[RASMB] Re: An early Christmas puzzle

[Arthur Rowe]

Hi John

Many thanks for your interesting thoughts. You should certainly know more
than most of us on the topic of temperature control.

I am sure you are correct to emphasise the distinction between the level of
control attainable and the absolute accuracy of what is measured by your
detector. As the detector in the instruments does pick up signal - when the
vacuum is down - more or less in the relevant place (rotor, more or less
where the cell is), and as that signal is steady to around ±0.1º, maybe we
can have a bit of confidence that whatever is being controlled is controlled
pretty steadily and reproducibly (see my mail to Marc Lewis). What we do not
know is just what systematic error is involved.

Anyone from Beckman want to comment on this? What is the basis for that ±0.5
degree figure, for example?

Kind regards

Arthur



Arthur Rowe <arthur.rowe at nottingham.ac.uk> was asking a question about
temperature control in his apparatus.

I can't pretend to know about AUC- I joined the list as a lurker to try to
understand a bit about what some of our customers do with our software.
However, I did  spend about 25 years in a lab, some of that time spent
using temperature controllers.

1. The temperature controller's spec (unless there is something to say
otherwise) tells you how closely the controller will control the measured
temperature. It does not include things like calibration error in your
thermocouple (or RTD or thermistor, or whatever).

2. In my experience, the largest source of error in temperature control is
not in the controller, but in temperature gradients. The temperature may be
known to within 0.5 degree where it is being measured, but what is the
temperature at the ends of the sample? (Hopefully the sensor is placed near
the middle of the sample chamber)

>Given the type of error distribution expected, can we really accept that
>it is better, using 2 instruments, to get 2 estimates for an s value that
>dis-agree than 2 which agree? Or is this believing in Santa Claus?

I think it's Santa Claus. As I said, you know that the controller is
keeping the measured temperature within a half degree, but you don't know
much about other sources of error.

I will now remove myself from an argument where I don't really have
expertise :)

Regards,
John Weeks

WaveMetrics, Inc.
Phone (503) 620-3001
Fax   (503) 620-6754
email   support at WaveMetrics.com

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