[RASMB] Re: XL-A/I temp control

Arthur Rowe arthur.rowe at nottingham.ac.uk
Fri Dec 10 09:41:01 PST 2004


Hi Everyone {this is a second (now 3rd!) try at getting this mail out -
first attempt got lost in cyber-space, it seems}

Mei-Ling Chien gives us a very useful review of the nature of the
temperature measurement and control system in the XL-I/A instrument.
However, I do not think that this fully addresses the problems which one has
in determining what the absolute temperature of one's sample actually is
when it is going round in the rotor at speed.

It is, of course, only a worry to those (very limited) number of people for
whom an absolute s value is of importance, normally for hydrodynamic
modelling purposes (although formulation issues should not be forgotten).
When I raised this issue on RASMB a week or so back, my concern was not  "to
ensure their operation within the published specification".    . I am trying
to get the accuracy of the temperature read-out to be close to the precision
of which the system is capable. I have no evidence at all to suggest that
the accuracy is outside the quoted spec of 0.5º. It is just that I - in my
greedy way - want 0.1º.

Even the method mentioned (equilibrate for 3 hours - under vacuum - and then
check  "with a calibrated external temperature sensing device to verify
accuracy"   is not unambiguous in what it will yield. Quite apart from
matters such as adiabatic effects when one releases the vacuum to use an
"external temperature sensing device", can one be sure that the thermal
emissivity of a spinning rotor surface, averaged over everything that is
passing by, is equal to that of a piece of the rotor surface 'seen' in a
stationary rotor? 

None of these are new concerns, and I certainly lay no claim to the IPRs! I
imagine, from what Mei-Ling Chien has communicated, that we at least know
clearly that the ±0.5º refers to the accuracy of the temperature as measured
by the defined procedure. Walter Stafford's colorimetric method (Stafford &
Liu) did not suggest the presence of errors outside the stated accuracy
limit, and is surely a valid way to approach the absolute temperature issue.
But is is pretty tedious to use as a procedure, and certainly as a routine
QA method is not feasible.

As an approach to the size of the problem, would there be support for
Borries Demeler's suggestion (a single sample to be circulated and multiple
users on multiple machines to report an s value under defined conditions)?
After all, the NCMH + Borries's Lab gives us 6 machines for starters.

Any way, we here keep trying here to locate the holy grail - a simple,
cheap, effective method for determining the in-cell temperature to ±0.1º

Regards to all (and many thanks to Mei-Ling Chien)

Arthur

-- 
*************************
Arthur Rowe
Lab at Sutton Bonington
tel: +44 115 951 6156
fax: +44 115 951 6157
*************************

From: mchien at beckman.com
Date: Fri, 3 Dec 2004 10:11:46 -0800
To: "'rasmb at rasmb-email.bbri.org'" <rasmb at server1.bbri.org>
Subject: [RASMB] Re: XL-A/I temp control


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Hi All,

Below is response regarding XL-A/I temperature control from our Technical
Support Department.

******************************************************
Mei-Ling Chien PhD
Staff Development Scientist, Centrifugation
Platform & Automation Business Center
Beckman Coulter Inc.

mchien at beckman.com
(650) 859-1948
******************************************************


The basis for temperature control specifications were instrument design
specifications for temperature control and dynamic system testing during the
prototype phase of the product.

If there is a discrepancy in temperature control and measurement between
instruments of the same design then a dynamic calibration check should be
performed on both instruments to ensure their operation within the published
specification.

First the physical condition of components within the temperature control
and
vacuum system should be verified through inspection.  Then an electronic
calibration for temperature control and vacuum can be performed.  Lastly a
dynamic test or rotor dunk test is performed (rotor should be precooled or
preheated to avoid testing delay).  The rotor and its contents must be
allowed
to equilibrate for up to 3 hours or more.  When set temperature equals
indicated
temperature at the instrument interface, the rotor temperature is then
checked
with a calibrated external temperature sensing device to verify accuracy.

If the checks fall out of specification then appropriate troubleshooting is
required to isolate the electronic or mechanical fault in the temperature
control or vacuum system.  Once the fault is corrected the temperature
control
checks are performed again.

Quote from Bob Giebeler, Analytical Ultracentrifugation in Biochemistry
  and Polymer Science, 1992,16-25 for the Optima XLA/I.
"Temperature  control is considerably more stable, provides more rapid
cool-down
and  heat-up  rates,  is  thermally more uniform, and has equivalent
accuracy as
compared  to previous models including the Model E.  This control system
uses an
isothermal radiometer temperature-sensing system to sense the temperature of
the
rotor  that  is  emissivity-independent  ad  view  factor-corrected in
software.
Heating  and cooling of the rotor are accomplished by the refrigeration can
that
surrounds  the  rotor,  which  is  in  turn  heated and cooled by
thermoelectric
modules.   This environment is very isothermal, and at equilibrium,
irrespective
of  speed  or  temperature,  rotor temperature is within about one degree of
the
refrigeration can temperature.


The  control  system  that  regulates  rotor  temperature,  as  monitored by
the
radiometer,  is  highly  software-intensive.   This  software encompasses
triple
proportional-integral-differential  control algorithms and
proportional-integral
smoothing  algorithms.  In addition, radiometer view factors are measured
during
rotor   cool-down  to  allow  more  rapid  rotor  cool-down  and  more
accurate
temperature  monitoring  during  cool-down.  While at equilibrium,
refrigeration
can   temperature   fluctuation   does  not  typically  exceed  +0.5C,  and
the
corresponding rotor temperature fluctuation is less than +0.2C.











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