[RASMB] density meter accuracy/precision

[Jo Butler]

Dear Steve,

Christine is of course absolutely correct and reminded us of an 
important point.  In fact, we also use a thermostatting bath rated to a 
constancy of temperature of 0.01°C.  The precision of setting is more 
like 0.1°C, but the really important point is that we always make the 
measurements of the buffer and solution(s) together, so that the 
constancy of temperature means that their temperatures will be very 
close.  This is important because of the great variation of water 
density with temperature in the ranges of interest to most biologists.
One practical point is that I have found that when one loads a new 
sample, you can use the Paar to watch it reach temperature equilibrium 
by simply starting to time counts.  At first the time varies rapidly, 
but then (usually within a few minutes) it slows and settles to the 
point where there is essentially random variation in the last decimal 
point.  Once this happens, one can take a series of readings to give a 
mean and SD for the value.

Best wishes,

Jo

Christine Wandrey wrote:
> Dear Steve,
>
> a practical comment. The description of the instrument says a 
> precision of 10-6. But practically it is very difficult to get. You 
> need then  an excellent instrument calibration and high temperature 
> stability, less than 0.01°C temperature fluctuation. The practical 
> case is a precision of 10-5, but this you really need as mentioned by Jo.
> Best,
> Christine
> On Feb 20, 2008, at 4:18 PM, Jo Butler wrote:
>
>> Dear Steve,
>>
>> If all you want is a solvent density, then you really only need (at 
>> most) 3 decimal places and the DMA5000 is over the top, since you are 
>> most unlikely to know the partial specific volume to this precision 
>> (since the product v-bar*rho will only have the precision of the 
>> least precise term).
>> However, if you think that you might ever want to get into the 
>> business of measuring density increments, and hence being able to 
>> calculate meaningful apparent partial specific volumes, then all the 
>> precision you can get in density is wanted, as one is looking at 
>> density differences between solution and solvent where the solute 
>> concentration is likely to be only mg/ml (i.e. 10–3), so a precision 
>> of 10–6 is by no means too high.
>>
>> Good luck,
>>
>> Jo
>>
>> Stephen Eyles wrote:
>>> Hi AUCers.
>>>
>>> We are contemplating purchasing a densitometer to more accurately 
>>> determine densities of our buffer solutions for AUC.
>>> What precision/accuracy is really necessary? I know the de facto 
>>> standard instrument seems to be the Anton Paar DMA5000 but is this 
>>> overkill? Aside the significant extra expense, is there a real 
>>> benefit to 6-decimal place measurement of density or could we do a 
>>> reasonable job with a hand-held instrument? Weighing known volumes 
>>> on a balance clearly isn't cutting the mustard!
>>>
>>> Thanks for any advice and helpful tips.
>>>
>>> Steve
>>>
>>> ------------------------------------------------
>>> Dr Stephen J. Eyles
>>> University of Massachusetts - Amherst
>>> Amherst, MA 01003, USA
>>> eyles at polysci.umass.edu <mailto:eyles at polysci.umass.edu>
>>> Tel: (413) 577-1528
>>> Fax: (413) 545-0082
>>> ------------------------------------------------
>>>
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>>
>> -- 
>> Dr P.J.G. Butler,
>> MRC Laboratory of Molecular Biology,
>> Hills Road, Cambridge, CB2 0QH, UK.
>> Tel. +44 (0)1223 402296
>>
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>
> --------------------------------------------------------------------------------
> PD Dr. habil. Christine Wandrey, MER
> Ecole Polytechnique Fédérale de Lausanne (EPFL)
> Laboratoire de Médecine Régénérative et de Pharmacobiologie (LMRP)
> http://people.epfl.ch/christine.wandrey
>
> tél.: (+41) 21 693 9661
> fax: (+41) 21 693 9685
> email: christine.wandrey at epfl.ch <mailto:christine.wandrey at epfl.ch>
>
> Postal address:
> EPFL-SV-IBI-LMRP
> AAB 042, station 15
> CH-1015 Lausanne
> Switzerland
>
>
>
>
>
>
>

-- 
Dr P.J.G. Butler,
MRC Laboratory of Molecular Biology,
Hills Road, Cambridge, CB2 0QH, UK.
Tel. +44 (0)1223 402296