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Hi Andy
From your questions:
> 1) Does the selected resistor actually impact the amplification or is this more of a bookkeeping feature?

The selected resistor will only impact the amplification if you have more than one resistor to chose from (providing you have switchable resistors i.e. Delta Advantage, XP, 253 or V). If you only have one set of resistors on an amplifier board then the impact of choosing the wrong one will be in the calculations for actual ratios (13C/12C) and At%; in other words any calculation that involves actual quantification.  For the delta calculation the error is on both the sample and the reference and thus cancels itself out.

> 2) Is the first resistor always default and the second resistor only used
when "high amplification" is selected, or does the instrument know which resistor has a higher amplification?

No, you have to know what is on your amplifier board for any given cup. Also you have to be careful on complex cup configurations (i.e. atmospheric gases like O2, Ar, N2, CO2 (8+ cups).  The cup and the amplifier channel may not correspond.  For example cup 5 may be on channel 7 due to the physical geometry of the wiring inside the collector array to the flange feedthrough.  Normally you would have received a cup configuration sheet that shows this relationship with your instrument.  If you have a simple configuration then the cups and channels match. If you want to have two gain resistor/capacitor pairs on a board then you have to create a second gas configuration for the same gas with the High amplification relay set to ON when you want the higher signal; for example on a standard Delta configuration where cups 1 and 8 are for H2:
Configuration name    cup1  cup2     cup3      cup4        cup 8
- CO2 normal               3E8(44)  3E10(45) 1E11(46)   
- CO2 High                 1E9(44)  1E11(45) 1E11(46)
Here cups 2 and 3 have double resistors/capacitor pairs.  Note that the resistor choice is not "one OR the other" but "one AND the other".  To set them properly you would highlight the mass of one of the configurations (CO2 Normal, mass 44 or 45) then right click on the mass and chose the channel (2 or 3). You would then set the relay ON for the high resistor and OFF for the two of them together (Kirchhoff's rule of parallel resistors) which is a slightly smaller value for the smaller resistor.  Thus 1E9 is as is, but 3E8 becomes a 2.31E8 ohm resistor in parallel with the 1E9 resistor.

When you chose the "CO2 high" gas configuration you will see a black triangle next to the masses 44 and 45 on ion beam readout.

Regards
Gilles



Gilles St-Jean
Chercheur / Research Scientist
Université d'Ottawa / University of Ottawa
Sciences de la Terre / Earth Sciences
140 Louis Pasteur, Ottawa, Ontario, Canada K1N 6N5
Tel: 1-613-562-5800 xt 6830 (Bureau / Office)
                    xt 6839 (Bureau / Office Lab)
                    xt 6836 (IRMS lab)
Téléc. / Fax: 1-613-562-5192
Courriel / E-mail: [log in to unmask]
Toile / Web: www.isotope.uottawa.ca


> -----Message d'origine-----
> De : Stable Isotope Geochemistry [mailto:[log in to unmask]] De la
> part de Andrew Schauer
> Envoyé : 24 janvier 2008 14:38
> À : [log in to unmask]
> Objet : [ISOGEOCHEM] Resistor Settings in IsoDat Configurator
> 
> When installing IsoDat, after one chooses the instrument, one must then
> indicate which cups are installed, what cup should have peak centering
> capacity, and what the resistors are associated with each of those cups.
> One can also go back and change these resistor settings using
> Configurator.
> 
> 1) Does the selected resistor actually impact the amplification or is this
> more of a bookkeeping feature?
> 
> 2) Is the first resistor always default and the second resistor only used
> when "high amplification" is selected, or does the instrument know which
> resistor has a higher amplification?
> 
> Thank you!
> 
> andy
> 
> 
> Andrew Schauer
> ISOLAB
> Department of Earth and Space Sciences
> University of Washington
> Seattle, WA 98195
> 206.543.6327
> [log in to unmask]