Tech Note 1078 Detail

Ethanol Sensor Calibration Procedure

Affected Products:

PS-2194 PASPORT Ethanol Sensor

Problem/Symptom:
Ethanol Sensor Calibration Procedure

PASCO Solution:

Theory of Operation

The probe of PS-2194 Ethanol Sensor consists of Pt-W electrodes printed on a multi-crystaline alumina (Al2O3) subtrate with an integrated RuO2 heater.

When a metal oxide such as alumina is heated to a sufficiently high temperature while exposed to air, oxygen from the air is absorbed on the surface of the metal oxide, transferring negative charge to the oxygen and leaving a positively charge surface barrier to electron flow at each grain boundary in the metal oxide.

Many solvent vapors (such as ethanol) act as reducing agents for this absorbed oxygen, decreasing the charge barriers and making it easier for electrons to flow past the grain boundaries. The conductivity of the metal oxide is related to the concentration and chemical composition of the reducing gas by

R = R0 (C)−α

  • R = electrical resistance of the metal oxide
  • R0 = resistance with absorbed oxygen
  • C = gas concentration
  • α = empirical constant that depends on the particular reducing gas

Any of the following solvent vapors will significantly reduce the metal oxide:

  • Xylene
  • Toluene
  • n-Heptane
  • Benzene
  • Ethanol
  • Acetone
  • Hydrogen
  • Methyl Mercapatan
  • Iso-Butane
  • Formaldehyde
  • Trimethlamine
  • Hydrogen Sulfide
  • Acetic Acid
  • Propionic Acid
  • Ammonia
  • Carbon Monoxide
  • Methane
  • Water Vapor

In addition, the sensor should be expected to be sensitive to chemically related solvent vapors.

The resistance of the aluminum oxide is converted to an output voltage by means of a voltage divider.

Usage Notes

  • The teflon membrane is designed to prevent the sensing element from getting wet. If the sensing element accidentally gets wet, rinse and dry it quickly. The probe may be degraded or destroyed if the membrane is ruptured or compromised.
  • Calibration data is stored in the sensor’s memory and it persists until the sensor is calibrated again, even if the sensor is unplugged from the interface.
  • Avoid exposure to
    • concentrations in excess that 3% of any of the vapors to which it is sensitive
    • silicone vapors
    • corrosive vapors (H2S, SOx, Cl2, HCl,...)
    • salts
    • liquid water
    • high pressures
    • high-temperatures
    • high-humidity
    • air flow
  • If the probe accidentally gets wet, dry it quickly. Do not immerse the probe.
  • Replace a damaged teflon membrane immediately.

Calibration (for Ethanol measurements)

Calibration data is retained in non-volatile memory in the sensor, but daily calibration is recommended for best results.

Material

  • 1% ethanol in water at the same temperature as the solutions to be measured.

Procedure

  1. Ensure that the membrane is installed and intact. (If not, refer to the manual for membrane replacement.)
  2. Ensure that the measurement environment has a fairly constant temperature and humidity.
  3. Power on the sensor for ~10 minutes before use.
  4. Connect the sensor to an interface.
  5. Place the probe either in a sealed container above the 1% ethanol solution. Do not immerse the probe. Avoid getting the probe wet.
  6. Wait until the reading stabilizes.
  7. Press and hold the 1% CAL button on the sensor for at least 4 seconds. (Immediately after a successful calibration, the sensor’s output reads 1%, and the button is illuminated.)

Measurement

  1. Ensure that the Teflon membrane is intact and not discolored.
  2. Place the end of the probe just above a solution, or in a gas, containing less than 3% ethanol.
  3. Click the Start button to begin recording data.

Storage

The sensor should be stored in a sealed polyethylene bag with clean air at ambient humidity. Do not use silica gel or store in any bag with a perceptible odor, such as a bag made of PVC, which can off-gas phtalates.

Creation Date: 03/30/2010
Last Modified:
Mod Summary: