Oxygen Sensor FAQs

Which oxygen sensor do I need?
What does an oxygen sensor do?
Oxygen availability determines the rate of many biological and chemical processes and is required for aerobic respiration. It is the absolute amount of oxygen (measured as partial pressure in kilopascals) that nearly always determines oxygen availability, but we think of oxygen as a percent of the total number of molecules in the air (20.95 %). The best example of this is the oxygen on top of Mount Everest, which is 20.95 %, but most climbers need supplemental oxygen to get to the top.

There are two types of oxygen sensors: those that measure gaseous O2 and those that measure dissolved oxygen in a solution. The Apogee sensor measures gaseous O2 as a percentage. Gas sensors read out in percent because this value does not change with temperature or pressure.

There are multiple techniques for measuring gaseous oxygen. Three widely used approaches for environmental applications are galvanic cell sensors, polarographic sensors, and optical sensors. The Apogee sensor is a galvanic cell type that operates by electrochemical reaction of oxygen with an electrolyte, which produces an electrical current. The electrochemical reaction consumes a small amount of oxygen in the reaction in order to produce the current flow and subsequent mV output. The current flow between the electrodes are proportional to the oxygen concentration being measured, and an internal bridge resistor is used to provide the mV output. The mV output responds to the partial pressure of oxygen in air.
What is the purpose of the oxygen sensor's built-in heater?
In order to keep condensation from forming on the sensor's Teflon membrane (where oxygen diffusion occurs), the built-in heater is designed to warm the sensor to a temperature slightly above the ambient temperature. This is particularly important in soil applications where the relative humidity is normally at 100 %. For this reason, it is recommended that the heater be continuously powered. Once condensation forms on the membrane, the sensor must be removed from the humid environment and allowed to dry before the condensation evaporates and the signal returns. If the heaters are turned off and condensation forms, the heaters don't supply enough energy to evaporate the condensation once they are turned back on. The heater requires a 12 V DC input and consumes about 74 mW of power. This works out to about 6 mA of current draw.
How often should I recalibrate the sensor?
Sensor recalibration can be conducted periodically and should be determined by the level of measurement accuracy required for the application. Apogee recommends the SO-100 series be recalibrated every 2-3 years and the SO-200 series be recalibrated every year. SO-100 and SO-200 series sensors decrease by approximately 1 mV (SO-100 series) and 0.8 mV (SO-200 series) or approximately 2 % (SO-100 series) and 6 % (SO-200 series) of signal output per year at 20.95 % oxygen. This signal decrease yields increases in calibration factor of approximately 2 % (SO-100 series) and 6 % (SO-200 series). Further information can be found in the Maintenance and Recalibration section of the manual.
What should my sensor read in ambient air?
SO-100 series sensors should read approximately 60 mV at sea level in ambient air (20.95 % O2). SO-200 series sensors should read approximately 12 mV at sea level in ambient air. These voltages will decrease by approximately 1 % per 100 meters of elevation increase above sea level.
How does atmospheric pressure affect my oxygen readings?
Lower pressure can result in higher readings from your sensor. A barometric pressure correction should be applied to all oxygen sensors that are calibrated to read relative oxygen concentration. Further information and the equation to correct relative oxygen measurements for barometric pressure can be found in the Operation and Measurement section of the manual. Additionally, pressure below the 60 kPa (8.7 PSI) limit may cause excessive evaporation of the electrolyte and lessen the life of the sensor.
How does temperature affect my oxygen readings?
The ideal gas law shows that absolute gas concentration decreases by 0.341 % at 1 C increase in temperature from 20 C. For a sensor that measures absolute gas concentration, but is calibrated to read out in relative units, a 1 C temperature increase from 20 C results in an apparent decrease of 0.0714 % and a relative oxygen concentration of 20.878 %. A correction should be applied to compensate for temperature effects. Further information and the equation to correct relative oxygen measurements in air for temperature effects can be found in the Operation and Measurement section of the manual.
What is the minimum airflow required to get oxygen readings?
Flow-through applications should have a minimum of 200 to 300 mL/min flow rate.
Does this sensor measure dissolved oxygen?
No, it measures the absolute concentration of gaseous oxygen.
Will it damage the oxygen sensor to get it wet?
No it won't. It might read "0 %" oxygen when it is wet, but will return to normal once it dries out. The housing of the sensor is polypropylene plastic, potted solid, and is waterproof.
Can I add additional cable to the sensor?
Yes, if properly spliced, the signal from the sensor is not affected by splicing on additional cable. Please check the Owner's Manual for details and limitations on adding cable. For information on how to create a proper waterproof splice, please visit our instructional webpage or watch our video. We offer custom cable lengths of our high quality cable and splice kits that can be ordered at the time of purchase or a later date.
How do I get data off my meter?
Handheld Meter Video Guide
The AC-100 communication cable is used to download saved measurements from any of our handheld meters. This USB cable includes a built-in circuit board to convert voltage levels to be compatible with the meters-normal USB cables will not work. The AC-100 also comes with a flash drive that includes the necessary computer software, drivers, and instructions. If you already have the AC-100 and just need the software files please click here.
Do I need to order a sensor to go with my meter?
No. All Apogee meters (MO, MP, MQ, and MU series) either have sensors built into the meter or attached via two meters of cable. If you order a sensor only (SI, SO, SP, SQ, and SU series) you will need to have your own datalogger (or, depending on the sensor, a voltmeter) to collect information form the sensor.
Can I add cable to my meter?
Although it is possible to splice additional cable to the separate sensor, note that the cable wires are soldered directly into the circuit board of the meter. Care should be taken to remove the back panel of the meter in order to access the board and splice on the additional cable, otherwise two splices would need to be made between the meter and sensor head. Click here for further details on how to extend sensor cable length.
I received an error code on my meter. How do I fix it?
Error codes will appear in place of the real-time reading on the LCD display and will continue to flash until the problem is corrected. For steps on completing a fix please refer to the manual.
Err 1: Battery voltage out of range. Fix: replace CR2320 battery and perform master reset.
Err 2: Sensor voltage out of range. Fix: perform master reset.
Err 3: Not calibrated. Fix: perform master reset.
Err 4: CPU voltage below minimum. Fix: replace CR2320 battery and perform master reset.
What kind of battery does my meter take?
The meter takes a CR2320 coin cell battery.
Can I measure soil respiration?
Apogee oxygen sensors can be used in conjunction with carbon dioxide sensors to help improve the characterization of soil respiration. Typically, soil oxygen sensors use a galvanic cell to produce a current flow that is proportional to the oxygen concentration being measured. These oxygen sensors are buried at various depths to monitor oxygen depletion over time, which is then used to predict soil respiration rates. Apogee oxygen sensors are equipped with a built-in heater to prevent condensation from forming on the permeable membrane, as relative humidity can reach 100 percent in soil.
What should I do if my meter becomes non-responsive or experience anomalies?
A master reset can be performed that may correct the problem. *Note: a master reset will erase all logged measurements from memory.

First press the power button so that the LCD display is activated. While still powered, slide the battery out of the holder, which will cause the LCD display to fade out. After a few seconds slide the battery back into the holder. The LCD display will flash all segments and then show a revision number. This indicates the master reset was performed and the display should return to normal.

Is each logged or sampled reading time stamped?
No – user should record the starting time when the meter is placed in LOG mode and take note of time when sampled readings are taken.
How many readings will the meter take in LOG mode?
When in LOG mode the meter will power on/off to make a measurement every 30 seconds. Every 30 minutes the meter will average the sixty 30 second measurements and record the averaged value to memory. The meter can store up to 99 averages and will start to overwrite the oldest measurement once there are 99 measurements. Every 48 averaged measurements (making a 24 hour period), the meter will also store 99 integrated daily totals in moles per meter squared per day (mol m-2 d-1).
How many measurements can I take in SMPL mode?
When in SMPL mode press the sample button to record up to 99 manual measurements (a counter in the upper right hand corner of the LCD display indicates the total number of saved measurements).