Air Condition Examples
Internal Sensor Signal
0-5 Volt Linear Output Signal
Clean fresh air
1. 50 volts
1.0 volts = 10 % pollution
Early morning in an un-occupied office
1.75 volts
1.0 volts = 15 % pollution
Smoke from one cigarette in a 300 ft
2
room
2.00 volts
1.6 volts = 40 % pollution
Coffee break, eight persons in a 300 ft
2
lunchroom
2.25 volts
2.2 volts = 65 % pollution
Occupied classroom with 30 persons
2.40 volts
2.6 volts = 68 % pollution
Workshop with solvents in use
3.00 volts
4.1 volts = 80 % pollution
Exhaled smoke towards the sensor from 3 ft distance
3.35 volts
5.0 volts = 90 % pollution
Strong chemicals near sensor (ethyl alcohol, nail polish, etc)
4.00 volts
5.0 volts = 100 % pollution
Manual Mode
The manual mode is selected by using the on board keypad and is explained further in the menu section. In this mode the Air Quality
Monitor will operate in the Manual base/Fixed scale mode. In this mode the scale factor (sensitivity) is factory set to 0.2 volts (wrt-
sensor) and cannot be changed by the user. However, the user can set the base level on the keypad. The base level can be set in the range
of 1.0 to 3.5 volts (wrt-sensor) and is displayed on the LEDs as the 5 least significant bits of a 6 bit binary number (63 steps) with the
OK LED being the LSB. During the set-up step, this voltage level can be measured on the ASO terminal with a DC voltmeter.
The processor measures the sensor output 1.50 – 4.00 volt signal and compares it to the base level as set above. If the sensor reads a
value lower than the base level + 0.2 volts the OK LED will be lit, if the sensor reads a value between the base level + 0.2 volts and
the base level + 0.4 volts then the LOW LED will be lit, etc.
The default base level is set to 20 of 63 or about 1.8 volts so the following settings apply: OK
Sensor < 2.0 V
LOW
2.0 V < Sensor < 2.2 V
MID
2.2 V < Sensor < 2.4 V
HIGH
Sensor > 2.4 V
This mode of operation is useful if the user wants to pre-set a level of air quality which must be maintained. The scale factor has been
optimized for contaminants such as solvents or cigarette smoke. Note that in the manual mode of operation the base level remains fixed
and sensor contamination, temperature or humidity extremes, etc. may cause dampers to stay open for long periods of time.
Auto Mode
The auto mode (factory default) is selected by using the keypad and the Air Quality Monitor will operate in the Auto base/Manual scale
mode. In this mode the base level is automatically adjusted by the program to ensure optimum air quality based on the surrounding
environment and the air handling system. The user can adjust the sensitivity of the device by changing the scale factor via the keypad.
The scale factor has a range of 0.05 to 0.36 V (wrt-sensor) and is displayed on the LEDs as the 5 least significant bits of a 6 bit binary
number (63 steps) with the OK LED being the LSB. See the following example of an LED sequence. The binary range of the scale
factor is (0)00000 to (1)11111 and each count represents about 4.9 mV. This value can be measured on the ASO output terminal.
LED
Extra Bit
Auto
High
Mid
Low
OK
Status
Assumed
Off
On
On
On
On
Binary
0
0
1
1
1
1
Scale Factor
15 x 4.9 mV + 0.05 V = 0.12 V, 001111 = 15
The scale factor represents the sensitivity of the device, a smaller scale factor means more sensitive operation and it will take a smaller
change in the pollution level to give a change in output state. On power up, the base level is initially set to the current air reading. During
operation the device constantly monitors the air quality and compares the contamination level to the base level. If the sensor level falls
below the base level, the base level is adjusted after one minute to equal the air level. This one minute interval of checking is indicated
by a flashing OK LED.
This feature always keeps the base level current to the present cleanliness of the air.
If the sensor level increases above the base level, the degree of pollution is determined by comparing the sensor level to the base level
plus multiples of the scale factor. When the pollution level is determined the corresponding LED is lit and the ASO output is updated.
This is the same as the example shown for manual mode except the scale factor can be changed by the user on the key pad.
After pollution has been detected the level must drop below the current level for a period of one minute before the output pollution level
is updated to help ensure against false indication. If the pollution level does not decrease within one hour, then the air is deemed to be
