Field Particle Count Measurements

In his comments regarding my initial post discussing the filter field trail I am involved with, William Lull, in addition to observing that our efforts to weigh the amount of dust the filters picked up would tell the story also asked if I had a particle counter and if so, would I be taking readings up and downstream of the filters. That’s a good question; after all, as William points out, the whole point of filters is to remove particles. To give you a feel for what your typical filter is up against, here is a plot of atmospheric particle count by weight and size that I developed from a couple of industry resources.

Basically, the graph says that there are zillions of little tiny particles in the air all around us, but they are so small that they don’t contribute much to the total weight of the particles suspended in the air. So, that means, in a general sense, that eliminating the big stuff; i.e. the stuff that tends to be pretty easy to filter out, really won’t eliminate much of the fine stuff that is really our target. A low efficiency filter does great job of catching the occasional “boulder” that is in the air stream, but by and large, doesn’t do much to stop the “pebbles” that comprise the bulk of the contaminants its exposed to.

To give you some perspective on the numbers in the graph, a human hair is generally taken to be 100 – 150 microns in diameter (although I found some research on the internet that says a human hair could be as small as 17 and as large as 181 microns). Looking at it another way, there are 25,400 microns or micrometer per inch.

Getting back to William’s question, while FDE and I personally do not have a particle counter in our tool inventory, Jeff Shapiro (the Kaiser facilities engineer I’m working with on this) and I plan to try taking some particle readings at some point in the near future and I will share the results when I get them.

That said, I believe any results we get will probably be qualitative at best and may turn out to not be very meaningful. In other words, it’s virtually impossible to demonstrate MERV ratings in the field for a number of reasons, (which I will discuss in a subsequent post). At best, I think we may be able to demonstrate things like the quality of the air leaving the filters relative to each other and possibly, how the movement of the flexible bag filters with air flow impacts particle count. But even then, things like frame leakage, the relative cleanliness of the different chambers we will be taking measurements in, measurement technique, and the test team’s very presence all have the potential to skew the results.

Most of my experience with particle counters has been in clean rooms for the purposes of troubleshooting particle related quality control problems or to certify HEPA and ULPA filters after they have been installed.  Conditions tended to be very controlled and particle counts very low.  For instance,  if I was in one of the wafer fab’s Class 1000 clean rooms with a particle counter set to measure 0.5 micron particles and hit 1,000 (particles per cubic foot) or more, I was in trouble. (That’s a bit of a simplification but you get the idea). In contrast, with the same instrument with the same setting operating in my office in the construction trailer would easily yield a count of 50,000 or more given all the activity going on outside.

That’s not to say that we haven’t taken a stab at checking particle counts in the field on some of the projects where we are experimenting with different filtration strategies. We have and I’ll share one of those experiences in a subsequent post. But its not so easy and certainly not like sticking a thermometer in a duct to measure a temperature. I’m pretty sure that obtaining consistent, quantitative, meaningful results requires more time and equipment than is typically available in the field. So at best, I think my down and dirty efforts on the Kaiser field trial will be qualitative at best and may be so dirty as to be meaningless.

On the bright side of things, Joe Brennan, the Camfil Farr Director of Branch Operations recently pointed out to me that ASHRAE has developed and released Guideline 26-2008 — Guideline for Field Testing of General Ventilation Filtration Devices and Systems for Removal Efficiency In-Situ by Particle Size and Resistance to Airflow which targets field measurement of filter performance. In fact, Camfil Farr has developed a field test program called the Camfil Farr In-Situ (CFIS) Test System around it.

I’ve ordered a copy of the ASHRAE guide and will do a “book review” of it in a subsequent post. But from what I can tell, the process, while certainly desirable and informative, is a bit more of an undertaking than we can afford in my current field trial due to our somewhat limited engineering budget (it’s pretty much a volunteer effort on all of our parts).

So, bottom line, I do hope to have some particle count information to share with you at some point. But, it will be an indicator at best and may even end up being meaningless or simply invalid for any number of reasons. And, it probably won’t be directly comparable to the ASHRAE standard. But, I bet we’ll learn something anyway, even if its just something about how to assess filter performance in the field in a meaningful way with a small budget.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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