Question
Why is there a measured charge with beam-off some time after the background measurement?
Answer
Zero drift
For reference class electrometers there is always a "zero drift". It is defined in the relevant norm IEC 60731 and must be ≤ 0.5% according to the IEC norm. Our DOSE-X always achieves ≤ 0.25%.
Details
When the DOSE-X is being turned on, the low sensitivity range is selected, and the measurement time for the background measurement and the timer measurement is set to 1 minute.
After turning it on and performing the background measurement and then measure the charge for 1 minute, you will get approximately 150 pC. If you repeat the charge measurement an hour later, it will again be around 150 pC. In some cases, however, it may be also higher.
The smallest value that one can measure in the low sensitivity range is 2.4 nA. According to the specification the maximum allowed zero drift current would be 6 pA (0.25% of 2.4 nA). A charge of 150 pC over 60 seconds results in an average current of 2.5 pA which is clearly within specifications.
So, when measuring typical values of e.g. 20 nA in low sensitivity range, the zero drift results in a measurement error of ca. 0.1%.
Recommendation
It is always recommended to perform a background measurement directly before the dose or charge measurement itself.