The tinyfilt Manual



Introduction

Photographic 35 mm film cassettes generally come in cylindrical 
plastic containers, still known as 'cassette cans' from the days when 
they were aluminium. These 'cans' are the same the world over. 
Another high density polythene item common to many countries is the 
tamperproof bottle screw cap used on PET drink bottles from half a 
liter upwards. A lower ring splits off this cap the first time it is 
opened and stays on the bottle. Australia's major bottle makers for 
example have licenced the original US product, so the Australian 
moulded caps have the same dimensions and screw threads as their US 
counterparts. The European system, as always, has slight differences. 
I had been told the caps used by Coca-Cola (TM) differ, but I checked 
out a 3 liter 'Coke' bottle and although there are minor internal 
differences, on the outside the critical cap dimensions are the same, 
making them equally suitable for tinyfilt construction. 

Purely as a matter of luck, if you take a standard PET tamperproof 
cap off the bottle, and push it into a Kodak (TM) black plastic 
cassette can, you will find that the blank end of the cap inserts 
without trouble. Half way in you start to feel a resistance as the 
cap is wider at his point than the 'can' but if you push a little 
harder it will continue to insert. The cap flares out slightly at the 
very back, but push harder and the flare will insert in the 'can' and 
if you push a little farther you can see a ring of expanded plastic 
on the 'can' where the flare on the cap has forced the encircling 
plastic outwards. This deformation is elastic, that is when the flare 
moves down the can, the ring of deformation can be seen following it 
down. Air compression will stop it going too far, indicating a very 
high quality hermetic seal. Removing the cap is much easier if you 
screw the bottle neck back into it and pull the bottle neck and cap 
out. This tight fit is essential for the tinyfilt design to work.


Finding the bits

Most places offering on-site photographic film processing will have a 
waste bin full of cassette cans, and you may well be buying cotton 
wool (see later) at the same location. In Australia the standard 
black Kodak cans with grey lids are supplied with such well 
established films as 100 ASA colour negative, and are not yet marked 
with recycling symbols. The 'can' Kodak supplies with the very high 
speed PJC 1600 film is more up to date. It has a moulded recycling 
symbol and a black cap, but the 'can' diameter is virtually the same 
as the grey-top cans and either old or new types may be used for the 
tinyfilt. The fit of the 'new' type is slightly tighter. 

Once you have a handful of Kodak cans, proceed to your local 
supermarket and check the PET bottles in the drink section by 
dropping a can over the top of the bottle, and making sure the can 
top stops half-way down the cap. Don't conduct the full 'push' trial 
as it might upset the management!. If the caps are consistently the 
wrong size for the cans, you can try other brands of 35 mm film. I 
found a translucent can (Agfa?) that was just too small, and a black 
can with a black top (Fuji?) that was just too big. A black can is 
not essential. 

Empty the drink bottle down the sink if you don't like the flavour.


Making the tinyfilt

Drill a 5/16 inch or 8 mm hole in the bottom centre of a 'can'. It is 
much easier to drill from outwards in, because of a handy moulded-in 
locating hollow. This leaves much swarf and sharp edges pointing 
inwards that have to be removed by oversizing the hole slightly with 
a thin sharp hobby knife or scalpel blade. I filed a large 5/16 hole 
flat washer until it fitted inside the 'can' then used it as a 
drilling guide to drill from the inside out. This put the rough edges 
outside where they could be easily trimmed, but if half an hour is 
your goal, you cannot take time to file washers. You can make a 
functional but not pretty hole with a heated skewer, nail, bolt or 
tent peg. Warm it over a flame until it is just hot enough to melt 
the plastic of the 'can' and push it through from the inside. Again 
use the craft blade to enlarge, clean up or adjust centre if you want 
a better-looking result. 

You now want a similar hole but 1/2 inch (12 mm) in diameter in the 
centre of the drink bottle cap. You can use a smaller drill then 
enlarge the hole to size with a round file. Again you can melt your 
way through and just melt off the side of the hole until the shape 
and diameter are roughly right. You can again trim off ugly excess 
melt on both sides to neaten things and make for a better fit with 
the filter material. On the inside of the cap, make sure you have not 
disturbed the area where the neck of the bottle seats, or you will 
get leaks.

Cut the bottom off your drink bottle with scissors, tinsnips, 
scalpel, hacksaw or a hot knife (practice!). This will give you a 
parallel sided filter funnel. Remove the tamper-proof ring from the 
neck of the filter funnel by cutting or snipping it. The tamper-proof 
ring is normally prevented from dropping down the bottle by a wide 
step moulded into the bottle, and this step must be reduced in 
diameter so the funnel neck can slide right inside the cassette can. 
The best tool is a large folding camping-type toenail clipper. Clip 
away in small sections at the wide step as if it were a massive 
circular toenail. You can steadily reduce the diameter until the 
clipper will take no more off. These four steps (drill can, drill 
cap, cut bottle, trim wide step) are the only construction 
operations, and can all be done with a hot tent peg, a penknife and 
toenail clippers if necessary. 


A quick trial

Screw your drilled cap firmly onto the filter funnel. A working 
filter is formed when you push cotton wool into the cassette can, 
then insert the drilled cap into the can, with the filter funnel on 
the back, and push it down the can until it compresses the cotton 
wool and stops moving. The working filter is then turned so the 
funnel top is uppermost and the cassette can hole is at the bottom. 

Pour tap water in to almost fill up the funnel, and filtered water 
runs at a fast drip out of the bottom. Some control of the drip speed 
can be done by varying the pressure of the funnel cap against the 
cotton wool. More pressure cuts down the drip rate, but also improves 
water quality. If you apply excess pressure, as soon as you take your 
hands off the cassette can, the cotton wool expands and pushes the 
funnel cap back until the frictional fit in the can stops the 
movement. Water should only drip out of the end of the cassette can, 
and not from the cassette can/funnel cap joint, or from the funnel 
cap/funnel neck joint.

This completes the making of the tinyfilt. 


Use of the tinyfilt

In the sections following specialised cotton wool handling and 
practical filtration will be covered. 


Cotton Wool

If you went further than the quick trial covered above and gulped 
down the first water filtered through your tinyfilt, you would have 
had an unpleasant surprise in the form of a very nasty taste. Relax, 
you have not been poisoned!

Most cotton wool sold worldwide is produced by Johnson and Johnson, 
although the product varies according to where it is made. Ours is 
still Australian-made, but whether it contains Australian cotton is 
unknown. There has been little change in cotton wool for many years, 
according to my earliest memories. The product is made with cotton 
and cellulose fibre (rayon). I went chasing the source of the nasty 
taste, and was told in addition there is a water-soluble anti-static 
additive presumably added to facilitate processing. Large amounts of 
this additive have apparently been consumed without toxicity 
problems. This is all very well, but personally I don't like the 
taste, and I would be surprised if anyone else does. If you are using 
the filter at home, simply follow normal instructions for most 
domestic filters and discard the first liter that the tinyfilt 
delivers. This contains almost all of the antistatic additive, and 
the flavour is not noticeable after that. Cotton wool has been used 
for many years for the fast filtration of photo film processing 
chemicals, but in this application the additive has no effect.

If you plan on camping with the tinyfilt, you really want to carry 
one or two fresh dry filters with you. No matter how relaxing a time 
you plan to have, there is never enough daylight for all you have to 
do in a Summer or Winter camping day, and waiting for half an hour 
while you flush out your new water filter is quite impossible. 
Unfiltered water may even be scarce. You can flush out a new filter 
before you go and carry it up damp, which is quite a reasonable 
solution, but the best answer is to wash and dry a bulk batch of 
cotton wool, which will be enough for the next twenty or so filter 
changes.


Washing cotton wool

This is not an easy job, but however you plan to do it, you will need 
seven changes of tap water before the additive no longer tastes. A 
large roll of cotton wool (375 grams in Australia) is a rolled up 
ribbon of the wool interleaved with paper of the same width, so it 
may be unwound and cut easily. It is worth ordering this from your 
Chemist rather than buy the more easily available 100 gram rolls 
which are not paper-interleaved. Do not buy a bag of cotton wool 
balls. Unroll about 15 inches of the ribbon, which will weigh about 70 
or 80 grams and cut it off. You can use scissors or snips, but if you 
try and tear it you will make a mess. I tried three washing methods.

Method 1

Put the cut off wool in a flat tray-type dish, run water in, swish it 
about, then slowly drain off the water. Repeat for seven changes 
(this took forever). As you drain off the last change, make sure the 
wool is still lying flat and as even as you can manage. As the last 
of the water drains off, the layer of wool settles and sticks to the 
bottom of the tray. You can then put the tray out in the sun in a 
bird-free location, nearly vertical so draining continues. By the end 
of the day the wool has started to fluff again, and when it is 
completely dry it comes free of the dish pretty suddenly, so watch 
things if the dish is still vertical. The wool produced was in very 
nice condition, but handling and washing stages were nearly 
impossible.

Method 2

Put the cut-off wool into a small clean plastic bucket, and with 
clean hands just hand wash, squeeze out the water and repeat for 
seven changes. This leaves you with a damp ball of cotton wool in 
about ten minutes, but you cannot spread it out so how do you dry it?

Put the wool in a clean dish into an oven set at 120 or 130 degrees 
Celsius. If the oven is cooler the wool will not dry, and if it is 
hotter you will start to get overheated cotton smells familiar from 
ironing cotton cloth. The cotton wool will take about four hours to 
dry. The final product is clumped together, but may be teased out to 
its original form if you take the time. The wash is easy, but the 
drying is tricky and the end result is inferior.

Method 3

The best place to do this is on a waterproof kitchen table out in the 
garden, but I improvised by putting a sheet of polythene over a 
small-table sized piece of plywood to keep it dry, and supporting the 
plywood just above the grass. Lay the cotton wool sheet on the table, 
and flood it carefully with clean water from a clean watering-can 
fitted with a fine droplet diffuser. The wet wool bulks up, but keeps 
its shape very well, and the fine water stream does not disturb it. 
Make one or two passes with a kitchen rolling-pin to roll out the 
water. When the wool starts sticking to the roller, it is more than 
dry enough, and you can put the next spray of water on. Again seven 
water applications are required. After the final roll the top layer 
of the sheet has spread a little, but most of the shape has been 
retained. Tilt the table nearly vertical, and leave the sheet to 
drain, dry and fluff up. Washing time is about ten minutes, there are 
no drying problems and the texture of the dried sheet is better than 
the original. Unless you have a better idea, method 3 is recommended.

Once your cotton wool is clean, dry and bagged to keep out the dust 
your problems are over. You can opt to fill one or two cans with wool 
at home, and save any messing about in the field, or take up a very 
light polythene bag full of the washed cotton wool and make up as 
many or as few filters as you need. If the water is murky, 
prefiltration may be needed (see later), and in that case you will 
certainly need washed cotton wool with you.


Filling the cans

Cotton wool is much more springy dry than wet, and this permits the 
correct maximum load for the can to be put in without weighing or 
other estimation. Push cotton wool (preferably one piece, although it 
is not critical) under moderate finger pressure into a dry can until 
the can is full. If the cotton wool is over pressurised it will 
spring back and start working its way out of the top of the can. Tear 
off the excess and the can is full with the maximum load. This 
corresponds to about 4 grams of cotton wool. You can weigh it if you 
have sensitive scales, or alternatively put a steel, plastic, or wood 
one foot rule so it balances over a round pencil held to the table 
with sticky tape. The empty can also weighs 4 grams, so you can 
balance the empty can at one end against the cotton wool at the 
other. If you are going to store the can, put the original film cap 
back on the can to keep the dirt out and the cotton wool in. The 
capped can weighs 10 grams.


Starting a dry filter

Have your constructed filter funnel ready, with the 1/2 inch hole 
filter cap firmly screwed on. You cannot push the filter cap into the 
filled cassette can far enough to use it because the cotton wool is dry 
and it will not compress sufficiently. Pour a little (preferably 
drinking) water into the top of the filter can, and within a few 
seconds the cotton wool will wet and the hole at the end of the can 
will start dripping. Push the filter funnel and cap down into the can 
until it stops as it reaches and compresses the wet cotton wool. Your 
filter is ready to run. Pour water in the top of the funnel until it 
is nearly full, and rest the funnel in your water collector. Discard 
the first minute or two of water for safety. If the cotton wool is 
unwashed and straight from the packet, discard the first half an hour 
of filtered water, or fifteen minutes if you are dying of thirst. 


Finishing off

With all your filtered water bottled, empty out the filter funnel, 
and push the funnel and cap hard down into the can with the funnel 
still vertical. This squeezes the cotton wool like a sponge and 
excess water drips out of the hole in the can. The cotton wool is 
left damp for the next filtering session. Pull the funnel and cap 
right out. You can expect to see a discoloured 1/2 inch diameter spot 
in the centre of the white cotton wool where muck has been caught, 
but if this spot is jet black you are filtering pretty dirty water. 
Filter blockage may occur before long unless you use prefiltering 
techniques. Put the film cap back on the can to keep out the dirt, 
and the tinyfilt is ready for the next run, when you just insert the 
funnel cap until it contacts the cotton wool once more and off you 
go, discarding the first minute of filtered water to be on the safe 
side. Commercial filters are often kept wet or damp for years, and to 
be honest we do this at home with our commercial unit. This is not a 
good or particularly safe practice. If your tinyfilt filter is damp 
for more than two weeks, throw out the cotton wool and use a new dry 
one. Can you afford ten cents?


Prefiltering techniques

Sooner or later out in the wilds it will be dirty water or nothing, 
and we all know which one to choose. There will be excessive muck in 
the water, and possibly bacteria too. Any filter run with dirty water 
is likely to 'block' as the material filtered out of the water builds 
up on the upstream side of the filter and stops water passing 
through. In a gravity filter like the tinyfilt the drop rate on the 
output slows dramatically, and in a pump filter you find yourself 
pushing much harder. Total blockage in each case is not far off. 
Big industrial filters incorporate rotary scrapers to get rid of the 
build-up plus 'backwashing' which is reversing clean water through 
the filter to force the build-up off the dirty side. Neither is an 
option with a small filter, although you can stop filtering with the 
tinyfilt and with your fingers pull out the 'black spot', reassemble 
and carry on.

Prefiltering is using a coarse, large area filter that the water 
flows through before it reaches the main filter, to pull out most of 
the garbage and let the main filter deal with the fine material. Make 
a prefilter plug out of about half the amount of washed and dried 
cotton wool you would use to fill a can, and with a finger push it 
down into the neck of the empty filter funnel until it contacts the 
half inch hole in the cap. Since this prefilter is not under any 
pressure, the pores are large, and it will only catch big material. 
The top of the prefilter has a large surface area to spread out the 
caught muck, so it does not block easily. When you pour water in the 
top of the filter funnel, the prefilter may lift in the funnel neck 
and swirl around, but this does no harm as it resettles very quickly. 
Really dirty water may block the prefilter within about 5 liters of 
filtering, but a change is easy and keeps the flow going. It is 
sobering to look down into the funnel as filtering proceeds, and 
watch the prefilter turning black. At least that is something you 
won't be drinking. A prefilter will slow your water throughput. A 
rough guess would be from 2 liters per hour to 1.5 liters per hour. 


Bacteria etc

Bacteria are smaller than two microns. The prefilter will hang on to 
a few, and if you compress the can filter for minimum drip rate, 
since this is a 30 mm thick labyrinth filter a remarkable number will 
stay in the cotton wool plug. The firm compression of the plug after 
filtering is over squirts many of these lodged bacteria out, but this 
is why changing the filter often is good technique. What you are 
aiming at is filtered water with reduced bacteria, and other solids 
reduced as far as possible to cut down the load on whatever system 
you use for sterilising. There are numerous tablets available for 
sterilising water. Read the instructions, try not to listen to the 
loud noises made by any one group of devotees, and above all do not 
follow my example and keep the tablets around for ten years before 
you use them. 

If you think these cute tablets are an expensive pharmaceutical 
swindle (no comment) in Australia you can still buy generic 'bleach' 
at one dollar for 2 liters. The plain variety (not lemon scented) 
contains sodium hypochlorite (NaClO) which is an excellent source of 
free chlorine. Try it out on your tap water and find how many drops 
you need per liter to make the water smell faintly of chlorine, 
assuming it does not do so already. Carry a small bottle of bleach 
into the wilds and do likewise, leaving the sterilised water as long 
as you can before drinking it. 


Speeding up

If you find the water at your campsite looks very good, but are 
giving it a quick filter to pull out small wrigglers, liver flukes 
etc, the normal tinyfilt filtration is excessive. You can remove a 
third or even half the cotton wool from the filter can since with the 
wide step clipped off, the funnel can be pushed much farther into the 
can if required. This gives you a thinner filter with a faster 
throughput. Fastest of all is to remove the can completely, and just 
use a prefilter plug of cotton wool as outlined above in the neck of 
the funnel. Keep the funnel at least half full of water while you 
filter to avoid disturbing the prefilter plug when you refill with 
water. Only use the prefilter plug for one session. 


General hints

Since water bottles have to be carried, and a recycled PET bottle is 
light, tough and free, the extra bulk associated with the filter 
funnel is the only penalty for using the tinyfilt. I use the most 
common 1.25 liter PET bottle to make the funnel, and this produces a 
funnel and drilled cap that together weigh 32 grams. In the pack the 
funnel can be crammed with clothes, small items or socks so it takes 
up no appreciable space, but if it is to be a repository for dirty 
socks and underwear a light poly bag could be used in addition. 

Where the funnel has to be smaller and lighter a half-liter or 750 ml 
bottle can be used for the funnel. The penalty is less free time 
since the funnel has to be topped up much more often, and slower 
filtration times since the water height, and thus the effective water 
pressure on the filter has been reduced. 

If you are going to be filtering much of the day at one campsite, 
make up a string sling so the funnel dangles vertically from a tree 
branch. Anchor the funnel neck with two tent-pegged cords and you 
have a fixed installation, if the wind keeps down, that produces a 
drop stream going straight into the narrow neck of any bottle located 
underneath it.

A rugged survival exercise that does not involve ropes, cliffs and 
friendly instructors happens every time you jet to an exotic 
destination, and discover at the far end you have parted company with 
your luggage. The old trick of buying a 750 ml PET bottle of spring 
water and refilling it at every drinking fountain you pass works in 
the airport, but sooner or later you must find a hostel in town, and 
drinking the local tap water may add to your troubles. With (of 
course) your penknife and nail clippers in your cabin bag, you could 
try knocking up a tinyfilt from local resources, but a half-liter 
filter funnel and two filled tinyfilt cans in your cabin bag could be 
of more use to you than a pair of pyjamas. 


This concludes the tinyfilt manual. Any comments are welcome at

breck@permaflate.com


APPENDIX - test methods used on the tinyfilt.

The water filtered for testing was a 'bad' sample from an Australian 
country public water supply reservoir. This water is normally 
extensively treated before it reaches drinkable standards.

The water was plainly both cloudy and yellow. A few liters were 
filtered through the tinyfilt with no prefilter. The top 1/2 inch 
circle on the cotton wool turned black, the drop rate slowed 
slightly, but filter blocking was not apparent.

The filtered water was crystal clear, with perhaps the faintest tinge 
of yellow. It tasted good, and I drank an unsterilised glassful with 
no apparent ill effects.

200 ml of the water filtered as above were syringe-pumped through a 
Gelman water analysis filter with a 0.25 micron pore size. The 
analysis filter did not block, which showed almost all the solids in 
the water had been removed by the tinyfilt. When filtration was over, 
examination of the white circle of the Gelman filter showed a pale 
grey circle of filtered-out material. This grey circle was scraped 
off wet, transferred to a microscope slide and examined under high 
power with a Wild (pronounced Villt) professional microscope. There 
were a very few 2 micron sized particles, none bigger apart from the 
occasional 'freak', and a significant concentration of 1 micron 
particles. Smaller than 1 micron particles were not resolved by the 
microscope. 

The water after passage through the Gelman filter tasted almost the 
same as after the tinyfilt, with an added trace of 'plastic' flavour 
that came probably from the syringe pump, or the soft plastic tube 
used to take water into the syringe. 

On the basis of these measurements, an effective pore size of 2 
microns looks like a reasonable claim for the tinyfilt.