Re: Digest Number 221

Hi Andrew,

The system of choice in this situation would be one or two heat detectors
directly over the machine (depending on its size).
These should be the type used in commercial kitchens etc which are basically
a tube full of a high melting point wax and which I believe work on the
expansion of the wax closing a pair of contacts although it could be the
other way round and they could work by the wax melting and running away and
allowing the contacts to close - I never actually took one of these to
pieces. Other alternatives would be simple fusible links - two bits of metal
strip soldered together with a low melting point solder and sprung apart so
that if the solder melts either a set of contacts close and work an
electrically triggered extinguisher or a spring loaded wire simply lets a
water valve open, or, a glass bulb type of water sprinkler head where a
small vial of water simply bursts and allows the sprinkler head to work
(messy and difficult to shut off sometimes) or a 'rate of rise' detector
which is electrical again and consists of two bimetallic strips coiled one
within the other and each having a contact on the free end. During normal
temperature changes the two strips uncoil at the same rate but, in the event
of a sudden temperature rise, the inner strip is designed to react faster
and so contacts the outer strip before it has had time to uncoil out of the
way. These detectors usually have another, fixed, contact on the base so
that they do also have an upper temperature limit in case a small
smouldering fire fools the differential bit. Water would be an appropriate
extinguishant and you could rig something up yourself quite easily using
standard or salvaged components. In fact, if you hang a length of plastic
hose over the machine and firmly seal the open end and then connect the
other end to a tap and turn it on, you will have a simple but effective
system for almost zero cost. When the tap is turned on, the air in the hose
will be compressed and so will not let water into the end bit over the
machine but, if a fire gets the hose hot enough to melt, this will let the
air escape and the water will flow.

Ian
--
Ian W. Wright
Sheffield UK

----- Original Message -----
From: Andrew Werby <drewid@...>

> Would a smoke detector
> work to trigger the thing, or would it be fooled by a haze of sawdust? Any
> other ideas?]