On Thursday, January 1, 2015 9:25:28 AM UTC-5, dpb wrote:
On 12/31/2014 7:52 PM, trader_4 wrote:
...

IDK what flame sensor you have there, but from the dimensions it looks
like it's a foot or more long, 1/2" in diameter and certainly not for a typical
home furnace. The one Danny has pics of does look like a small rod,
mounted on an insulator, connected to a wire. The descriptions I've
read about rod to flame conductivity also don't say anything special
about the reqts for the rod. They just say that a metal rod inserted
into a flame will conduct a current when voltage is applied. That would
seem to coincide with what he has, ie a metal rod on an insulator mount.

Those are commercial for larger boilers, etc., but the physics is the
same. There must be an isolated section for the current flow; otherwise
it would also be grounded. The _precise_ configuration is immaterial...

--

I agree. The isolated section is the metal rod with a wire attached.
It's housed in a ceramic insulator that holds it and keeps it isolated
from the furnace metal. The flame completes the circuit. The controller
board sends it a small current, at about 90V?. If the flame is there,
the controller detects that current is flowing, because the flame can
conduct a small current, it actually rectifies it. I had no idea how
this worked before, but that's what I learned from googling. I haven't
seen anything about any special requirements beyond that. And the pics
Danny posted of his flame sensors sure look like just a metal rod housed
in a ceramic insulator. So, from everything I've seen so far about
how it works, the physics, etc it sounds like just a metal rod.

So, if that's right, then the question remains, what failed in Danny's
detector? There have been so many posts, I'm unsure of is what exactly
he measured, what the readings were, etc.
I think he measured that there was infinite resistance between rod and
furnace metal, ie no shorting of the insulator. I think he verified
conductivity from end of wire to tip of rod, but not sure what exactly he
measured there. So, I don't understand what could be wrong with the old
one......