Thread: Budget level dust exposure measurements

For some time I have been experimenting with a way for wood workers to check their dust exposure without it; costing too much, requiring too great an amount of time, and the need to develop significant expertise in the area. This rules out devices like dust particle counters.

At work about 20 years ago we developed a small kit consisting of a small aquarium air pump and a set of small 50c size air filters. Some 200 of these kits were sent around the world to sample air borne dust. We ask the people who we sent pumps, to collect air for a month and just send filters back by mail and they got to keep the pump. Based on this I decided to see if a similar setup could be used for wood workers to measure their own wood dust exposure.

For wood work all we would need to do is measure the weight of dust picked up on a small filter - sounds easy but there are quite a few complications to consider

Firstly you need to be able to reliably collect the dust. This requires a cheap efficient filter that will collect dust down to the 0.3 microns. Initially I experimented with a miniature HEPA filter from a robot vacuum cleaner like these.
The dirty looking one has about 6 months of moderate use and the one below that is brand new.
These filters are available on eBay for about $2 each
Rhoombafilters.jpg

To hold this type of filter I made a filter holder and placed in in a sealed lunch box like this
The white filter folder plate is 6 mm white PVC screwed and silicon sealed to the sides of the container.
I have to stress that sealing everything well is important if you want reliable results otherwise the fine dust will want to sneak past the filter and not be captured.
To seal between the filter and the filter holder there is some soft self adhesive foam rubber - see a couple of pictures further down.
Rhoombafilter1.jpg

Here is a side view.
Its not essential to mount the filter on an angle but it does help prevent the filter from loading up too quickly with dust.
You can see the air inlet at the top and the exit or suck point at the left (black PE fitting)
The lid just protects the filter from being damaged but the open PVC spout still allows it to collect air and dust
BTW that top spout should not be too long as it could influence the result.
For best results remove the lunch box lid.
Side.jpg
It's better to suck than blow air though the filter because sucking can be arranged to help self seal a filter and when a filter is under pressure the air and dust will try and bypass it where it can.

However, when I tested the robot VC filter it was only about 95% efficient for particles down to 0.3 microns.
The particle counter is connected to the filter holder and it sucks air though the filter and counts the particles per litre com ingthrough.
Before and after this I measure the numbers of particles per litre in the shed air and the efficiency is simply the ratio of the filtered to non filtered air
Testing.jpg
Interestingly the dirty filter performed slightly better (96%) than the new filter - probably because it was "conditioned".
95% is still perfectly OK to perform wood dust measurements as it would just involve multiplying all final results by 1/0.95 to get the actual result.

Anyway I decide to try and see if I could find a low cost alternative with a better efficiency so I decided to use a piece of P2 mask.
P2 masks cost ~$2.40 at Bunnings plus you can get two filters out of one mask
To hold the mask in place I made up a small frame in white PVC - in this you can also see the adhesive foam seal on the filter holder plate.
FILTERH1.jpg

And here it is with the piece of P2 Face mask material in place
P2filter.jpg

This gave efficiencies of between 98.5 and 99.5% for all particles of 0.3 microns or larger which is as good as we will probably get.

So now we have a simple cheap way of filtering particles.
Now we need a pump to continually pump a known amount of shed air (with dust in it) through the filter over the entire period of time you are in the shed.

Compressors, VCs and DCs are too noisy and are also expensive to run. VCs and DCs would also clog the filter too quickly so that the flow would slow down and you would have to stop work every 15 mins or so to measure the weight the filter and clean it out. In some circumstances you might want to do this but my idea is you could come into a shed and turn on a small pump, collect airborne dust and at the end of the session check the weight of the filter and that would be it.

So a small low cost, quiet pump that collects enough air (and dust) that can be weighed is what is needed.
Aquarium air (not water) pumps are ideal for this however there are two things to consider.
Most aquarium pumps blow and we really want ours to suck. As well as holding the filter better against its base plate a "suck" removes the pump out of the way of the air/dust stream. Most aquarium pumps are also quite dirty and put out some dust so you don't want this messing up your measurement.

So whatever pump you use is should be able to be converted to be able to suck and be reasonably quiet and powerful enough to keep sucking even when the filter clogs up a bit. The other thing is that it should have enough flow to collect enough dust so that it can be weighed.

The OHS limit for hardwood dust is 1mg/m³ .
To measure that you have to collect 1 m³ of air and be able to measure a 1mg change in the weight of the filter.
1 m³ of air is 1000L of air.

Small aquarium air pumps are in the 2 to 10L/min pumping range, which means it would take 500 to 100 minute just to collect 1 m³ of air.
A better pump would be in the 20 - 50 L/min range so 50 to 20 minute per cubic meter so during a work session several cubic meters of air could be collected and measured.

Ideally something like a mens shed or workshop would have two setups, one with a high volume pump for spot or short tests of high dust concentrations, and a slower pump that would collect all day for daily exposure measurements.

The pump and filter should be placed as near as possible to where an operator works and at operator head height to simulate the air they would be breathing.
Too high up in the shed would not capture sufficient heavy particles and VV.

This is the pump I bought to test the mens shed situation out.
It's supposedly an 85 L/min pump but its more like 70L/min.
It's s also noisier than I would like it to be and it vibrates so much I mounted it on rubber shocks to a board and I will be placing rubber feet on the board.
COMPSETUP1.jpg

The last and very importantly you will need a milligram (mg) balance. These can be bought on eBay for <$20.

So to summarise this is what you would do

1) Weigh filter
2) setup filter and run for known time e.g. 240 minutes
3) Using the pump flow rate convert the time into volume (m³ )
4) Weigh the filter to determine how many mg's of dust have been collected
5) dived 4) by 3) to determine the mg/m³

The weakest link in all this is the reliability of the flow rate of the pump. This can be measured in a variety of ways, including sending it to me to be calibrated using my air flow gear.
I feel reasonably safe in not being swamped by requests but I will of course pull the pin on this offer at any time if it gets too demanding.

I would like to report some measurements but that will have to wait until I go to the mens shed.

Costs
1) Lunch box $5
2) Filters box of 3 $7.20
3) Pump $20 - $50
4) mg balance $20
So all up for ~$80 you could have quite a reliable dust measuring setup.

Sorry for the length of this post but I wanted to get it all into the one post.