watson.fawkes - 9-9-2010 at 05:42
Earlier this week, my brother told me he needed help fixing his electric dryer. He'd been on line already a bit, had learned enough there to test his
door switch, and found out that wasn't it. I asked him to get me a schematic for his model, Maytag PYET244AYW. A little coaching over the phone led to
the diagnosis that he'd blown a "THERMAL FUSE / NOT RESETABLE" [sic] as the schematic shows. Not resettable!--I hate these hidden costs. It gets
worse. It used to be that you could just get that one part by itself. Whirlpool now sells the parts, and now you can only get it in a "replacement
kit" of three parts: the thermostat, a resettable fuse for the electric model, and a resettable fuse for the gas model, thus completely assuring
waste. Of course, the price tag went from $7 to $27. Time to stick it to The Man.
If I was going to find an alternate source, I needed to know just what this thing was. My brother sent me all the numbers printed on the fuse itself:
"36F14 20021 L300F 53-1182 A0421". Here's where this gets relevant. "L300F" seems to mean "Limit 300° Fahrenheit", the temperature when
the fuse goes. Excellent; there's a number of homemade apparatuses, such as tube furnaces, that could use a thermal safety limit. Turns out I had some
thermostat relays in the parts box labeled "L155F", a pretty good temperature for protecting electronics next to a furnace. "53-1182" is the Maytag
part number. I never did figure out what "A0421" is, but I'm guessing it's a date/plant code.
Digressing a bit, the thermostat in that kit is labeled "L248-80F". Presumably this means "make at 248° F, break at 280° F". This is
about exactly the right temperature range to manage the coolant in the condenser of a sulfur still. The melting point of sulfur is just below the
lower limit of this switch, meaning "add heat now". The upper limit is below the polymerization point of liquid sulfur, meaning "cool off now".
The magic starts with "36F14 20021". I found a reference to this in some page of an industrial supply catalog. This company sells a replacement part. The number is designated "Therm-O-Disc Style No.", so I presume
that's the original part number this unit is replacing. Incidentally, the part of the code "36F14" seems to designate a form factor. There's a column
in the table that verifies the 300° limit temperature.
But hey! That same table has another column that says "Auto Reclose Below -31° F". Not resettable my ass! Dry ice is at -109.3 F. So I had my
brother pick up a bit of dry ice. He had his wife pick up the ice and cool off the fuse.
And it worked.
I'm guessing the innards of this thing has a bimetallic disc with convexity that's geometrically stable in each curvature direction, but with large
hysteresis in changing configuration. Unfortunately, the part's already installed again. I never got a chance to handle the thing.
Wizzard - 9-9-2010 at 05:51
Love this story 
psychokinetic - 9-9-2010 at 13:07
Hmm, these temperature dependant fuses could be well useful for the chemist who doesn't want to get his/her face too close to a thermometer in a
sample to check that X temperature has been achieved....