[time-nuts] Need some wisdom from the cesium beam tube gurus out there
kb8tq at n1k.org
Fri Nov 11 09:50:52 EST 2016
Even *with* all the fancy stuff in my Fluke thermometers … they still are only rated for
a bit worse than 0.1 C. When I send them in for calibration, the thermometer generally
comes back “calibrated fine”. The thermocouple I send in with them often comes back
with a note about “you need to buy a real thermocouple …”. On a simple lash up, you
would use a thermocouple that is lying around as your cold junction. If you get yours
from eBay (like I do) … who knows what you have.
Lots of gotcha’s.
> On Nov 11, 2016, at 9:16 AM, Scott Stobbe <scott.j.stobbe at gmail.com> wrote:
> If you want sub degree precision, you will need to make your connections to
> dissimilar metals on an isothermal boundary, a terminal block is better
> than clips in free air.
> On Fri, Nov 11, 2016 at 8:28 AM, Bob Camp <kb8tq at n1k.org> wrote:
>>> On Nov 11, 2016, at 8:02 AM, jimlux <jimlux at earthlink.net> wrote:
>>> On 11/10/16 10:28 PM, Mike Millen wrote:
>>>> It would work as well if you used a pair of regular copper wires to
>>>> connect the meter to the thermocouple...
>>>> The junctions created by all the new connections will cancel out.
>>> as long as the temperatures are "exactly" the same,
>>> (Seebeck coefficient varies with temperature)
>>> and the two metals at the junctions are the same,
>>> (ditto, but the curves are different for different materials)
>>> and the mechanical configuration is the same
>>> (current density also affects it)
>> The gotcha is that few of us weld copper directly to the thermocouple
>> leads. The far more
>> common approach is to grab clip leads. At least around here, the clips on
>> the leads are
>> not made of copper. They are some sort of (badly worn) plating over
>> (oxidized) base
>> I grab a “copper wire” clip lead and hook up to the thermocouple. There
>> isn’t a lot of
>> delta T in most bench situations. In this case you have a heated gizmo
>> warming things up ….
>> Who knows what the delta T may be or how small the contact area actually
>> Simple answer: Don’t trust the first number you get. Try it a couple of
>> times with *different*
>> leads. Make sure you do indeed get within a degree or three on each of
>> them. Depending on
>> how you have your cold junction set up, that may also need the same
>>> For run of the mill "measure to 1 degree at room temperature" you can
>> probably make that assumption.
>>> But if you're looking for precision, you need to take this stuff into
>> account (that's what "cold junction compensation" is all about.. )
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