the particle size down to a low value.  Though I have no information regarding your system setup, so I don't know how much of a role the offline filtering plays.

You have a report for viscosity, are those within the new or used fluid limits?  Is there anything else that's measured or controlled?  For Phosphate Ester hydraulic fluid, there are several indicators that the hydraulic fluid should be replaced.  Colour, acid level, viscosity, elemental contamination, etc.  If these are outside of the limits, they should be replaced.

If the fluid is still within the limits, I see no reason why it should be replaced.  Have you tried looking online or asking the manufacturer if there's a life limit on the fluid you've used?

Hydromechdude

Thanks for your input.
See attached spec for the Mobil DTE 25. As you can see it is within the limits although there are a few characteristics missing such as Particle count, TAN number and Oxidation data.
I cannot find any Life span anywhere but I might contact Mobil to see if they will help.

More opinions are welcome.

• http://files.engineering.com/getfile.aspx?folder=c0837304-76e2-4075-a8b9-0d

All right, so that fluid also uses TAN.  TAN stands for the Total Acid Number and is the amount of acid in the solution.  Less familiar with Oxidation, because we don't use that.

At any rate, I would suggest getting both of these sampled and comparing to the standard.  Fluid can be filtered to remove particles, and depending on setup, moisture can be eliminated.  However, to my knowledge, the only way to get the TAN back in order is by replacing the fluid.

I am very familiar with Mobil DTE 25. It is a fairly high end non-synthetic hydraulic oil for non-aerospace use. One very good thing about it is that it resists varnish build-up, so much so that it is the only oil MTS recommends for their servo-hydraulic systems.

"It is my belief that the existing oil is cleaner than new oil and oil change is not necessary."
There are two reasons to change oil - one is particle contamination, the other is additive depletion. (and if you have really abused oil the viscosity will go away.) As Hydromechdude said you should be checking the TAN. You should also monitor the water content, especially in a marine environment.

At the end of the day it really comes down to maintenance costs vs. potential repair/downtime costs. What are you comfortable with?

ISZ

I see two big gaps in your oil testing.
1)  Water content: The result of "NIL" indicates that the lab is performing a "crackle" test, that will only tell you if you have 1000+ ppm of water.  For most oils, you should have a Karl Fischer test performed (don't have the ASTM # handy) to get the actual ppm.

2) Particle count: This is going to let you know what amount of contamination you have in your system. The cleaner your system, the better off.  "Typcial" ISO 4406 codes for hydraulics are 17/14 or better.

When to change oil based on oil analysis
alarm bells: wear debris. (Al) 5ppm, (Cr)9ppm,(Cu)12ppm,(Fe) 26ppm.(Si) 15ppm
             Viscosity. 12% of new fluid.
             TAN higher than 2 change oil.
new oil TAN 0.2mgKOH grm.The rate of change of TAN number is important more than absolute number.Every oil has a max Tan ask supplier.
herewith a few samples of wear metals, iron,chrome,nickel,aluminium,copper,tin,lead, a PQ Index is good to have. Contaminants silicon,sodium, water, there are also additves.

I would also ask why the zinc counts were on their way down, and suddenly jumped back up - what was added to the system, or what is breaking down?

Looks like something was done between september and october that caused a change in a lot of the numbers - like fluid was added maybe?

Where are the numbers of the oil when new?  Given this data we can only trend the system - but I don't see the OEM numbers to compare to the oil as it sits "on the shelf".

Chris

Swedishrigpig,

You may want to check out the Hydraulic Supermarket.  It has a wealth of info on hydraulic systems and will help you with your question.  There are several acticles written there that will give you the information about what to check for in your hydraulic fluid.     

• http://www.hydraulicsupermarket.com/technical.html

Aggie, I would treat the info at the Hydraulic Supermarket like I do the advice I get here - with a measure of skepticism. I have read many of his "articles." In fact I was involved with one, and lets say his facts are not what I remember. Not saying it is all bad.

ISZ