hey everyone I just
picked up this
thermocouple vacuum gauge on eBay and I
thought I'd take it apart and show you
how it works so the idea is that you
connect this box up to one or two of
these and this is a thermocouple vacuum
gauge which I'll talk about in a minute
and right now I have that another one of
these connected into this manifold and
then the manifold is connected to this
digital gauge so I'll switch the pump on
then hopefully the digital gauge will
match the this old analog one so as you
can see it pumps down pretty quick
because there's not much volume here is
there's no vacuum chamber it's just just
the hose I suspect that the rubber in
this blue hose here is actually
responsible for most of the pressure as
you can see it's sort of closed the
gauge the analog gauge is reading maybe
500 and the digital gauge is reading 620
or 610 but I haven't quite finished
calibrating this or setting it up but
it's the thermocouple gauges are not
really known for high accuracy and it's
also entirely possible that the pressure
at the digital gauge is in fact slightly
higher because it's behind this blue
hose
okay let me show you how this thing
works the vacuum sensor itself is a four
wire device and two of the wires are
connected to a heater which is inside
this envelope and you connect this
envelope to your vacuum system and then
on the heater itself there's a
thermocouple which is a two wire device
so there's four total and the idea here
is that you run a specified current
through the heater circuit in this case
it's something like 165 milliamps and
the resistance of the heater is about an
ohm and a half and if the if the vessel
here is full of air the heater will be
cooled down by all of the conduction
mostly in the air but if you suck all
the air out of this the heater has no
way to lose energy except for radiation
which you know comes right off the
filament and conduction through the wire
leads and inside there this structure is
actually very tiny so the amount of heat
loss through conduction is quite low so
when you pull all the air out of here
this thing gets quite hot and the
thermocouple will respond with about 10
to 15 millivolts of voltage there but
when this thing is full of air the air
does such a good job of conducting heat
away from the heater the thermocouple is
putting out barely a millivolt maybe
like you know half a millivolt or
something like that so they're not
particularly accurate vacuum gauges what
they're good for is telling you if
you're able to turn on your high vacuum
device so after your mechanical pump is
done doing most of its work you'll be in
the range that this can measure and then
you can turn on your high vacuum pump
what's neat about this circuit is how
amazingly simple it can actually be if
you were to design something like this
with modern electronics you know you'd
use a constant current source and a
couple of op amps and maybe a you know
microcontroller to run the whole show
but this box is actually very very
simple it's really just a power source
that's unregulated with a variable
resistor to control how much current
goes through the heater and the
thermocouple input is connected right up
to the to the meter here so right now I
have it connected just to this
thermocouple laying on the gauge and
it's not even plugged in right now
and if I heat up the thermocouple a
little bit you can see the gauge is
starting to come up so the interesting
thing about thermocouples is that
they're very low impedance output so
even though the signal is only you know
0 to 10 millivolts in this case there's
enough drive current to actually push
the meter up all by itself and if I cool
this back down you can see the meter
starting to come back down so let's open
this up and take a look at what's inside
it's fun to take apart electronics that
are this old because the style of doing
things was so different back then
they've got this ganged up a knob here
and the back selector this one actually
switches 110 volt AC to the entire
circuit and believe it or not this red
wire here it's unplugged right now by
the way this red wire here is actually
carrying five current and when you turn
the switch on it passes the current to
here which energizes the transformer so
the amount of safety distance between
this grounded post which goes right to
the front panel and this hot lead is
just ridiculously small but any case
when you finally get the thing on and
don't electrocute yourself the AC
current from the transformer flows
directly through this pot and then out
the side of the unit to power up the
heaters in the in the vacuum sensors the
return path is through this 30 ohm
resistor so the 6.5 volt source Plus
this pot and the 30 ohm resistor pretty
much set how much current is actually
going to flow through the heaters really
curiously someone has been in here
before and they wired both this this
device has two ports here because it's
meant to be able to handle two sensors
and the front panel has a knob to select
which sensor you want however strangely
someone wired the two heaters in
parallel so I don't really see how that
was ever going to work maybe that's why
there's someone through this meter out
but obviously if you put the two heaters
in parallel there the the current flow
through through them is not going to be
correct so I imagine when this device
was originally set up it came with like
a bypass plug there aren't enough
positions on this knob for it to switch
the current to both sensors they
has to flow through both for this to
work the return path is dead simple
there the other two sections of this
knob just switched the return from the
thermocouple directly into that analog
meter that's why it works with with the
power even switched off the designers of
this circuit knew that you would have to
have some way of setting the actual
current because it is unregulated in
fact so you'd have to have some way of
checking it and then adjusting it so the
rest of this circuit which I've drawn
here is just a it all it is a voltmeter
really and they're just measuring the
voltmeter they're the voltage across
this 30 ohm resistor and they included a
diode and a capacitor so that you could
actually read the value on that meter
since they're feeding AC current through
the sensor heaters they had to rectify
it and smooth it out with a cap so that
you could actually read the value on
that meter which which needs to have a
DC value the meter itself has an
impedance or at least the DC resistance
of about 26 ohms so I you know kind of
quickly went through the numbers and
everything seems to check out it okay
hope that was interesting see you next
time byeonline income site, best online earning, money online, earn money from home, earn dollars online, earn money online, earn money online 2019, earn money online by typing pages, earn money online daily, online work at home and earn money, online earning, earn money online free, online money earning sites, earn real money online, e commerce ideas to make money, easiest way to earn money online, best way to earn money online, make money from home, make money online free, money making sites, earn from home, real ways to make money from home, online income ideas, make real money online, top online earning websites, earn money online fast, top 10 money earning websites, earn money without investment, earn money online for students, best sites to earn money, make money online 2019, earn easy money online, earn money online instantly, real money earning sites, make money from home online, make money online fast, earn dollar online free, online earning ways, best online money earning, earn money online with surejob, make money online, online money making sites, best sites to make money online, earn money online paypal, online earn money by typing, best survey sites, ways to earn money online, ways to earn money from home, make money online without investment, make money online surveys,
s
Posts
No comments:
Post a Comment