Particle Counters - Sources of Inaccuracy 1

[DarrenJ]

We currently use a Hydac optical particle counter to measure, in house, the level of contamination in hydraulic and transmission oils used on our production line. We are finding that there is a large descrepancy between the results we obtain, and those obtained by an external lab for the same samples. Our 'in house' reading is generally very much higher than that measured by the lab.

Example (Coding as per ISO 4406):
Particle Counter : 17/10 @ 6/14 micron size
Lab : 12/7 @ 5/15 micron size

Previously we drew samples into sterile bottles. We have now added sample ports to our hydraulic and transmission systems to draw the samples directly into the particle counter. This has made little difference to the results.

It has been suggested that our counter could be misinterpreting air bubbles as contaminant?

Can anyone suggest where we are going wrong? Comments appreciated....

 

[plawrence]

99% of the particle counters on the market use some kind of light (laser) blockage technology to determine the size and number of particles. Anything opaque will count, including air bubbles, water, filter strings, slime, etc.

We have a vacuum pump and bell jar that we use to degas our samples (we use bottled samples). This helps a lot. As far as water, if you have a high water content (from a crackle test or Karl-Fischer, etc.) and a high "particle" reading then chances are you may be reading more water than dirt--pull a slide sample and verify.

Also, I'd check with your outside lab and compare your procedures and equipment. If you're getting a sample right from the source, I'd trust your results over the lab. Maybe they're not shaking the sample before they test. Are all your samples higher than the labs or just a couple points? Are you bleeding the sample lines first to make sure you're getting a representative sample? Have you run a calibration fluid through it to verify the counts? Just some thoughts.

Patrick