[time-nuts] Need some wisdom from the cesium beam tube gurus out there

Scott Stobbe scott.j.stobbe at gmail.com
Fri Nov 11 09:16:11 EST 2016


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:

> Hi
>
> > 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
> material.
>
> 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
> is.
>
> 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
> treatment.
>
> Bob
>
>
> >
> >
> > 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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