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

[Bob Camp]

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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