Envco – Environmental Equipment

Apogee oxygen sensors are galvanic cell sensors that have a lead anode, a gold cathode, an acid electrolyte and a Teflon membrane. The current flow between the electrodes is proportional to the oxygen concentration being measured. An internal bridge resistor is used to provide a mV output. Being a galvanic cell type sensor, a small amount of oxygen is consumed in the reaction in order to produce the current flow and subsequent mV output. The oxygen consumption was measured to be 2.2 µmol O2 per day when the O2 concentration was 20.95% (3240 mmol) at 23 C.

The mV output responds to the partial pressure of oxygen in air. The standard units for partial pressure are kPa. However, gas sensors that respond to partial pressure are typically calibrated to read out in mole fraction of the gas in air, which has units of moles of oxygen per mole of air. These units can be directly converted to % O2 in air or ppm O2 in air.

Gas sensors read out in percent because this value does not change with temperature or pressure. The concentration of oxygen in our atmosphere is 20.95%, and this value, to 4 significant digits, has not changed for decades. This means that we are surrounded by calibration gas for this sensor (provided you are not breathing on the sensor when it is being calibrated. Our exhaled breath is about 17% oxygen).

Two Models
DIFFUSION 3.5 cm long; 3.5 cm dia.; 125 mesh screen creates air pocket
FLOW THROUGH 3.5 cm long; 3.5 cm dia.; 1/8" barbed adaptors for hose connections

Two Applicaitons
SOIL
OUTPUT: ~50 mV SIGNAL
DECREASE: <0.2% /year
LIFE
EXPECTANCY: 10 years, continuous
RESPONSE : 60 seconds

FAST RESPONSE
OUTPUT: ~12 mV
SIGNAL
DECREASE: <4% /year
LIFE
EXPECTANCY: 5 years, continuous
RESPONSE : 12 seconds

Two Options for Temperature Output
THERMISTOR
Datalogger Channels:
(1) differential
(1) single-ended
Recommended for best accuracy

THERMOCOUPLE
Datalogger Channels:
(2) differential
Requires a thermocouple reference

Ordering Information
SO 1 - Soil Sensor
SO 2 - Fast Response Sensor

Temperature Option (Must select)
A – Thermistor
B - Thermocouple

Application Option (Must select)
1 - Diffusion Head
2 - Flow-through Head
3 - Both Heads

Example: SO-1A2 = Soil Sensor, Thermistor, Flow-through Head