[RASMB] Re: XL-A/I temp control
John Correia
jcorreia at biochem.umsmed.edu
Fri Dec 10 13:49:28 PST 2004
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
----------------------------------------------------------------------------------
The older archived RASMB emails can be found at:
http://rasmb-email.bbri.org/rasmb_archives
and current archives at
http://rasmb-email.bbri.org/pipermail/rasmb/
Search All the Archives at:
http://rasmb-email.bbri.org/rasmb_search.html
----------------------------------------------------------------------------------
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.
_______________________________________________
RASMB mailing list
RASMB at rasmb-email.bbri.org
http://rasmb-email.bbri.org/mailman/listinfo/rasmb
This message has been scanned but we cannot guarantee that it and any attachments are free from viruses or other damaging content: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://list.rasmb.org/pipermail/rasmb-rasmb.org/attachments/20041210/a72c9b40/attachment.htm>
More information about the RASMB
mailing list