Zirconia oxygen analyzers operate based on the Nernst Equation using a solid electrolyte cell made of Yttria-Stabilized Zirconia (YSZ).

• Mechanism: At high temperatures (>600°C), zirconia becomes conductive to oxygen ions.
• Construction: The cell usually separates two gases: the process gas (unknown O₂ concentration) and a reference gas (usually instrument air, 20.9% O₂).
• Migration: Oxygen ions migrate from the higher concentration side to the lower concentration side through the crystal lattice.
• Voltage Generation: This ion movement generates a potential difference (EMF) between the two electrodes (Platinum coated). This voltage is inversely proportional to the logarithm of the oxygen concentration ratio.

The primary difference lies in the physical principle used for detection:

1. Paramagnetic Analyzers:
   • Principle: Oxygen is strongly attracted to a magnetic field (paramagnetic). Other common gases are diamagnetic.
   • Operation: Uses a dumbbell suspension or magneto-dynamic wind. When O₂ enters the magnetic field, it displaces the dumbbell or creates a flow, which is measured.
   • Pros: Non-depleting sensor, very accurate, linear response.
2. Electrochemical (Fuel Cell) Analyzers:
   • Principle: Chemical reaction. O₂ enters the cell, reacts at the cathode, and generates a current proportional to concentration.
   • Operation: The cell acts like a battery that runs on oxygen.
   • Cons: The sensor is a consumable (depletes over time), shelf-life issues.

Zirconia analyzers operate at very high temperatures (typically 700°C to 800°C). Ideally, we want to measure excess oxygen.

• If the sample gas contains unburnt combustibles (like CO, H₂, or hydrocarbons) and Oxygen mixed together, they will combust on the hot surface of the zirconia cell.
• The analyzer consumes some of the oxygen to burn these combustibles before the measurement takes place.
• Therefore, the reading provided is the "Net Oxygen" (Total Oxygen minus Oxygen consumed by combustibles), not the gross oxygen content.

Thermal shock refers to the cracking of the ceramic Zirconia cell due to rapid temperature changes.

• Cause: If a cold process gas or liquid water (condensate) hits the hot ceramic cell (700°C), the material contracts unevenly and cracks.
• Prevention: Ensure the probe is heated slowly during startup and that the process gas is not introduced until the cell temperature is stable. Avoid liquid carryover.

The Zirconia cell measures the ratio of partial pressures between two sides of the cell, based on the Nernst Equation.

• To calculate the unknown concentration on the process side, the analyzer must compare it against a known constant concentration on the other side.
• Instrument Air is typically used because it provides a stable, clean source of 20.9% Oxygen.
• Without a reference gas, the differential ion migration cannot be quantified into a measurement.

This is a purely physical measurement method:

• Two nitrogen-filled glass spheres (dumbbells) are suspended on a taut platinum wire inside a non-uniform magnetic field.
• Nitrogen is diamagnetic (repelled), while Oxygen is paramagnetic (attracted).
• When sample gas containing O₂ enters the cell, the O₂ molecules are attracted to the strongest part of the magnetic field.
• This displaces the dumbbells, causing them to rotate. The torque required to restore the dumbbells to the null position is proportional to the O₂ concentration.

While highly selective, paramagnetic analyzers can be affected by other gases with magnetic susceptibility:

• NO (Nitric Oxide) and NO₂ (Nitrogen Dioxide) are also paramagnetic, though weaker than Oxygen. They can cause a positive error.
• Most common background gases (N₂, CO₂, Ar) are diamagnetic (repelled by magnets) and can cause a slight negative offset if the instrument is zeroed on Nitrogen but used on CO₂.

• In-situ (Zirconia): The probe is inserted directly into the stack/duct.
  Pros: Fast response, no sample handling system, handles high dust.
  Cons: Maintenance is difficult (hot location), cannot easily verify with test gas without specialized ports.
• Extractive: Sample is drawn out, conditioned (cooled/dried), and sent to an analyzer shelter.
  Pros: Analyzer is accessible, easier maintenance, can use paramagnetic technology.
  Cons: Slower response time (lag), sampling system requires heavy maintenance (filters, pumps).

• Single Point (Span): Often done using Instrument Air (20.9% O₂) if the process can be exposed to air or if the probe is pulled out.
• Two Point:
  1. Zero Gas: Usually a low concentration O₂ mix (e.g., 1% O₂ in N₂). Note: Zirconia cells cannot measure true "zero" (0%) effectively because the log of 0 is undefined; we use a low value like 0.4% or 1%.
  2. Span Gas: Usually Air (20.9%) or a specific mix near the process range (e.g., 8% O₂).

TDL is an optical absorption technique.

• Principle: Oxygen absorbs light at specific wavelengths (typically around 760 nm).
• Beer-Lambert Law: The amount of light absorbed is proportional to the concentration of gas and the path length.
• Operation: A laser is tuned across the specific absorption line of Oxygen. The receiver measures the loss of intensity.
• Benefits: Non-contact, extremely fast response (milliseconds), no cross-sensitivity, measures average concentration across a stack.

This is the most common probe for trace moisture in petrochemicals.

• Structure: An aluminum base is anodized to form a porous oxide layer, then coated with a thin layer of gold. This forms a capacitor (Aluminum - Oxide Dielectric - Gold).
• Operation: Water vapor molecules permeate the gold layer and adsorb onto the porous oxide.
• Effect: Water has a high dielectric constant. As water adsorbs, the capacitance of the sensor changes drastically.
• Measurement: The analyzer measures the impedance/capacitance, which correlates directly to the water vapor pressure (Dew Point).

• Dew Point (°C/°F): The temperature to which a gas must be cooled for saturation (condensation) to occur. It is related to the partial pressure of water vapor. It is pressure-dependent.
• PPMv (Parts Per Million by Volume): The ratio of the volume of water vapor to the total volume of gas. It is an absolute concentration and is pressure independent (as long as the gas remains a gas).

Also known as the Electrolytic method.

• Principle: Faraday’s Law of Electrolysis.
• Construction: Two electrodes are wound on a glass rod and coated with P2O5 (a strong desiccant).
• Operation: P2O5 absorbs all moisture from the sample stream, turning into phosphoric acid. A voltage applied across the electrodes electrolyzes the water back into H₂ and O₂, regenerating the P2O5.
• Result: The current required to electrolyze the water is directly proportional to the mass flow of water entering the cell. It is an absolute measurement method (no calibration theoretically needed).

Glycol is commonly used in dehydration units (TEG units) upstream of moisture analyzers.

• The Problem: Glycol is polar, just like water. If glycol mist coats the sensor, it clogs the pores of the aluminum oxide.
• Symptoms: Slow response time and permanently high readings (the glycol traps moisture on the sensor).
• Solution: Use extensive sample conditioning (coalescing filters, glycol absorbers) before the sensor. Cleaning the sensor is rarely successful; replacement is usually required.

QCM is a highly accurate method for trace moisture.

• Principle: A quartz crystal oscillates at a specific resonant frequency. The crystal is coated with a moisture-sensitive polymer.
• Operation: As moisture adsorbs onto the polymer, the mass of the crystal increases.
• Sauerbrey Equation: The change in frequency is proportional to the change in mass.
• Benefit: Very fast response to wet-up and dry-down compared to Al-Oxide sensors.

Tunable Diode Laser Absorption Spectroscopy (TDLAS) is becoming the industry standard.

• Mechanism: A laser emits light at a specific wavelength absorbed by H₂O molecules (e.g., 1854 nm or 1392 nm).
• Selectivity: The laser line is extremely narrow, avoiding interference from Methane, CO₂, or H₂S.
• Non-Contact: The laser passes through the gas; the sensor never touches the process (no glycol poisoning!).
• Maintenance: Minimal. Optical windows need cleaning, but no sensor drift occurs.

• Field calibration is difficult due to hysteresis and drift.
• Standard Practice: Swap the probe with a refurbished/calibrated one from the factory and send the old one to the lab.
• Verification: A portable moisture analyzer (reference) is connected in series to verify readings.
• Single Point Adjustment: Some systems allow a "wet gas" or "dry gas" offset, but full recalibration requires a humidity generator in a controlled lab.

Sample lines are critical in trace moisture measurement.

• Adsorption: Water molecules stick to the walls of the tubing. Long lines act as a moisture reservoir.
• Desorption: If the process dries out, the tubing walls release moisture, causing the analyzer to read "wet" long after the process is dry.
• Mitigation: Use Electropolished Stainless Steel tubing (reduces surface area), keep lines short, and heat trace the lines to prevent condensation and aid desorption.

A primary standard method used often in calibration labs.

• Principle: A polished mirror is cooled thermoelectrically (Peltier cooler) while a light shines on it.
• Detection: When the mirror reaches the dew point, condensation forms, scattering the light. An optical detector sees this change.
• Measurement: The temperature of the mirror at that exact moment is the fundamental Dew Point.
• Pros/Cons: extremely accurate but easily fouled by dirt or hydrocarbons; generally not for dirty process gas.

Yes, but the principle is slightly different.

• Aluminum Oxide sensors can be used in liquids (like Naphtha or Gasoline).
• They measure Henry's Law partial pressure of water vapor dissolved in the liquid.
• The output is typically converted to PPMw (Parts Per Million by Weight) using solubility constants (K-values) specific to that liquid hydrocarbon.

This is the traditional standard for trace H₂S.

• Principle: H₂S reacts specifically with Lead Acetate to form Lead Sulfide (brown stain).
• Process: A paper tape impregnated with lead acetate moves through an exposure window. Sample gas flows over it.
• Detection: The tape turns brown. An optical system measures the rate of darkening (color change).
• Reaction: Pb(CH₃COO)₂ + H₂S → PbS (Black/Brown) + 2CH₃COOH
• Pros: Specific to H₂S, no interference from other sulfurs.

• Conversion: First, all sulfur compounds (including H₂S) are oxidized in a high-temperature furnace (approx 1000°C) to form SO₂.
• Excitation: The SO₂ is exposed to UV light, exciting the molecules to a higher energy state.
• Fluorescence: As the molecules relax back to the ground state, they emit light at a specific wavelength.
• Measurement: A photomultiplier tube measures this emitted light, which is proportional to the sulfur concentration.

Similar to moisture TDLAS, but focused on H₂S absorption lines.

• Challenge: H₂S has weaker absorption lines in the Near-Infrared (NIR) spectrum compared to CO₂ or H₂O.
• Solution: Modern analyzers use "Herriott Cells" (multi-pass cells) where the laser bounces back and forth mirrors 50+ times to increase the path length (e.g., 30 meters path length in a 1-meter cell), increasing sensitivity to low PPM levels.

H₂S is lethal.

• Personal Monitor: Always wear a portable H₂S detector.
• Purging: Thoroughly purge the sample lines with Nitrogen before opening fittings.
• Scavenging: The vent of an H₂S analyzer must never vent to the atmosphere inside the shelter. It must be connected to a flare or a chemical scavenger (charcoal/permanganate filter).
• Leak Check: Use Snoop or detectors after reassembling lines.

In Differential Absorption analyzers or when measuring Total Sulfur vs H₂S:

• A scrubber is a chemical filter used to remove specific components.
• Example: To measure Total Sulfur minus H₂S, you might run the sample through a scrubber that removes only H₂S, measure the result, and subtract it from the total.
• Also used on vents to prevent toxic gas release.

• While H₂S itself is a gas, it is often measured in streams containing heavy hydrocarbons or moisture.
• If condensation occurs, H₂S is soluble in water and hydrocarbons. The H₂S will dissolve into the liquid condensate and disappear from the gas phase measurement (False Low Reading).
• Heating maintains the sample above the Dew Point.

• H₂S absorbs UV light strongly in the 200-300nm range.
• A source lamp sends light through a flow cell. A spectrometer separates the wavelengths.
• Challenge: Other compounds (Mercaptans, SO₂) also absorb in this region. Advanced algorithms (chemometrics) are required to deconvolute the overlapping spectra.
• Tail Gas: Very common in Sulfur Recovery Units (SRU) for measuring H₂S/SO₂ Ratio (Air Demand Analyzer).

• Gas: Direct measurement (Tape, TDL, UV).
• Liquid (e.g., Crude Oil, Water): Usually requires a "Headspace" or "Stripping" system.
  • The liquid is heated or sparged with a carrier gas (Nitrogen).
  • Henry's Law dictates that the H₂S will transfer from liquid to the gas phase.
  • The analyzer measures the H₂S in the gas phase, and the liquid concentration is calculated.

• Symptom: The tape motor runs continuously, wasting tape.
• Cause: High concentration of H₂S causing the tape to darken instantly, or a dirty optical gate/sensor. The system keeps advancing tape trying to find a "clean" spot to start the measurement cycle.
• Fix: Check sample dilution (range is exceeded) or clean the optical transmitter/receiver photocell.

• It is generally very specific. However:
• Methyl Mercaptan: Can cause a slight yellow stain which might be read as H₂S.
• Extreme Humidity: Can affect the tape reaction rate or cause mechanical jamming.
• Hydrogen: High concentrations of H₂ can sometimes dry out the tape chemistry.

• Zero Calibration: Sets the "bottom" of the scale. Used to correct the offset error. (e.g., using pure N2 for an O2 analyzer).
• Span Calibration: Sets the "slope" or gain of the analyzer. Used to correct sensitivity errors. (e.g., using 100ppm H2S gas for a 0-200ppm range analyzer).
• Order matters: Usually, Zero is adjusted first, then Span.

A standard metric for analyzer speed.

• It is the time taken for the analyzer reading to reach 90% of the final value after a step change in concentration is applied at the inlet.
• Example: If you switch from 0 to 100ppm, T90 is the time it takes to hit 90ppm.
• Includes both the sensor response and the sample transport lag time.

• Usually, we regulate pressure upstream to protect the analyzer.
• However, for Trace Moisture or reactive gases, reducing pressure upstream can cause Joule-Thomson cooling, potentially causing condensation.
• Also, reducing pressure increases the volumetric flow velocity, but if you measure at atmospheric pressure, putting the regulator downstream (backpressure regulator) keeps the sample under pressure through the lines (preventing ingress of ambient moisture) until the very end.

• A sampling technique to reduce lag time.
• A high flow rate is drawn from the process tap to the analyzer shelter and immediately returned to the process (bypass).
• A small "slipstream" is taken from this fast moving loop to the analyzer.
• Ensures the sample at the analyzer is fresh and representative of the current process conditions.

• A filter designed to remove liquid mists (aerosols) from a gas stream.
• Operation: Fine fibers capture small droplets, which merge (coalesce) into larger drops and drain down by gravity.
• Importance: Vital for protecting optical analyzers and gas chromatographs from liquid carryover.

• Adsorption: Plastic absorbs moisture and H₂S, causing memory effects.
• Reaction: Copper reacts with H₂S and Ammonia (corrosion).
• Integrity: SS316 offers high pressure handling and corrosion resistance.
• For H₂S and low-level Sulfur, SilcoNert (sulfur-inert coated) SS tubing is often required.

1. Check the rotameter (flow meter) physically.
2. Check sample supply pressure at the tap.
3. Check filters (particulate and coalescing) for plugging.
4. Check pumps (diaphragm rupture is common).
5. Check for frozen lines or hydrate formation (in cold weather or high pressure drops).
6. Verify the flow switch/sensor itself is not faulty.

• It is used to protect gas analyzers from liquids.
• Mechanism: Uses a hydrophobic membrane. Gas molecules pass through, but liquid molecules (water/hydrocarbon) are blocked due to surface tension.
• Unlike a coalescer, it acts as a final barrier stop.

• Volume inside the tubing, filters, and fittings that is not actively swept by the flow path.
• Problem: Traps old sample, causing slow response times and cross-contamination when switching streams.
• Fix: Design systems with minimal tees and dead legs; use block-and-bleed configurations.

• Mixtures, especially those with heavy hydrocarbons or trace components (ppm level), can stratify over time (heavier molecules settle, though gas laws suggest mixing, surface adsorption plays a role).
• Standard practice is to ensure the mix is homogenous before use, especially if stored for long periods in cold weather.
• For reactive gases (H2S), check the expiry date. H2S concentration drops over time as it reacts with the cylinder walls.

• Analyzers used for Safety Instrumented Systems (SIS) (e.g., closing a valve if O2 is too high) must be SIL rated.
• It defines the reliability and Probability of Failure on Demand (PFD).
• Requires redundant analyzers (e.g., 2oo3 voting) and strict proof testing schedules.

• A device that uses the Venturi effect to create vacuum.
• Used to draw sample from a low-pressure process (like a stack) without moving parts (pumps).
• Motive air (instrument air) speeds through a constriction, creating low pressure that sucks the sample in.
• Advantage: Highly reliable, no electrical parts.

• % Volume: Absolute concentration (e.g., 5% Methane).
• % LEL (Lower Explosive Limit): A safety scale. 100% LEL is the concentration where the gas becomes explosive.
• Example: Methane LEL is 5% Vol. Therefore, 2.5% Vol Methane = 50% LEL.
• Combustion analyzers (Catalytic bead) measure LEL. IR analyzers often measure Vol.

• The electronics (light source and detector) are kept in a safe area or central control room.
• Only the passive sensor head (optics) is in the hazardous field area.
• Reduces electrical noise (EMI) and eliminates the need for explosion-proof housings for the main unit.

• Maintains sample temperature above the Dew Point or Bubble Point.
• Prevents heavy hydrocarbons from polymerizing or clogging lines.
• Cold Spots: Even a 1-inch uninsulated gap can cause condensation, leading to erratic analyzer readings ("spiking").

• A configuration used to prevent cross-contamination between different streams connected to one analyzer.
• Ensures that if one valve leaks, it leaks to a vent (bleed) rather than contaminating the sample going to the analyzer.
• Essential for multi-stream analyzers.

• Poisoning agents: Silicones, Lead, Sulfur.
• Symptom: The sensor will zero correctly but will have very low sensitivity to span gas (e.g., reads 10% when exposed to 50%).
• Test: Apply span gas. If the response is sluggish or fails to reach value, the catalyst is dead. It cannot be cleaned.

• Often grouped with O2 analyzers in CEMS (Continuous Emission Monitoring Systems).
• Reaction: NO reacts with Ozone (O₃) to form NO₂ in an excited state.
• Light: As the excited NO₂ decays, it emits light.
• Detection: Intensity of light is proportional to NO concentration.

• Accuracy: How close the measurement is to the true value (requires a certified standard).
• Repeatability (Precision): The ability of the analyzer to give the same result under the same conditions multiple times.
• In process control, Repeatability is often more important than absolute Accuracy.

• Generally maintenance-free regarding the sensor element itself.
• Key Tasks: 1. Verify zero/span regularly. 2. Protect from liquid carryover (liquids destroy the suspension). 3. Protect from overpressure (can snap the suspension wire). 4. Maintain the sample conditioning system (filters).