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[IP] Re: insulin-pumpers-digest V10 #868

On Mon, 28 Apr 2008 11:33:39 -0700, you wrote:

>> I could never understand this either. If I fill it with 300 units and prime
 >>with 7 units why does it show 286 units or 280 units. I called MM many times
 >>they were not concerned and tried in a very confusing way to explain this to
>>but it never made sense. It should be what it should be but it is not.

 It's pretty simple, really. The reservoir holds a bit more than 300 units. Some
of it is a small reserve, perhaps 10 or 15 units in my experience (possibly
 varies from pump to pump) that is still there even when the pump says it's
And it also varies because the end stop to which you pull the plunger out when
filling it's totally exact.  It's not hard to imagine that you might fill
slightly past, or not quite to,  the measurement the pump will think is 300
 units. So if you think you filled 300 units but actually put in 290, then prime
with 10, the result won't add up to 300.  Also, it's probable that the pump's
 detection of the starting point to count from may be a little hazy. It's got to
screw it's internal plunger into the reservoir's plunger, detecting from back
pressure/motor load when it's seated, and it starts counting units from there.
Probably this is not the most exact detection, and could be off a little.  Once
it's actually primed, by then the pump knows the exact position of the plunger,
so from there on, it knows where it is, and can count consistantly.  But the
 initial count of how many units it primed might be inexact, as might your
fill level you give it to start with.   

 The bottom line is you can reasonably trust the ongoing count after it's
but remember that part of the reason to prime the pump is not just to fill the
tubing, but also to make sure the pump's plunger is fully seated in the
reservoir, slack is taken out of the system, and the pump now knows where it's
counting from.  The priming operation is removing the slack and innacuracies in
the system, and because there ARE such inaccuracies, the initial math might not
be what you expect.  To eliminate all those innacuracies right from the start
would require more complex engineering in the pump itself, as well as, most
likely, more precise manufacturing of the reservoirs.

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