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Enabling better global research outcomes in soil, plant & environmental monitoring.

Apogee Soil Oxygen SO-200 Series: Fast Response

Apogee oxygen sensors are designed to measure 0 - 100% oxygen and come with tinned leads to connect to a data logger. The AO-001 Diffusion Head is used for field or soil measurements of oxygen concentration. The AO-002 Flow Through Head is used for laboratory applications.

The SO-200 Series of Appogee’s Soil Oxygen sensors have a fast response time of 14 seconds using SDI-12 output, but have a shorter life span; making them ideal for laboratory soil applications.

Typical applications include measurement of O2 in laboratory experiments, monitoring gaseous O2 in indoor environments for climate control, monitoring of O2 levels in compost piles and mine sailings, monitoring redox potential in soils, and determination of respiration rates through measurement of O2 consumption in sealed chambers or measurement of O2 gradients in soil/porous media.

The SO-210 comes with a thermistor temperature sensor to correct for temperature changes; while the SO-220 comes with thermocouple temperature corrections. And both the SO-220 and SO-210 incorporate a restive heater to raise the temperature of the membrane approximately two degrees above ambient temperature to keep condensation from occurring on the Teflon membrane and blocking the diffusion path of the sensor.

Oxygen (O2) is the second most abundant gas in the atmosphere and is essential to life on Earth. Absolute oxygen concentration determines the rate of many biological and chemical processes. Oxygen is required for aerobic respiration. In addition to measurements of absolute oxygen concentration, relative oxygen concentration is also often measured and reported.

Oxygen sensors are used to measure gaseous or dissolved oxygen. There are multiple different techniques for measuring gaseous oxygen. Three of the more widely used sensors for environmental applications are galvanic cell sensors, polarographic sensors, and optical sensors. Galvanic cell and polarographic sensors operate similarly, by electrochemical reaction of oxygen with an electrolyte to produce an electrical current. The electrochemical reaction consumes a small amount of oxygen. Unlike polarographic oxygen sensors, galvanic cell sensors are self-powered and do not require input power for operation. Optical oxygen sensors use fibre optics and a fluorescence method to measure oxygen via spectrometry.

Typical applications of Apogee oxygen sensors include measurement of oxygen in laboratory experiments, monitoring gaseous oxygen in indoor environments for climate control, monitoring of oxygen levels in compost piles and mine tailings, and determination of respiration rates through measurement of oxygen consumption in sealed chambers or measurement of oxygen gradients in soil/porous media.

Apogee Instruments SO-100 and SO-200 series oxygen sensors consist of a galvanic cell sensing element (electrochemical cell), Teflon membrane, reference temperature sensor (thermistor or thermocouple), heater (located behind the Teflon membrane), and signal processing circuitry mounted in a stainless steel housing (or ABS plastic housing for use in acidic environments), and lead wires to connect the sensor to a measurement device. Sensors are designed for continuous gaseous oxygen measurement in ambient air, soil/porous media, sealed chambers, and in-line tubing (flow through applications). SO-100 and SO-200 series oxygen sensors output an analogue voltage that is directly proportional to gaseous oxygen.

Measurement Range 0 to 100 % O₂
Sensitivity (at sea level, 101.3 kPa) 12-13 mV in 21 % O₂, 0.6 mV per % O₂, 6 µV per 0.01 % O₂
Response Time (time required to read 90 % of saturated response) 14 s
Output at 0 % O₂ 2 % of output at 20.95 % O₂ or 0.2 ± 0.1 mV
Measurement Repeatability Less than 0.1 % of mV output at 20.95 % O₂
Non-linearity Less than 1 % Long-term Drift (Non-stability) 0.8 mV per year
Oxygen Consumption Rate 2.2 µmol O₂ per day at 20.95 % O₂ and 23 C
Operating Environment -20 to 60 C, 0 to 100 % relative humidity (non-condensing); 60 to 140 kPa
Input Voltage Requirement 12 V DC continuous (for heater), 2.5 V DC excitation (for thermistor)
Heater Current Drain 6.2 mA (74 mW power requirement when powered with 12 V DC source)
Thermistor Current Drain 0.1 mA DC at 70 C (maximum, assuming input excitation of 2.5 V DC)
Dimensions 32 mm diameter, 68 mm length
Mass 175 g (with 5 m of lead wire)
Cable 5 m of six conductor, shielded, twisted-pair wire
Warranty 4 years against defects in materials and workmanship

 

Heated Detector
The protective membrane in front of the oxygen sensor can be heated to prevent water from condensing on the membrane and blocking the diffusion path. The heater is typically used when sensors are deployed in soil or compost where relative humidity is close to 100%.

Rugged Housing
Housed in a polypropylene body and electronics are fully potted, ideal for long-term deployment in porous media, including acidic environments (mine tailings). Two head options are available: a diffusion head that creates a small air pocket for measurement in porous media and a flow-through head with two adapters for tubing that allows measurement of gas flowing in lines.

Internal Temperature Sensor
All oxygen sensors have an internal thermistor (type-K thermocouple is available upon request) that allows for temperature monitoring and correction of signal for temperature effects.

Simple Calibration
Voltage output is linearly proportional to absolute amount of oxygen. Calibration is accomplished by measuring the voltage under ambient conditions (atmosphere is 20.95% O2) and deriving a linear calibration factor (slope). A zero offset can be measured with N2 gas (recommended for measurements below 10% O2).

Typical Applications
Applications include: measurement of O2 in laboratory experiments, monitoring gaseous O2 in indoor environments for climate control, monitoring of O2 levels in compost piles and mine tailings, monitoring redox potential in soils, and determination of respiration rates through measurement of O2 consumption in sealed chambers or measurement of O2 gradients in soil/porous media.