Gas Sensor Specifications and Cross Sensitivities - RAE Systems

Disclaimer: Technical Note TN-114 presents specifications, cross-sensitivities, and calibration information on select RAE Systems sensors. All specifications presented in this Technical Note reflect the performance of standalone sensors. For instrument specifications, please refer product datasheets and manuals.


Actual sensor characteristics may differ when the sensor is installed in different instruments. As sensor performance may change over time, specifications provided are for brand new sensors.


All specifications have been verified under the following environmental conditions:


Temperature: 68° F (20° C) Relative humidity (non-condensing): 50% Ambient pressure: 1 atm (1,013 mbar)


SENSORS FOR COMBUSTIBLE GASES AND VAPORS







OXYGEN SENSORS



ELECTROCHEMICAL SENSORS FOR TOXIC GASES
























NDIR SENSORS FOR CARBON DIOXIDE



PID SENSORS FOR VOLATILE ORGANIC COMPOUNDS (VOCS)




SENSOR CROSS-SENSITIVITIES


Electrochemical sensors, like many other sensors, are known to have cross-sensitivity to gases other than its target gas. Depending on the nature of the reaction in the sensor, the gas can either decrease the signal (negative cross-sensitivity) or increase the signal (positive cross-sensitivity). The cross sensitivity data listed here are based on at most a few batches of electrochemical sensors. The actual values may vary between batches because the cross sensitivity is not typically controlled during the manufacturing process.


When calibrating a multi-gas instrument that has two sensors which gases have significant cross-sensitivity, be sure to allow adequate time between calibrations to allow the sensors to clear.


When calibrating sensors with cross-sensitivities, calibrate the most cross-sensitive first, followed by the least cross-sensitive. Wait for both sensors to recover to zero, then expose both to gas again with most cross sensitive first and least cross sensitive second. For example, 50 ppm of NH3 produces 0 ppm response on a Cl2 sensor and 1 ppm of Cl2 produces about -0.5 ppm of response on a NH3 sensor. So calibrate the NH3 sensor first with 50 ppm of NH3. This should have no affect on the Cl2 sensor. Then calibrate the Cl2 sensor on 10 ppm Cl2. This will send the NH3 sensor negative for some period of time. After calibrating the Cl2 sensor, return the meter to clean air and wait until the most cross-sensitive sensor (NH3) fully recovers and/or stabilizes (if it stabilizes to a number other than zero then re-zero the meter). After both sensors return to zero apply calibration gas in the same order (NH3 first then Cl2) and note the sensor response. If both sensors are within 10% of the value on the gas cylinder then the calibration of the cross-sensitive sensors was successful.

The variety of sensor combinations, including corrosive, chemically active and highly adsorptive do not always address the proper calibration gas sequences and the calibration check tests that could potentially lead to actual gases. This can create misleading data in the field and cause possible threats to workers/responders’ personal health and safety. Calibration sequence of such sensors in different combinations were experimentally confirmed in TN-203 for the new MultiRAE family instruments.


Use extreme caution with mixtures of gases!

The following table and data are based on % cross-sensitivity to a single gas. Mixtures of the gases were not tested and results with mixed gases are unpredictable.


EXTENDED CALIBRATION AND SOAK TIMES


Most RAE systems instruments (MultiRAE family, ToxiRAE Pro, etc.) incorporating electrochemical or NDIR sensors should follow the instrument calibration time and soak time as outlined in the table below. Teflon tubing (1ft is recommended) should be used for calibration and verification testing for reactive gases.


For more comprehensive soak and calibration time for the MultiRAE instruments family, see TN-203


Automatic vs. Manual Calibration:


When used in automatic calibration mode (with AutoRAE2), soak time is required to ensure proper gas concentration delivery during calibration. The soak time can be set through ProRAE studio II using the guideline in the table below.

When used in manual calibration mode, instrument calibration time is sufficient.


Guideline for Manual Calibration of Classic Instruments:


Some classic instruments (AreaRAE, MultiRAE Plus, QRAE Plus, etc.) have a fixed calibration time (typically 60 sec.) programmed into the instrument. When doing manual calibration it is recommended to extend the amount of time the sensor is exposed to gas to match values summarized in the above table. As a reminder newer instruments like the MultiRAE and ToxiRAE Pro family automatically adjust the programmed calibration time in the instrument to those listed in the table so there is no need to apply gas in advance of starting the countdown sequence.


For example, when performing calibration of the MultiRAE Plus for Cl2, apply gas for 140 seconds prior to pressing the [Y/+] to start the programmed calibration time countdown of 60 seconds. In this case the sensor is exposed to Cl2 gas for a total of 200 seconds during the calibration process ensuring an accurate calibration.

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