Calibrating Optical Oxygen Sensors

NeoFox oxygen sensing systems from Ocean Optics are the only oxygen sensors of their type that can be calibrated to a specific experiment environment. While other sensing systems are sold with a single, factory-defined calibration, or require users to take their own on-site references, NeoFox is available with custom calibration regimes that cover the precise oxygen pressure and temperature ranges you expect to encounter during your measurements.

About NeoFox Optical Oxygen Sensors

Advances in sensor materials and optoelectronics have enabled novel optical oxygen sensors for applications in life sciences, pharmaceuticals, biotechnology and more. The principle of operation is to trap an oxygen-sensitive fluorophore in a sol-gel host matrix that is applied to the tip of a fiber, an adhesive membrane such as a patch, or a flat substrate such as a microtiter plate. The indicator materials change optical properties in response to specific analytes and electronics measure the response. For oxygen, the NeoFox phase fluorometer measures the partial pressure of dissolved or gaseous oxygen.

Optical Oxygen Sensor Calibration

Sensor calibration in gas or liquid phase is necessary to ensure that your system reads oxygen levels accurately over the oxygen pressure and temperature range of your measurements. You can calibrate the system in your sample environment or have Ocean Optics calibrate the system prior to shipment.

Because the raw sensor signal is affected by the oxygen partial pressure, temperature and physical properties of the sample, it is important for measurements in non-isothermal environments to have temperature compensation worked into the calibration.

In NeoFox sensor setups, temperature affects both fluorescence intensity and excited state lifetime, resulting in a change in the calibration slope. To avoid false partial pressure readings, the sample must be held at constant temperature (±3 °C), or corrected for temperature. As such, Ocean Optics offers temperature compensation options including a discrete thermistor and a probe with integrated thermistor.

Oxygen Sensor Calibration Services from Ocean Optics

Ocean Optics oxygen sensors produce a fluorescence lifetime, or tau value, as a function of partial pressure of oxygen and temperature. To account for these temperature effects, we calibrate our sensors across a range of temperatures. By generating oxygen response curves at various temperature levels, a set of polynomial relationships are determined to provide precise oxygen readings despite changes in environmental temperature.

To calibrate your oxygen sensors in-house, we use robust, fully automated environmental chambers that support up to 8 NeoFox channels each. A custom gas dilution system feeds four ultra-high purity (UHP-grade) gas standards to a series of mass flow controllers that output oxygen concentrations with 0.05% precision. The gas flows through a manifold with fittings designed to integrate all our oxygen sensor products. The manifold sits in a Thermotron® environmental chamber that quickly moves the sensors across the calibration temperatures.

Standard and Custom Calibration Options

Ocean Optics oxygen calibration services are offered in three versions: standard (O2-CAL-STANDARD), custom (O2-CAL-CUSTOM) and liquid-phase calibration (O2-CAL-LIQUID).

The O2-CAL-STANDARD option runs the sensors through the most common ambient-range conditions. This includes a 0-25% oxygen range at 1 atm and 15-35 °C temperature range, using 11 oxygen concentrations and 9 temperature points across this distribution:

The O2-CAL-CUSTOM option allows users to select the exact oxygen and temperature ranges for their sensor, and allows grouping of points for higher accuracy in select conditions. The following ranges are available for the various oxygen sensor chemistries:

The O2-CAL-LIQUID option allows consistent liquid-phase calibrations in your unique fluid environment. Like the O2-CAL-CUSTOM option, the liquid calibration can be selected for exact oxygen and temperature ranges as shown in the table above. The following restrictions apply to the desired calibration fluid:

• Must have freezing/boiling points outside the requested temperature ranges (remains liquid during entire calibration regime)
• Must be safe to handle outside of a fume hood
• Must be available from a qualified U.S. supplier
• Must be chemically compatible with the oxygen sensor formulation