I measured the AC signal to estimate the noise in the following conditions:

No sample installed ;

Chopper frequency: 478 Hz ;

Lock-in time costant: 100 ms ;

Tuned to maximum DC signal ;

Front door closed ;

Motors moving but time wait 500 ms ;

Measure are taken in 3 different situations

 - Pump open at 30mW --> figure empty20150612.jpg ;

 - Pump  closed --> figure emptyclosed20150612.jpg ;

 - My voice singing around 478 Hz like here --> figure manuelvoice478hz.jpg ;

I measured 60 microV with silent, and 2.2 mV with acustic noise at chopper frequency.

The manual  says, at 380 Hz and with motors not moving, the noise is between 5 and 25 microV.

I will make measurement for many chopper frequencies and look for the minimum.