An ink jet printer comprising a replaceable cartridge having a supply of ink and a controller in operative communication with the cartridge. The controller keeps an estimate of an amount of ink remaining in the cartridge and provides a signal indicating that the cartridge is to be weighed in response...http://www.google.com.au/patents/US6024429?utm_source=gb-gplus-sharePatent US6024429 - Mailing machine including ink jet printing having ink availability checking
