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We use Mobil DTE-26 hydraulic fluid in our Hydraulic presses. Just got back an oil analysis which shows Tan numbers. What is an acceptable number. The highest reading was 3.4 for one press. The Lab doesn't know what is acceptable or not. I called mobil tech services and the guy I spoke to couldn't understand why we were doing TAN on hydraulic oil. The manufacturer recomends TAN testing annually. Any comments on what is an acceptable number. Thanks, Chris Ward, Entegris Inc.
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A practice you may want to consider is to send a new sample to your lab to find what the new lube sample starting acid number is. An increase of 0.2 should raise concern and be monitored, and an increase of 1.0 should be considered critical. You also want to ensure that you are following a good trend with your results. You should also try to find out why your AN results are high. Is water present, increasing the possibility of corrosion? Was the system run hot? There are several possibilities. Being and ISOVG 68 fluid, was the viscosity high on the fluids with higher acid numbers? I'd also expect that the lab ran FTIR and reported an oxidation value. I'd expect this to be higher than the previous sample as well. There are so many factors to take into consideration. I'd definitely begin by sending a new lube sample to your laboratory for a baseline of the acid number.

TAN is used to measure the total acidity of the oil, every lubricant has an neutralizing agent to react with this acidity and that is being measured through TBN. TAN will determine if the oil needs to be change or not, An oil analysis test which will determine the remaining useful life of oil is the RULER, but this is somewhat expensive, hence an alternative will be the TAN. TAN should always be less than 1. A high reading on TAN means that additives used to neutralized the acid had already been depleted and the oil must already be change.

Many people believe that you have to change oil regularly. The only indicator that would tell you that you need to change your oil is the TAN (Total Acid Number). If your TAN is greater than 2 then you had been shot and your oil
needs to be changed. If TAN is under one , then the oil is in excellent condition unless you have an unusual amount of water in the oil. If your oil has a high amount of particulates don’t change it if the TAN number is good, filter it.
Hi Chris, rather than just change the oil check your test history and if you are increasing the TAN by more than 0.75 per month then the previous replies are correct,
if the TAN is above 2 but not increasing I would complete a WDA and particle count and if both normal, don't change but monitor,
The key for change appears to be a steady increase in TAN value every test and of so rectify by change.

Regards Rob S
Not sure who you talked to at Mobil but TAN is a routine analysis test for DTE 20 series oils in Hydraulic applications.

Bear in mind that fresh oil has a new value of approx 1.2 (due to additive response), an unsatisfactory level would be >2.7

As with all oil analysis, you would need to look at the trend of other parameters to decide whether an oil change was required or not.
Viscosity increase and oxidation levels would be the other critical parameters to monitor.
Chris as mentioned above the new value is approx 1.2. We do quite a few of these samples and a reading of up to 1.8 is normal. The trend is a very important feature ie the increase should be very gradual over time. Sharp increases indicate problems. As mentioned 2.7 would be an upper limit not to go over. Also look at your bearing metals ie lead,copper and tin for increases as the TAN increases. Often there is a direct relationship.

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