The 9103 USB picoammeter is often equipped with a +90 V bias option which improves the accuracy of electron and ion current measurements (by reducing the number of low energy secondary electrons that are generated by the beam from leaving the target).
The +90 V bias comprises two 45V batteries in series which are located inside the 9103 chassis. This blog post will explain how to test and replace the batteries in a 9103 picoammeter.
1. Connect a DVM (digital volt meter) to the 9103 Input BNC connector. A BNC to double banana cable works well. Set the DVM to DC volts.
2. In Actuel (the 9103 software), select the input Grounded and bias On.
3. Sample the current.
4. When the bias ON is checked there will be about +90V DC on the input of the 9103.
5 .The input impedance of most DVMs when measuring DC voltage is 10 meg ohms. The two 45 volt batteries should total 90 to 95V DC. The bias voltage divided by the input impedance of the DVM will equal the current. In this case the voltage of the two 45 volt batteries totaled 94V and the current was 9.414 uA.
6. It is recommended that the bias batteries be tested every 6 months and replaced when the voltage drops below 80 volts. It is normal for the batteries to wear out over time and with use. Once the bias voltage drops to less than 50 V the effectiveness of preventing secondary electron emission is greatly reduced, which in turn reduces the accuracy of electron and ion beam current measurements.
The RBD part number for the 45 V battery is BAT-45-213.
Whether you have an RBD 9103 USB picoammeter or an older Keithley with a PHI model 78 bias box, you should test the batteries as part of your preventive maintenance procedure and replace them as needed.
To replace the batteries in a 9103:
Unplug the 9103 USB power and input cables.
Using the Torx wrench that was included with the 9103, remove the screws from the front and rear 9103 chassis covers.
Slide the board out from the front of the 9103. You will need to rotate the back cover to feed it in. The back cover has a ground wire that is attached to the 9103 board. Also note which groove the 9103 board is in as you will need to put it back in the same groove.
Remove the battery support bracket (white plastic).
Carefully remove the old batteries.
Install the new batteries.You may need to adjust the contacts on the batteries to get them to fit onto the board snaps more easily.
Reattach the battery support bracket.
Carefully slide the back cover and board back into the chassis. Make sure that you put the board back in the same groove that it came out of. If the front cover does not line up with the chassis then you are not in the correct groove.
Reattach the screws to the front and back covers. Do not over tighten the screws!
Once you have installed the new batteries, test the voltage. You should have 90 to 95 Volts.
