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

Arthur Rowe arthur.rowe at nottingham.ac.uk
Thu Dec 16 07:18:00 PST 2004


Hi Everyone

I entirely agree that the temperature on machines needs to be calibrated.
Equally, I agree with Jack's  statement that "most users . . . never
calibrate the temperature". As to whether it is actually easy to do same
using the Stafford & Liu method, opinions might differ. We ourselves find
that the integration required to get the area under the curve is easy enough
to estimate with data from the spec, but less easy (baseline definition,
noisier values) with data from the XL-A.

The suggestion that a quick set of standard runs be undertaken on multiple
machines would serve to clarify as to whether it all really matters, at the
level of doing careful calibration - if one is talking at the precision (not
accuracy) level of the instrument. After all, with a (presumably
standardised) production line, one might have supposed that any errors at a
given temperature might have been pretty much the same from one instrument
to another. The sketchy data which we have to hand suggests that that is not
the case.

Measuring an s value of a given solute via an agreed procedure is about as
objective a procedure as one can define. Variations are easily transformed
into temperature uncertainties. If that is what we are looking for, then
this is the easiest way to find it. The numbers of instruments volunteered
to date (10) looks enough to give a pretty good idea of what the variation
level actually is - but any more would bolster the stats nicely . . . .

Regards to all

Arthur

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

From: "John Correia" <jcorreia at biochem.umsmed.edu>
Date: Fri, 10 Dec 2004 11:16:43 -0600
To: <mchien at beckman.com>, <arthur.rowe at nottingham.ac.uk>,
<rasmb at server1.bbri.org>
Subject: Re: [RASMB] Re: XL-A/I temp control


I suspect other will give technical responses about the ability of these
temperature controllers to function at the +/- 0.1 C level.  My take is
slightly different.
 
What matters is not the set point but knowing the actual temperature.
 
To my knowledge most users never use the Stafford & LUS method to calibrate
the temperature on their machine.  (can we vote on the RASMB in some way?)
It is no more tedious than waiting three hours before starting a run.  I
have checked the calibration on my machine twice, once in '93 when I got it,
and again in 1999 when a result caused me to question the temperature
accuracy.  A series of measurements up to 40 C and back down to 4 C (with
parallel measurements in a spec) takes about two days.  It does not take
three hours to go from 20 to 25 C, as verified by the stability of the area
under the CoCl2/ethanol curve.  At a setting of 4 C I get 3.6, at 20 I get
19.7, at 40 I get 39.8.  The values drifted by 0.1 C in 7 years.  So if we
mean by accuracy +/- 0.5 C of setting, my machine is within spec.  When I
fit data I use the actual temperature, not the set point. & I trust the
setting on the screen at low vacuum and hit start when it reports the set
temperature.
 
Comparisons between uncalibrated machines honestly make no sense to me.
Calibrate & be done with it.  (Arthur, if you do that in Nottingham & still
get different values of S between machines I would be concerned, & amazed.)
 
I also use the calibrated temperature values when I measure density in an
Anton Paar DMA 5000.  It has a peltier cell good between 0 - 80 C, so I dial
in 19.7 instead of 20 C and measure away.  I suspect the error from a
calculated density (viscosity & Vbar) is larger than (albeit coupled to) an
assumed temperature.
 
At this point the inquiring "student" should assume errors in various
parameters and propagate them into S or MW by the appropriate equations - my
favorite book for teaching this is Bevington, "Data Reduction and Error
Analysis for the Physical Sciences".
 
PS - why Beckman has never joined up with Anton Paar to bundle Density
Meters into XLA/XLI quotes amazes me.
 
PPSS - Walter claims to keep the original solution around, in a cell, from
the CoCl2 calibration work, and years later it still gives the same results.
Quick checking may not be as difficult as one might think.
 


-------------------------------------------------------------------
Dr. John J. "Jack" Correia
Department of Biochemistry
University of Mississippi Medical Center
2500 North State Street
Jackson, MS  39216
(601) 984-1522     
fax (601) 984-1501 
email address: jcorreia at biochem.umsmed.edu
homepage location: http://biochemistry.umc.edu/correia.html
dept homepage location:    http://biochemistry.umc.edu/
-------------------------------------------------------------------




>>> Arthur Rowe <arthur.rowe at nottingham.ac.uk> 12/10/04 08:17AM >>>
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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