>Wes A Brzozowski wrote:
>>
>> On Wed, 28 Oct 1998, Mark Willis wrote:
>>
>> > Wes A Brzozowski wrote:
>> > >
>> > > > Yep <G> Those parts I remember (I may series connect a bunch of type
>> > > > S thermocouples myself, to raise the voltage, which is good.)
>> > >
>> > > One little problem there... when you connect thermocouples in series, you
>> > > generate a whole lot of reverse thermocouples in the process of
>> > > connecting them. These will mostly counteract the thermocouples you're
>> > > interested in, and unless you can keep the unwanted thermocouples at a
>> > > very controlled temperature, they'll add uncertainty to your measurement.
>> >
>> > Other end of the thermocouples are cooled to a known, stable, measured
>> > temperature, I thought that went without saying <G> (Chilled water,
>> > probably?)
>>
>> We each have our own personal style of working, and have found various
>> methods and procedures that have served us well. Myself, I've found that
>> *nothing* goes without saying when talking to someone I don't know,
>> and whose experience in the matter is unknown to me. Also, a peek at my
>> earlier comment, which you've quoted above, contains an allusion to the
>> controlled temperature of the unwanted thermocouples, and hints at this
>> as a possible disadvantage, rather than a design solution. Still, you
>> may want to investigate this first hand. More on that in a moment.
>>
>> It's possible that you may find otherwise, but when you consider the cost
>> of the plumbing, the lower reliability and cost (not to mention the
>> inconvenience!) of the numerous lengthy "thermocouple extension cords"
>> you'll need for the distance to keep your water from boiling, let alone
>> keep it chilled, you may start thinking instead in terms of the cost
>> and convenience of one hot thermocouple, possibly a single remote cold
>> junction. and a decent instrumentation amp. Count also the number of
>> commercial temperature measurement solutions that use series
>> thermocouples. This may prove helpful.
>>
>> Now, the fact that no one else uses a particular solution doesn't make
>> it a bad thing. If your solution is novel, it may be a very good thing.
>> But, getting back to that "personal style" thing again, I've found that
>> it's been only a small percentage of the time that my "novel" solutions
>> were truly novel. More often, I've found that the reason no one uses a
>> particular solution is not because no one's tried it, but rather because
>> they already have. Your track record may well be better than mine in this
>> area, and that would be great, but it's still prudent to view a novel
>> idea with suspicion and skepticism, at least until getting a little data.
>> That's not to say that I follow my own advice in this matter nearly
>> enough. The excitement of a new idea often has an amazing amount of
>> momentum, and I continue to learn "the hard way"...
>>
>> If the data ends up looking good, then it's full speed ahead!
>>
>> > That or I use an amplifier to raise the single
>> > thermocouples' voltage to near the A/D's max. range, if needed (I need
>>
>> Yep. Start with just a few "back of the envelope" preliminary designs,
>> just comparing the costs. It's fairly straightforeward to estimate the
>> amount of radiant heat you'll be dealing with at that temperature, for
>> different distances from a certain sized melt; consider what you'll need
>> to do to minimize the error you'll get from heat coupling into those
>> unwanted junctions. Again, you may have an interesting solution, or not.
>>
>> > to go stare at spec sheets for type 'S' thermocouples <G>) Doesn't do
>> > to have an A/D converter that gives 8 bits 0-5VDC and then feed it a
>> > signal that only changes from 0.45VDC to 0.78VDC or something, giving 1
>> > bit ADC resolution with 0 bits accuracy <G> (and most thermocouples will
>> > source loads of current, and are quite low impedance IIRC, but do not
>> > source much voltage at all.) So an Op Amp insulating the couple(s) from
>>
>> Think "low offset instrumentation amp".
>>
>> > the melt are both probables unless I go a different route (I'll have a
>> > couple months to learn here <G>)
>>
>> Taking the time to learn is an excellent idea. I mentioned taking some
>> data. If you're a fairly "hands on" kinda guy, you might want to peek
>> through rec.crafts.metalworking, under a series of threads with titles
>> like "coffee can foundry". You'll find you can melt small amounts of
>> aluminum for almost no cost, and you could use such a setup to get a
>> vague feel for the kind of environment you'll need to deal with. Don't
>> try to actually pick up a melt using this apparatus, no matter what
>> they say, though. While fascinating, I do think it's quite crazy.
>> Just produce a melt, perhaps try a measurement or two, (MAKE SURE THAT
>> YOUR SENSOR IS **ABSOLUTELY** DRY!!! BAKE THOROUGHLY BEFOREHAND!) and let
>> it cool, undisturbed. This will of course be a very lightweight version
>> of what you'll need to handle. This melt will merely radiate enough heat
>> that it's difficult to get near. Your target melt will probably radiate
>> enough heat (and light!) that it may be difficult even to view from some
>> distance. You'll have to fill us in on the details, there. I'd be quite
>> interested, at least.
>>
>> Again, you may see things very different from myself, but such an
>> experience may just turn you into a minimalist when it comes to
>> deciding how much "stuff" should be placed in the vicinity of the melt.
>> If so, you'll want to know as early as possible.
>>
>> In either case, if you're the type of person who relishes seeing and
>> learning new things, this should be a real eye opener.
>>
>> Best of luck & be careful out there...
>>
>> Wes B.
>
> (Well, hoist on my own petard, I guess - I'm usually only that "goes
>without saying" way on safety stuff <G>)
>
> I've cast lead & so on so I do know about "pre-heating" <G> I'm
>looking forwards to this one, I'll be challenged <G>
>
> One person mentioned (off list) that some folks just run things on a
>"fourier" manner (i.e. knowing mass, BTU's in, and desired temperature,
>just cook things for yea long "because it's always worked before", &
>then turn off the heat.) Too low-tech for me - BUT, I guess I'll solve
>the problems the client *wants* me to solve, whatever they are <G>
>
> Mark,
RemoveMEmwillisTakeThisOuT
nwlink.com
>
>