Diamond DA 40 Flight Manual page 201

DA 40 AFM
b) Long Range Tank:
Read the fuel quantity indicated on the fuel quantity indicators.
At an indication of 16 US gal the amount of auxiliary fuel can
be determined by switching the AUX FUEL QTY switch to the
respective position (LH or RH). The indicated auxiliary fuel
quantity is added to the 16 US gal.
An auxiliary fuel quantity of less than 3 US gal cannot be
indicated by the system. In this case the quantity must be
determined by means of the fuel quantity measuring device
(see Section 7.10 - FUEL SYSTEM).
The correct indication of the fuel quantity takes 2 minutes after
actuation of the switch.
Doc. # 6.01.01-E
NOTE
CAUTION
Rev. 8 01-Dec-2010
Mass & Balance
Page 6 - 7