Hartmann Masks & Dew Shields

In ,
adm typed:

Thanks Jez. What I like about this kind of arrangement is that as you
say - you hardly notice it's there. I think I'll have to have a go at
building one myself....I might go the resistance wire route as
opposed to using individual resistors as it may give a more even
spread of heat and be easier to construct.


I have a homebrew resistor-based heater using microbore tubing but it only
dissipates about 5 watts at 12v. I'm interested in a resistance wire version
and I like the idea of wrapping the wire around a flexible core. The whole
thing could then be wound with PTFE tape (from a plumbers merchant) before
insertion onto the copper microbore tubing. The only material not in my junk
box is a suitable flexible core. I imagine some sort of rope like material
that is reasonably heat resistant and has a diameter of a few mm. Any ideas,
anybody?

BTW, note that it is not possible to reliably solder to resistance wire, you
will need some sort of screw or crimped connectors.

Jo

Jo wrote:


I have a homebrew resistor-based heater using microbore tubing but it only
dissipates about 5 watts at 12v. I'm interested in a resistance wire version
and I like the idea of wrapping the wire around a flexible core. The whole
thing could then be wound with PTFE tape (from a plumbers merchant) before
insertion onto the copper microbore tubing. The only material not in my junk
box is a suitable flexible core. I imagine some sort of rope like material
that is reasonably heat resistant and has a diameter of a few mm. Any ideas,
anybody?

Hi Jo,

We wind things like that on hard silicone rubber tube - I assume we
aren't going to red heat with it ! We have done it by pushing the tube
onto a metal bar as a mandrel, and spinning it in the lathe. Then you
can slip another tube on top if you want, to seal it all in.

Have you got any suitable wire ?

BTW, note that it is not possible to reliably solder to resistance wire.
Actually you can solder it well, but only if you use "Baker's Fluid" as
the flux - then wash off the residue.

Steve

In ,
Steve typed:

We wind things like that on hard silicone rubber tube - I assume we
aren't going to red heat with it ! We have done it by pushing the tube
onto a metal bar as a mandrel, and spinning it in the lathe. Then you
can slip another tube on top if you want, to seal it all in.


Thanks. I estimate about 15 watts around a circumference of about a meter so
temp will not be too extreme. I overran my 5 watt rated dew heater at this
output for a short time. It got quite hot, but probably less than 80C. This
would be even lower when fixed to the scope.

I may be able to get some silicon rubber tube or whatever...maybe auto shop?
I seem to recall that some types of HT ignition cable have silicone rubber
insulation.

Have you got any suitable wire ?


Yes, some 28 SWG constantan at 4R2 per meter.

Actually you can solder it well, but only if you use "Baker's Fluid"
as the flux - then wash off the residue.


OK. From a hardware shop, plumbers merchant ? Anybody know what Bakers fluid
actually is? Borax ? Boric acid? Zinc chloride?

Jo

Jo wrote:

I may be able to get some silicon rubber tube or whatever...maybe auto shop?
I seem to recall that some types of HT ignition cable have silicone rubber
insulation.

RS sell it. Brewing shop perhaps ? If you aren't aiming for radiant
heating (!!) then how about using a piece of pyrotenax (MICC) insulated
cable, which has a copper sheath surrounded by PVC insulation.

Yes, some 28 SWG constantan at 4R2 per meter.

Isn't that a bit thin ? That's only about 2.3 metres of the stuff to
get your 15 W on 12 V, on say, what ? 10mm diameter former ? 31 mm /
turn. Thats only 75 turns over 1 metre. Is that going to be OK ?

OK. From a hardware shop, plumbers merchant ? Anybody know what Bakers fluid
actually is? Borax ? Boric acid? Zinc chloride?

Engineers merchants for sure, model engineers place certainly. Dunno
about the composition, but it smells evil when hot. If I could post it,
you're welcome to a 25mL of the stuff which is about 1000 years supply !
I don't remember when we bought ours.

Steve

In ,
steve Taylor typed:

RS sell it. Brewing shop perhaps ? If you aren't aiming for radiant
heating (!!) then how about using a piece of pyrotenax (MICC)
insulated cable, which has a copper sheath surrounded by PVC
insulation.


I'll do some shopping around this weekend. I understand that some types of
PVC insulated cable are designed for operation at up to 100C for use
on/around hot water cylinders when an immersion heater is fitted. A length
of this might do for my "core".


Yes, some 28 SWG constantan at 4R2 per meter.

Isn't that a bit thin ? That's only about 2.3 metres of the stuff to
get your 15 W on 12 V, on say, what ? 10mm diameter former ? 31 mm /
turn. Thats only 75 turns over 1 metre. Is that going to be OK ?


Not sure yet, but it is what I have several meters of. I can always wind
*two* lengths of the wire in parallel to get more turns.



Engineers merchants for sure, model engineers place certainly. Dunno
about the composition, but it smells evil when hot. If I could post
it, you're welcome to a 25mL of the stuff which is about 1000 years
supply ! I don't remember when we bought ours.


Thanks. I'll shop around this weekend and maybe get back to you if I can't
find any.

Jo

BTW is the wire already insulated ?

Steve

steve Taylor wrote:

Yes, some 28 SWG constantan at 4R2 per meter.


Isn't that a bit thin ?
THICK ! Sorry.

Steve

In ,
Steve typed:
BTW is the wire already insulated ?

Steve

No

Jo

Jo wrote:

No
Shame. Saves a lot of hassle - we get ours from Scientific Wire. You
could wrap it on a metal rod then and it would work just fine.

Steve

In ,
Steve typed:
Jo wrote:

No
Shame. Saves a lot of hassle - we get ours from Scientific Wire. You
could wrap it on a metal rod then and it would work just fine.

Steve

The attraction of the wire I have is that it is free, from my junk box,
however...I'm no great rush so I may look around for some insulated
resistance wire. You are suggesting a metal rod wound with insulated
resistance wire and bent into a circle to fit the scope ?

Jo