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Old 9th Jan 2020, 3:04 pm   #1
Wendymott
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Default PCB Etching tank

Hi peeps. This is not a purely electronic issue.....but one that I am sure a few of you learn'ed members may have an idea about.
I recently refurbished my etch tank...... It has a tube in the bottom which is perforated to allow a bubble screen.
The tube is 300 mm long with a dia of 8mm.
The problem is.....to get a reasonably even curtain of bubbles, there needs to be an even ish pressure along the tube.
At present I have approx 15 holes of 1.1mm dia , however.. as you can guess, the bubbles are confined to the area near the inlet end.
How do I work out the hole diameter gradient to give me a more even bubble field.
Over to you.. with fingers crossed.
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Old 9th Jan 2020, 3:21 pm   #2
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Default Re: PCB Etching tank

Can you not feed the tube from each end, like a ring main?
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Old 9th Jan 2020, 4:35 pm   #3
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Default Re: PCB Etching tank

I suspect that you could get a better dispersion of small bubbles by putting some kind of "sintered particle" devices ("sparging devices"?) on the gas line. The cheap ones for aquaria won't like the FeCl3 much, though that's what I used to pump air through hot oil to oxidise it. You would have to change it every so often. Ideally, a stainless one would be best and I'm sure they are out there...somewhere.

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Old 9th Jan 2020, 4:56 pm   #4
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Default Re: PCB Etching tank

The larger the diameter of the hole, the greater will be the volume of air emitting from that hole, but the lower will be the pressure, just as when you squeeze the end of a hosepipe. Hence, the smaller the holes, the smaller will be the volume and the higher will be the pressure. Further along the pipe the pressure will be progressively lower, so the holes will need to be larger to get a given volume of air at that point, but of course you know that anyway.

I think you could only determine the optimum hole sizes along the tube by experimentation (using water in the tank of course rather than etchant!) For example, maybe five holes 0.8mm, five 1mm and five 1.1mm, all of which you'll no doubt have to hand for PCB drilling. Ordinarily, 8mm annealed microbore copper tubing is sold in 10 Metres coils, but there are firms who will supply say a 1 Metre length, such as here, at £3.99 post free:

https://www.ebay.co.uk/itm/1m-of-8mm...-/251629048506

To save wasting tubing in your experiments, if one set of holes doesn't produce the desired result, you could tape over those and drill another set. I doubt whether the relationship of the pressure loss will be linear, so for example, you might need to drill say seven 0.8mm, five 1mm and three 1.1 mm.

Hope that helps a bit.

My own etchant tank is a rather modest affair -a cereal container, aquarium airstone and pump which works really well, two 75 Watt aquarium heaters with the thermostats rigged to go up to 45 degrees C (though when the pic was taken I only used an old heater), and an infra-red thermometer. (I only heat the etchant because I use sodium persulphate in preference to ferric chloride).

Have fun!
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Old 9th Jan 2020, 7:27 pm   #5
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Default Re: PCB Etching tank

It seems that the items I'm thinking of are called air stones, at least on ebay, which I think are porous stones (probably man made materials)

This product looks interesting, referring to "stones" but only showing what appear to be porous plastic tubes?

https://www.ebay.co.uk/itm/UK-Bubble....c100005.m1851

B
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Old 9th Jan 2020, 11:40 pm   #6
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Default Re: PCB Etching tank

Hi David and Bazz. David.. I guessed as much...the original pipe was discarded a while ago so I couldnt copy it...as you say... I will seal the holes with hotmelt and work from the "dead" end reducing the hole sizes as you suggest.
I dont have a lot of etchant in the tank anyway, it is rather small.. compared with the pro tanks I used to use.

Bazz those "air bricks" I am told leech away into the ferric after time... and the fact that its a cube makes it a "spot" source rather than a linear one. It not the fact that the bubbles stir up or move the liquid,, its the oxygen in the bubbles that rips off the copper.
I once saw an etch tank that was designed as a waterfall..at approx 45 degrees, the board has ferric pumped from the top in a linear waterfall, it ripped the copper in minutes. The problem with that tank was pumping ferric rather than air, one wonders how long the pump seals lasted. It was expensive, for a few bits of acrylic and a pump.
I will test friday and report back.

Just one teensy point David...we cannot use copper microbore.. it would disolve in a few minutes.. I use acrylic tube.
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Last edited by Wendymott; 9th Jan 2020 at 11:42 pm. Reason: Additional info
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Old 10th Jan 2020, 12:22 am   #7
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Default Re: PCB Etching tank

Those "airbricks" usually turn into a goopy mass after a while.

It also sounds like either too many holes or too large for the available air pressure.

Another trick is to have the air holes on the bottom of the tube so

a) there is a bit of resistance to overcome, helps equalise the pressure along the tube.

b) the air then rides up either side of the tube to give you a double screen of bubbles.

You get a much better and quicker etch if you can 'invert' the system so it sprays the etchant over the board as opposed to air bubbles.
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Old 10th Jan 2020, 12:09 pm   #8
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Default Re: PCB Etching tank

Hi Terry. I have just fitted the re drilled tube.. but with the holes on topI could mount the tube upside down with a couple of spacers between the pipe and its mounting.... I will try that and report back.
As it is.. the air dispersal is much better with graded holes from the "dead" end.
As I said earlier, that waterfall or spray method was much more efficient, but getting a pump to pump ferric may be a little difficult.. However as this is a DIY project, its sufficient for my needs. Thanks for your inputs guys.
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Old 10th Jan 2020, 1:02 pm   #9
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Default Re: PCB Etching tank

Keep an eye open for the sort of pumps they use in dialysis machines, basically a silicon tube type that has a couple of motor driven rollers that run around a near complete circle of the tube.

They show up in the usual auction places at times.

The other sort of pump uses a magnetic type arrangement where the actual pump section is chemical impervious and the motor is coupled magnetically.

I forget exactly what the two types are properly called, but I suspect someone here will know.

Edit:- homebrew beer places have the magnetic pumps - all plastic pump assembly and handles up to 120C temperature other option is farm chemical spray pumps although they may be a bit over the top at 100psi 5L/m rate.
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Old 10th Jan 2020, 3:36 pm   #10
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Default Re: PCB Etching tank

Quote:
Keep an eye open for the sort of pumps they use in dialysis machines, basically a silicon tube type that has a couple of motor driven rollers that run around a near complete circle of the tube.
They are called "peristaltic" pumps and are pretty common, they are used in dosing pumps, basically a rotating wheel and rollers squidging a silicon rubber tube, the usual "auction" site should prove fruitful.
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Old 10th Jan 2020, 4:26 pm   #11
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Default Re: PCB Etching tank

Quite a lot of small hobby type 'peristaltic' pumps on Amazon and elswhere for prices ranging from as little as £7.00, but on the basis that 'you get what you pay for', this one at £32.29 + free delivery looks to be a reasonable offering. It will pump air or circulate liquids. Given that the tubing is silicone, that could include ferric chloride or other etchants, provided that if heated, the temperature is kept below 40℃:

To quote the spec:

Tube material: Transparent silicone tube Shell material.
PC Rolling wheel: POM Roller type.
Ball bearing Motor: 12V DC motor. Motor current: 0.5-1.4A.
Flow range: 0-400ml/min.
Tube inner diameter: 6.4mm / 0.25inch Tube outer diameter: 9.6mm / 0.38inch Wall thickness: 1.6mm / 0.06inch.
Working temperature: 0-40℃.
Weight: Approx. 273g.
Ball bearings in the wheel frame, silent, long life time. Stainless steel screws, rust corrosion resistance.
Pump head designed for easy disassembly, easy to clean and maintain.
The direction of flow of the peristaltic pump depends on the connection way of positive and negative.

The pump is suitable for aquarium, chemicals, liquids and other dosing additives.

End quote.

https://www.amazon.co.uk/dp/B07NPJZF...NsaWNrPXRydWU=
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Old 10th Jan 2020, 4:31 pm   #12
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Default Re: PCB Etching tank

Quote:
Originally Posted by Wendymott View Post

Just one teensy point David...we cannot use copper microbore.. it would disolve in a few minutes.. I use acrylic tube.
Sorry Wendy - I should have made it clearer that I only meant for experimenting, which is why I said using water not etchant for the tests. But yes, if you're using hard acrylic tubing in the tank, it makes sense to experiment with such tube of course.
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Old 10th Jan 2020, 4:43 pm   #13
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Default Re: PCB Etching tank

Quote:
Originally Posted by Bazz4CQJ View Post
It seems that the items I'm thinking of are called air stones, at least on ebay, which I think are porous stones (probably man made materials)

This product looks interesting, referring to "stones" but only showing what appear to be porous plastic tubes?

https://www.ebay.co.uk/itm/UK-Bubble....c100005.m1851

B
That was well spotted Bazz - not seen those before, so thanks for that.

Up to now I've used the standard air-stones like the one in the link below. My little 'tank' is only in use for maybe 15 mins a time, maybe ten times a year and I clean the stone in clear water after each use so it lasts about a year, but it time, it does crumble away. Cheap enough to replace, so no dramas, but the type you've mentioned are porous plastic, so may be more immune to etchant than traditional airstones. I'll soon find out because I've just ordered a 7" one!

This is the sort of thing I've used till now:

https://www.ebay.co.uk/itm/CLASSIC-A...e2175db954dba7
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Old 11th Jan 2020, 2:28 pm   #14
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Default Re: PCB Etching tank

Hmmm.. Might look at these peristaltic pumps.....But as my "rip" time was 10 mins at 30 deg C I am now reasonably happy...Thanks guys for your inputs.
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Old 15th Jan 2020, 9:12 pm   #15
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Default Re: PCB Etching tank

Quote:
Originally Posted by Bazz4CQJ View Post
It seems that the items I'm thinking of are called air stones, at least on ebay, which I think are porous stones (probably man made materials)

This product looks interesting, referring to "stones" but only showing what appear to be porous plastic tubes?

https://www.ebay.co.uk/itm/UK-Bubble....c100005.m1851

B
Just a quick update to say that I bought one and it arrived today.

I've tried it by attaching a length of plastic tubing and gently blowing into it with the aerator submerged in a washing up bowl of clear water. The bubble pattern is excellent along the full length. It's just a little too long to fit into my tank and I'd rather hoped that the plastic end(s) would pull off so I could shorten it a tad, but the ends are stuck firmly on. I'll look into that further. (It does come with a joining socket to enable you to cut the end of the tube off to extend it with another length of tube, so maybe I'll cut it in the middle, take a couple of cms out and join the two halves together with the socket).

The good thing is that unlike the traditional aquarium air-stones, these aren't fragile and I doubt that etchant of whatever type will cause them to degrade. Cheap enough to replace if that does happen over time. I empty the etchant out of the tank after use, store it for re-use and clean the tank, air-stone and heaters. to it's only in use for about ten minutes at a time then thoroughly rinsed.
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Old 15th Jan 2020, 10:45 pm   #16
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Default Re: PCB Etching tank

Quote:
Originally Posted by David G4EBT View Post
Quote:
Originally Posted by Bazz4CQJ View Post
It seems that the items I'm thinking of are called air stones, at least on ebay, which I think are porous stones (probably man made materials)

This product looks interesting, referring to "stones" but only showing what appear to be porous plastic tubes?

https://www.ebay.co.uk/itm/UK-Bubble....c100005.m1851

B
Just a quick update to say that I bought one and it arrived today.

I've tried it by attaching a length of plastic tubing and gently blowing into it with the aerator submerged in a washing up bowl of clear water. The bubble pattern is excellent along the full length. It's just a little too long to fit into my tank and I'd rather hoped that the plastic end(s) would pull off so I could shorten it a tad, but the ends are stuck firmly on. I'll look into that further. (It does come with a joining socket to enable you to cut the end of the tube off to extend it with another length of tube, so maybe I'll cut it in the middle, take a couple of cms out and join the two halves together with the socket).
.
Thanks for that info; those are different from anything I've ever seen before, but obviously interesting for DIY projects.

Persitaltic pumps; the cheaper ones in particular depend upon the tubing being soft so they can squeeze it. I think that various grades of silicones work well in that roll. Inevitably, however, the failure mode of those pumps is the rubber tubing tearing (unless you replace them quite often) and at the that point, the pump may well continue to pump liquid out of the tear and let it go anywhere it can. If used with FeCl3, you'd best put the pump in a small plastic container so the etchant cannot get away too far.

B
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Old 16th Jan 2020, 12:21 am   #17
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Default Re: PCB Etching tank

Bazz.... I decided that Air is the way...as Im not making many pcb's....Its a means to an end.. not an end in its self.Thanks for your thoughts though.
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