Calibrating Florine Sensors with ACD Equipment

Many electrochemical fluorine (F2) sensors can be calibrated and bump tested with chlorine (Cl2), and ACD Cl2 generating sources deliver that Cl2 on demand without a cylinder.

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Purpose and scope

F2 calibration gas is difficult to obtain, short-lived, and hazardous to handle. Several sensor and instrument makers therefore approve Cl2 as a surrogate gas for their F2 sensors. This note explains the principle, lists published manufacturer guidance, and gives a general procedure for using ACD instruments and Cl2 sources for F2 sensors.

This note applies to electrochemical F2 sensors only. Always follow the sensor or instrument manufacturer's instructions where they differ from this note.

How Cl2 surrogate calibration works

Electrochemical F2 sensors respond to strong oxidizing gases, so a known Cl2 concentration produces a predictable, repeatable F2 reading.

The ratio between the two responses is the sensor's response factor. Each sensor manufacturer determines it for its own sensor design. The F2 span value is calculated from the Cl2 concentration applied:

F2 span value (ppm) = Cl2 applied (ppm) × response factor (ppm F2 per ppm Cl2)

Example: a sensor with a published factor of 1.4, exposed to 1.0 ppm Cl2, is spanned to 1.4 ppm F2. This example is illustrative only. Use the factor published for the specific sensor being calibrated.

Some instruments handle this automatically. For example, a sensor may be configured for F2 but calibrated directly with Cl2 because the conversion is stored in the sensor or transmitter. Check the instrument manual to see which method applies.

Published manufacturer guidance

Dräger and Honeywell both specify Cl2 for calibrating sensors that measure F2; factors and conditions differ by sensor, so confirm with the current manufacturer documentation.

Manufacturer / sensor F2 range Cl2 guidance Conditions or factor
Dräger XS EC Cl2 (68 09 165) 0–20 ppm Calibration with Cl2 test gas is sufficient for all target gases (Cl2, F2, Br2, ClO2) Test gas approx. 2–20 ppm Cl2
Dräger fixed-transmitter sensor (68 09 665) Per transmitter Factory-calibrated with Cl2; surrogate calibration permitted when target gas is not available Regular F2-target bump tests required; extra measuring error must be allowed for
Honeywell Midas MIDAS-S/E-HAL 0–4 ppm Cl2 is the specified calibration gas and bump test gas Cl2 range 0–2 ppm on the same cartridge
Sensoric F2 3E 1 (sensor element) — Calibrate with 1 ppm Cl2 1 ppm Cl2 reads 1.4 ppm; varies by sensor and with age

Related: some Honeywell Midas HF cartridges, such as MIDAS-E-HFU, are calibrated with HF but specify Cl2 as the bump test gas.

Open question: RKI, MSA, Industrial Scientific and Honeywell RAE offer F2 sensors, but no published Cl2 factors were found. Contact the manufacturer before using Cl2 with these sensors.

Recommended ACD equipment

The low-ppm Cl2 micro sources cover the 0–4 ppm F2 ranges typical of semiconductor and industrial F2 monitors.

Instrument Cl2 source Output range Best suited for
GENie EC Cl2 micro source 0.05–5 ppm Calibration and bump testing of low-range F2 sensors
CAL 2000 Cl2 micro source 0.05–1 ppm Very low-level calibration points
CAL 2000 / GENie EC Standard Cl2 source See instrument specifications Higher-range F2 sensors (for example 0–20 ppm)

Generating sources are instrument-specific and cannot be swapped between instruments. Cl2 sources are warrantied for 50 hours of generation.

Benefits over cylinder Cl2: no high-pressure reactive gas cylinders, no cylinder shelf-life expiry, and output adjustable to the exact concentration the procedure requires.

Procedure

Zero the sensor, apply a known Cl2 concentration, set the span to the F2-equivalent value, then confirm with a bump test.

  1. Confirm the sensor manufacturer approves Cl2 for the specific F2 sensor, and record its response factor or calibration method.
  2. Install the correct Cl2 source in the ACD instrument and allow it to warm up and stabilize per the ACD instruction manual.
  3. Connect the instrument to the sensor with the shortest practical length of PTFE or FKM tubing. Reactive gases are partly absorbed on other materials and on long tubing.
  4. Zero the sensor using the ACD calibrator set to 0 ppm as the zero gas.
  5. Select a Cl2 output within the range the sensor manufacturer recommends, typically 40–100% of the sensor's full scale.
  6. Apply the Cl2 to the sensor at the flow rate the sensor manufacturer specifies.
  7. Wait for a stable reading, usually 2–3 minutes, then set the span:
    • If the instrument converts automatically, span to the Cl2 value applied.
    • Otherwise, span to the F2-equivalent value: Cl2 applied × response factor.
  8. Remove the gas, allow the sensor to recover, and record the Cl2 concentration, response factor, F2 span value, and date.
  9. Between calibrations, bump test regularly with the same Cl2 source to confirm the sensor still responds.

Limitations and precautions

Surrogate calibration is less accurate than calibration with F2 itself, and the response factor drifts as the sensor ages.

  • Response factors vary between sensor models, between individual sensors, and over sensor life. Never apply one manufacturer's factor to another manufacturer's sensor.
  • Some manufacturers do not approve surrogate calibration for certain sensors. Honeywell, for example, advises against calibrating with cross-sensitivity factors on some cartridges.
  • Where accuracy is critical, verify periodically against a certified F2 standard or have the sensor calibrated by the manufacturer.
  • Do not exceed the sensor's expected service life when relying on surrogate calibration.
  • Use only in well-ventilated areas and vent the calibration exhaust safely. Cl2 is toxic; do not inhale the test gas.

Contact

For help selecting an instrument and source for a specific F2 sensor, contact Advanced Calibration Designs, Inc.
2024 W. McMillan Street, Tucson, AZ 85705 · 800-737-0223 · acd@goacd.com

Sources

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