Section 5: Integrated Gas Sampling
Airmetrics
39
1) Place the Overlap Jumper on the pins.
2) Plug ba g 1 solen oid into So lenoid O utput Co nnector 2 ; plug bag 2 soleno id
into Solenoid Output Connector 4.
3) Set the timer:
a) Set timer Program 1 to turn on at 10:00 am and off at 3:00 pm. During
this program, bag 1 will be filling.
b) Set timer Program 2 to turn on at 3:01 am and off at 6:00 pm. During
this program, both solenoids will be active and both bags will be filling
simultaneously.
c) Set timer Program 3 to turn on at 6:01 pm and off at 11:00 pm. During
this programmed step, only bag 2 will be filling.
A
DJUSTING
P
ULSE
F
REQUENCY AND
D
URATION
The rate at which the bags are filled is set by using a tunable intervalometer or
pulse circuit which can be adjusted both for frequency (continuously on to 1 pulse
in 15 seco nds) and for duration (50 ms to 750 m s). The pu lse freque ncy is
controlled by the
Intervalometer Frequency Adjustment
, while the duration of
each pulse is set by the
Pulse Du ration Adju stment.
The
Power o n/off
(sw-5)
enables gas sampling mode.
Adjusting the pulse circuit using the Intervalometer Frequency Adjustment and the
Pulse D uration A djustme nt is accom plished thr ough trial a nd error. A test period in
the labora tory is there fore requir ed before the sam pler can b e mov ed to the field
site. The o bject is to ac hieve a co mbine d pulse d uration an d interval th at will
integrate a sample of air over the programmed period of time. At the end of the
programmed period the bags should be 80-90% filled. That is, the bag should not
be tightly filled, since there would be no way of knowing at what point the bag
became filled.
The pu lse duration and frequ ency co ntrols can be adjuste d to suit the re quirem ents
of the task at hand. For example; one can take many small samples or a few large
samples ov er the same p eriod of time dep ending on the n eeds of the opera tor.
Airmetrics MiniVol Users Guide
40
Airmetrics
Pulse Interval Adjustment
The pulse interval (off time) of the circuit is adjustable over a range of 0-15
seconds by a 16-position rotary switch. This switch is located just to the right of
the solenoid valve output connectors. Switch positions are marked clockwise 0-9,
and continue A-F. The interval between pulses increases in 1-second increments as
the switch is rotated in a clockwise direction. Position "0" enables continuous
pulsing. Position "1" corresponds to a minimum delay time of one pulse per
second, and position "F" indicates the maximum delay of 15 seconds between
pulses. The fine adjustment potentiometer next to this control is for calibrating the
one-second interval (fully counterclock wise - decrease , fully clockwise - increa se).
Pulse Duration Adjustment
The duration (on time) of each pulse is adjustable over a range of 50-750
milliseconds. This adjustment is made by summing the calibrated interval values of
DIP switches 1-4 on the 5-position DIP switch located in the upper right corner of
the auxiliary board. Each switch has an assigned "On Time" value, and the on time
for each pulse is de termined by th e sum of the v alues of switches in the "off"
position. The "On Time" v alues for each switch are as follows:
SW-1 = 50 ms
SW-2 = 100 ms
SW-3 = 200 ms
SW-4 = 400 ms
For Example: If SW-1, and SW-4 are in the "off" position, the solenoid on time
for each pulse w ould be 450 m illiseconds. (50 + 400 ms = 450 ms).
The D OW N positio n is "on" fo r these sw itches. Th e fine adju stment n ext to this
control adds an extra 50 ms to the total pulse time (fully counterclockwise - adds
50 ms, fully clock wise - adds no time).
Calculate an approximate pulse duration and frequency that would fill the 6 liter
Tedlar® bag over the desired collection period.
Example:
Suppose you wish to fill a 6 liter bag over a period of 4 hours. The first
step wo uld be to d etermine the amo unt of air pu mped during a s ingle pulse . This
can be done by replacing the bag at the mini quick-connect with a bubble flow
meter, and starting the pump in the gas sampling mode. If 1 cc of air was pumped
