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#1: How does a CIS work? Author: bernado,

Posted: Sat Nov 18, 2006 11:28 am
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Hi everyone,

after reading through many of your posts, I am unable to find out how a CIS works.
I just bought a CIS (china made), it has 2 chambers for each ink tank, after I follow the instruction to suck out the air, and close the cap, the ink levels in the 2 chambers are not the same. Must they be the same?

Please explain to me how a CIS works. Like what is the pressure, what is an air filter for? how high or low my CIS have to be relative to the printer.

This is the picture of the CIS I got.

Thank you

#2: Author: Peter_G, Location: Hertfordshire, UK

Posted: Sun Nov 19, 2006 4:36 pm
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I'll try to keep this as short as possible, so here goes.

Obviously the aim of any CISS is to keep the cartridges in the printer continuously supplied with ink from the reservoirs located outside the printer so that these can be topped up as required, without the need to remove cartridges with the inconvenience and waste of ink that usually accompanies removal. The cartridges are sealed so that any ink used in printing is drawn from the reservoirs, leaving the amount in the cartridges unchanged.

A major problem faced by all CISS is to keep the hydrostatic head of ink (i.e. the difference in height between that ink surface open to the atmosphere in the reervoirs and the outlet nozzles of the print head) at a low and constant level. This is often achieved by sealing a small diameter tube, open to the atmosphere at one end, into the main ink reservoir, and arranging the other open end to be just above the bottom of the reservoir. The main reservoir is closed to atmosphere by an airtight stopper (critical that this is really airtight).

As ink is used from the reservoir, the only way in for air to replace the ink used is down this sealed in tube, so the ink level will fall until it reaches the bottom of the tube, at which point it will remain constant whilst the ink in the outside chamber is used and replaced by air bubbling from the bottom of the sealed in tube. More on this later!!.

The ink flowing from the reservoir to the cartridges does so along a group of thin flexible tubes linking the two. Ideally there should be no air bubbles in these tubes, as this will alter the hydrostatic pressure seen by the ink nozzles. In practice small air bubbles cause minimal change, and in any case will end up in the sealed cartridge where they cease to be a problem.

The difference in height (ie hydrostatic pressure) between the printhead ink nozzles and the bottom of the sealed in tube needs to be kept quite small, and can even be made negative. If the head (height difference) of ink is too large, then there is a risk that ink will flow unchecked from the reservoir and out of the print nozzles. This is bad news!!. It can mean empty reservoirs and a full printer!!

Normally the head should be kept to a maximum of 2 or 3 cms, as this is what OEM cartridges deliver. As you might have guessed this low level only occurs in a CISS system once sufficient ink has been used to lower the level in the sealed in tube inside the reservoir. To avoid having to use this amount of ink before achieving a correct head, and to get the system correctly set up once everthing is fitted, filled. sealed and ready to go, it is advisable to carefully and slowly tilt the whole reservoir group so that the open bottoms of the sealed in tubes are just above the level of the ink surface, this will ensure that the correct amount of air is trapped in the main part of the reservoir and will leave the ink level in the tube very close to the bottom end (where it ishould be). Some mauufacturers have coined the term "resetting" for this procedure.

Unfortunately this is not the end of the story, because as atmospheric pressure varies, so will the volume of the air trapped in the sealed reservoir. Rising pressure is OK, as air merely bubbles into the sealed reservoir from the tube, but if the pressure falls then the air in the reservoir expands and the ink level in the sealed in tube will rise. This increases the head of ink, and could cause problems - so if the pressure falls significantly (tropical storm!!), think about "resetting" your reservoirs. You will need to do this after refilling the reservoirs of course, as you will be removing the airtight stoppers to gain access to the main reservoir chamber (don't overfill, as you won't then be able to "reset" the system).

Finally the air filters are just that - they prevent dust, dirt and even the odd small insect or spider from invading your ink. More to the point, apparently some fungi can thrive in ink, so hopefully the filters are suffuciently fine to intercept the spores.

Well that's it for the time being, do come back to me if I haven't made myself clear. Happy CISSing!!

#3: Author: AlienSteve, Location: Tacoma, WA USA

Posted: Wed Nov 22, 2006 11:02 pm
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Very good description!

#4: Author: bernado,

Posted: Tue Dec 05, 2006 2:07 pm
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Hi, thanks for the detailed explanation. Now I am wondering, why is the hydrostatic head is measured from the inksurface exposed to air. Does that mean that the pressure at the opening tube which leads to the cartridge, is equal to that of the inksurface exposed to air?
Kindly explain why the hydrostatic head is measured from the inksurface exposed to air.

I am confused because I am wondering why the height in the main (sealed) reservoir does not affect the CISS.

Thank you. Will wait for your explanation

#5: Author: Martin, Location: South Yorkshire, UK

Posted: Tue Dec 05, 2006 7:33 pm
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Hi Bernardo.. not sure I understand your question completely but the best way to demonstrate the effect is to:
- get a glass
- full a sink/bowl with water.
- lower the glass into the water upside down (so air is trapped in it) and then gently tip it so that some of the air is replaced with water
- then try lifting the glass out of the sink/bowl slowly..

You will notice the pressure you need to lift the glass increases as you lift the glass further up until it finally pops at the surface.

This is the effect that these type or reservoirs rely on.. Instead of pressure decreasing as ink decreases, the reservoirs maintain a constant pressure.

In truth I don't understand it all that well myself but my father showed me that when I was a kid so it helped. Smile

#6: Author: Peter_G, Location: Hertfordshire, UK

Posted: Tue Dec 05, 2006 9:58 pm
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The hydrostatic head is simply the vertical distance between the surface of the ink in the reservoir or cartridge and the exit nozzles of the print head. Because the nozzles are so small, the surface tension of the ink is sufficient to resist the pressure of the ink caused by the hydrostatic head, so that no ink flows out without being forced by a pressure wave from the piezo "crystal".

(Surface tension is that force which causes a liquid such as water to rise inside a small diameter or capillary tube dipped into a container. It will continue to rise until the weight of liquid in the tube is balanced by the forces due to surface tension. Both liquid surfaces are at atmospheric pressure.)

In the case of your print head, the hydrostatic pressure will make the ink bulge out below the nozzle rim, and in doing so will generate a force resisting the hydrostatic pressure of the column of ink above it. As the hydrostatic head increases, the ink will bulge more and more, until finally ink will flow continuously from the nozzle. Obviously the larger the bulge, the less is the excess pressure required to cause a drop of ink to be ejected when triggered by a pressure wave.

So in the interests of consistancy of printing, the pressure (hydrostatic head) needs to be kept as constant as possible at the nozzles, especially as the use of variable drop size technology relies on the "bulge" remaining constant (or nearly so) throughout the life of the cartridge.

When you consider all the things which can go wrong, it is a miracle that inkjet printers work as well as they do!!

Hope this helps - shout if not!!

#7: Author: hadryan,

Posted: Wed Jan 31, 2007 5:21 pm
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very good explanation but " a picture is equal 1000 words " .
Please atach some drawing ,picture to understand better

ContinuousInk.info -> CIS systems

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