This is a good question…..

Mostly the level of ink shown on the screen is just an estimate. The only real accurate way to know the true level is with an optical sensor.

Canon’s older printers that used the BCI-3’s and BCI-6 range of cartridges detected the ink levels in this way – in the main this worked well – it was only let down if the sensor became dirty or obscured. Canon recently introduced their newer range of home and small office printers, that use the PGI-5bk and CLI-8bk/c/m/y range of cartridges. The new range of cartridges use almost the same casing’s as the older BCI ones, but have a chip on the cartridge itself which communicates with the printer. Between the printer and the chip they monitor or estimate the amount of ink left in the cartridge.

This new method is very similar to the way the ‘chipped’ Epson printer cartridges work. Epson introduced chipped cartridges 6 or 7 years ago, these are de-noted by their reference being prefixed with by a T, Eg the Epson C46 takes the T036 black and T037 colour cartridges. Basically the new cartridge chip has a counter in it – lets say it has 300 for an example – each time the printer prints a sheet of paper the counter ticks backwards. So you get your new printer and eagerly print off 20 sheets of homework, the counter on the chip has gone down to 280 and the graph on the computer screen reflects this use of ink by lowering the bar.

The printer manufacturers have worked out that on average a certain amount of ink will be used on a certain size of paper. It is quite accurate, but not foolproof and I think the manufacturer will always side with the cartridge being emptier than the reality. They have agreed that there is still ink left in many reportedly empty cartridges, but this is a safety feature to protect the printers head burning out. In most Epson and Canon printers the head is fixed in the printer and isn’t replaced each time a new ink tank is fitted, so if the printer runs dry and continues to print the excess heat in the head can burn it out, basically ruining the whole printer.

To sum up the levels of ink shown on your computer screen are not usually the actual levels, but an estimated amount. Also, problems can occur if the chip on the cartridge isn’t read properly by the printer – or the cached levels held by the computer disagree with the chip. Sometimes if this happens the printer’s software thinks that the cartridge has been refilled and blocks any further levels being reported and can even just say that there is a problem with the cartridge and not allow you to use the printer!