The ink levels go down because you have used ink to print documents. In order to refill the ink, you will need to buy a new cartridge then replace the old one.
A:
When it opens, there is a row of three large square icons. The center is labeled Help and Support. Click on that and you will find a Topic Heading called Tools/Utilities. Under that heading find Printer Toolbox and click on that. A box will open with four tabs at the top. Click on the one labeled Estimated Ink Levels and there you will find a graphic representation of approximate ink levels.
Cartridges such as HP 96 Black Ink and HP 97 Tricolor are the best ink cartridge for the HP Deskjet 6940. If one wants a less expensive cartridge, then it is best to use HP 99 Photo Ink Cartridge.
Deskjet 5740, 6540, 6840 & 6940
There are a wide variety of places where one can purchase a HP Deskjet 940C ink cartridge. The easiest places to buy the cartridge is from the official website or a local electronics store such as Best Buy.
Hp deskjet1360
The HP Deskjet 6940 series printer may be purchased at major electronics dealers as well as at the online sites. In addition the printer is available on Amazon.
The power consumption rate of the HP DeskJet 6940 is 50 watts. To put things into perspective, printers may consume anywhere from 20 to 60 watts on average.
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HP Photosmart printers use the line of HP 564 ink. Any of the Photosmart printers can be fitted with a HP 564 Black Ink Cartridge, HP 564 Photo Ink Cartridge, HP 564 Cyan Ink Cartridge, HP 564 Magenta Ink Cartridge, HP 564 Yellow Ink Cartridge, HP 564XL Black Ink Cartridge, HP 564XL Photo Ink Cartridge, HP 564XL Cyan Ink Cartridge, HP 564XL Magenta Ink Cartridge, or a HP 564XL Yellow Ink Cartridge.
No, an HP 920 ink cartridge can't substitute for an HP 934 ink cartridge. They are completely different.
One can purchase HP 94 and HP 95 ink at the HP website under the "Purchase" tab. One can also buy HP 94 and HP 95 ink as well as a wide variety of ink at the nearest HP ink refilling station and outlet store.