Is there a recommended procedure for storing the TDF Instrument during a period of non-use?

Yes, there is a recommended procedure for the TDF Instrument for periods of non-use in Appendix C of the Operator's Manual for instructions. Following this procedure will help maintain clear fluid lines.
 
The most current Operators Manual can be found at https://www.ankom.com/sites/default/files/document-files/TDF_Manual.pdf