GUARDED chamber configuration and UNGUARDED mode in 6517B (GUARD OFF)

Hello, 
I've been using the 6517B to measure the ionization current produced by a cavity ionization chamber. As most of us who do these type of measurements, in order to measure the ionization current generated by the cavity, we measure the electrical charge (in units of Coulomb) over a period of time (say for example 60 s). From the measured charge and time we can derive the current. 

Part B of Figure 2-12 of the 7th edition of the Keithley Low Level Handbook (https://download.tek.com/document/LowLevelHandbook_7Ed.pdf) shows a schematic of how a guarded chamber is connected for this type of measurement (see attached for figure). The schematic shows clearly the following: 
 

• The chamber’s guard electrode is connected to the INPUT LO of the Keithley triaxial connector  (Keithley 6517B),  and is connected to one end  of the Keithley electrometer 6517B internal Power supply (say for example at +300 V)
• The chamber’s collector electrode is connected to the INPUT HI of the Keithley triaxial connector  (Keithley 617 or 6517B), and because of how the feedback loop works, I believe that internally INPUT HI tries to be at nearly the same potential as INPUT LO (because of being the inputs to a differential amplifier with feedback loop). So in essence,  INPUT HI will also be (in this example) nearly at  +300 V.
• The chamber’s wall (outer shell) is connected to the other end of the internal power supply (so in this example would be Chassis GROUND)

All this is very clear. But here comes my question related to what appears to be the  use of GUARD and UNGUARDED terminology in the exact same application.    Although the configuration shown is for a guarded CHAMBER configuration, the Keithley electrometer 6517B is operated in the UNGUARDED mode.  This seems to me a little misleading, unless I am missing something.  I was wondering if someone can explain this apparent discrepancy in nomenclature.

The wiring shown in Fig 2-12 Part B is clearly a  a Guarded configuration.  But in order to achieve this configuration, the cavity chamber's guard electrode must be connected to the INPUT Lo of the Keithley electrometer. For this, the inner shell of the Triaxial Connector on the Keithley must be the Input Lo of the Keithley Pre-Amplifier. By design, the inner shell of the triax connector  of the 6517B electrometer is connected internally to the INPUT LO provided the electrometer is operated in the unguarded mode, that is, the GUARD is OFF (as described in the Keithley 6517B manual).   If the 6517B is operated in the Guarded mode, my understanding is that the inner shell of the triax connector is disconnected from the INPUT Lo in which case the configuration shown in Part B of Fig 2-12 can not be made. 

In closing below are my questions:
1)  why is it that when using a chamber guarded configuration as shown in Part B of Fig 2-12, the Keithley mode of operation is called unguarded. Wouldn't it be more appropriate to call this mode of operation GUARDED? 
2) Are my  general descriptions/interpretations  above  correct 
 
Thank you!
Ron



 

Your understanding of fixture and electrometer guard are correct. This is more closely related to misunderstanding related to the settings/configuration of the instrument itself and how it is identified in that nomenclature.
In the ideal sense, guard holds the low-impedance point in the circuit to nearly the same potential as the high-impedance input terminal.
There are two common implementations of GUARD, driven guard and 'feedback' guard inherent to the op amp input of the electrometer.
The 'guarded mode' of the Model 6517B, however, is designed for guarded voltage measurements and will define the inner shield to a driven guard, rather than the feedback guard.
The 'unguarded mode' of the Model 6517B is designed for more standard operations/measurements and will define the inner shield to Input LO to utilize feedback guard.
While you will get the inherent/feedback guard through the op amp input shown in both parts of the referenced figure, it is considered 'unguarded' from the 6517B perspective because it is using fixture/inherent feedback for providing that guard, instead of the driven guard.
This is likely the source of confusion related to our definitions in that figure and its implementation within the greater use of the term 'guard'.
This video demonstrates some of these differences in implementation of that inner shield as guard or input LO:
https://youtu.be/TBVeXlxNO-U

Your understanding is correct, but our nomenclature related to guarded vs unguarded is referring to a different application of the same principle, as describe.
The Low Level Handbook and instrument manual should provide additional details related to these modes and their specific use cases:
https://www.tek.com/en/manual/specialty-instruments/keithley-high-resistance-low-current-electr-0-electrometers