 |
 |
 |
 |
All of a sudden, our FACStar+ needed a differential pressure of 2.0 to get
a reasonable event rate. At 0.7, it gave 0 events. Rates at various
pressures were quite reproducible, arguing against an intermittant
problem (e.g. an o-ring fragment in a line somewhere). For years before
this, it gave nice event rates at 0.5 or even lower. The differential
pressure is reported by an LED display on the back of which are two gas
pressure tubes, one for sheath and one for sample pressures. After about 3
hours of head scratching and diagnostic testing on two different days, and
two calls to B-D, I discovered that when both tubes were disconnected, it
still read 0.7. I found that by adjusting the blue potentiometer nearest
the corner of the LED circuit board 2 and 1/3 turns counterclockwise, I
could zero the LED display when there was zero differential. The
potentiometer next to this one seemed to be set at the edge of where it had
no effect; I left it where it had been. Now, the event rate is a
reasonable function of differential reading. I have no idea why the
differential LED meter circuit evidently suddenly took a jump. An earlier
test I did was to clamp the sheath line to the nozzle holder with a
hemostat, so that the only fluid coming out of the nozzle was from the
sample tube. I timed the drip rates and they slowed only about 25% between
differential 2.0 and 0 (before I adjusted the pot). I don't think this is
a fair reflection of differential pressure because there is no back
pressure from the sheath in the nozzle, but it did show that there were no
evident obstructions in the sample flow anywhere. Has anyone else ever
seen such behavior? Is there any way to calibrate the differential meter
slope (now that I have the intercept set reaonably)? (You may have guessed
that this intrument is not on service contract! :-)
/*- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Eric Martz, Professor of Immunology emartz@microbio.umass.edu
Dept Microbiology Voice: 413-545-2325 FAX: 413-545-1578
Morrill IVN 203, Box 35720, Univ Massachusetts, Amherst MA 01003-5720
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -*/
|
|
|
|