Section 3: C ontrols and Adjustm ents
Airmetrics
15
that was set using th e Flow Rate A djustme nt and w hen the F low Co ntrol Circu it
can no longer maintain the desired flow rate. The size of the increment is adjusted
with the Low Flow C oarse and Fine Adjustmen ts.
NOTE:
This light is o n when ever the sa mpler is c onnecte d to a pow er source but is
not operating.
L
OW
F
LOW
C
UTOFF
I
NDICATOR
If the Low Flow Threshold Indicator is lit for an extended period of time (several
seconds), the flow sensor will shut off the sampler's pump and turn on the Low
Flow Cutoff Indicator light. The red light will remain lit to alert the operator that
the sampling was aborted because air flow could not be maintained at the desired
rate. The p ump is tu rned off be cause the cut-point o f the PM size selectiv e inlet is
determined by the air flow rate through the inlet. For the inlet to have constant
particle size cut-point, it is necessary to maintain a constant flow rate throughout
the sampling p eriod.
When a low flow cutoff condition arises, the sampler can be restarted only by
pressing the "R eset Switch" (see Figure 3.1).
L
OW
F
LOW
Z
ERO
&
C
UTOFF
A
DJUSTMENTS
The Low Flow Zero and Cutoff Adjustments set the increment below the desired
flow rate a t which p oint the sam pler will sh ut off. Tw o potentio meters a re used to
set the cutoff point: the Low Flow Zero Adjustment, with a range of adjustment of
approxima tely 0% to 10 0% below the desired rate, and the Low Flow Cutoff
Adjustm ent, with a n approx imate 0% to 20% adjustm ent range . In particula te
sampling, the desired flow rate is 5 lpm, and the flow rate below which sampling
should be terminated is between 4.5 and 4 lpm, a 10 - 20 percent decrease in flow
rate.
To set a low flow cutoff 10 - 20 percent below the optimal flow rate:
1.
Turn the sampler on.
2.
Using the Sampling Flow Rate Adjustment, set the desired optimal flow rate as
measured by the Flow meter.
Airmetrics MiniVol Users Guide
16
Airmetrics
L
3.
Set the Lo w Flow Cutoff A djustme nt to the zer o position (rotated fully
counterclockw ise).
4.
Adjust the Low Flow Zero Adjustment until the Low Flow Threshold Indicator
lights dimly but the pump continues to run. If the pump shuts off, turn the
Zero Adjustment clockwise and reset with the Lo-Flow/Lo-Battery Reset
Switch. At this point the cutoff flow rate is nearly equal to the optimal flow
rate.
5.
Rem ember ing that the L ow Flo w Cu toff Adjus tment turn ed fully
counterc lockwis e is equiva lent to a 0 pe rcent less c utoff flow r ate and a fu lly
clockwise setting (
.
180° rotation) is equivalent to a 20 percent less cutoff flow
rate, turn the Cutoff Adjustment to the position that will yield the desired
cutoff flow rate. For ex ample, if th e optima l flow rate w as set at 5
liters/minute and a cutoff flow rate of 4.5 liters/minute is desired, rotate the
Cutoff Adjustment clockwise by 90°.
The absolute low flow cutoff rate can be determined by running the sampler with a
variable re strictor (valv e) installed in the inlet tubin g. Wa tch the flow meter w hile
increasing the restriction un til the low flow thresho ld indicator lights. The cu toff
flow rate is the rate indicated by the flowmeter just as the indicator light turns on.
L
OW
B
ATTERY
I
NDICATOR
When lit, the Low Battery Indicator means that the battery voltage has dropped to a
limit too low (10.3 volts) to permit continued operation. When the low voltage
limit is reached, the pump shuts off and the low battery indicator turns on and
remains to alert the o perator. If the pum p was not turne d off and the battery
voltage continue d to drop, the battery c ould be perm anently dam aged or its life
significantly shortened.
When a low battery condition arises, the sampler can be restarted only by pressing
the "Reset Sw itch" (see Figure 3.1 ).
L
O
-F
LOW
/L
O
-B
ATTERY
R
ESET
S
WITCH
The Lo-Flow/Lo-Battery Reset Switch restarts the pump when the system has been
shut down due to low flow or low battery voltage conditions (see "Low Flow
Cutoff Indicator" a nd "Low Battery Indicator" ab ove).
