Gas Detector Sensor Technology

Joel Myerson – President

Safety Inc – ESafetyInc.comETA Process Instrumentation – etapii.comMartech Controls – MartechControls.comiFacilityServices – iFacilityServices.com

joel@ESafetyInc.com

Standard technology and new developments

• Electrochemical (EC) Sensors • Catalytic Bead (LEL) Sensors• Infrared (IR) Sensors• Photo Ionization Detectors (PID)• Ceramic Metal Oxide (CMOS)• Photoacoustic Infrared

SERVICES WE PROVIDE:- On-site or in our state of the art calibration lab- Calibration and repair of portable and fixed instruments- Respirator maintenance - SCBA, PAPR, airline, breather boxes- Level A suit testing - to ASTM F 1052 Pressure Test Method- Fall protection equipment inspection - Competent Person Fall Protection- Equipment Inspections per ANSI Z359.1-1992 requirements

PRODUCTS CATEGORIES:On-Line Analyzers - For gases and liquids. Technologies: electrochemical, catalytic, infrared, spectroscopy, spectrometry, chromatography, fourier transform infrared.Process Control and Measurement - Flow, pressure, temperature, humidity, moisture, prowe, level.Instrumentation - Gas detection, sound, noise, dust, relative humidity, combustion efficiency,Indoor air quality, calibration gases and regulators.Safety Equipment - Personal protective equipment, fall protection, first aid, spill control

Safety equipment and portable instruments - Safety Inc wws,ESafetyInc.comProcess Instrumentation :- New England - ETA Process Instrumentation www.etapii.com- Upstate NY - Martech Controls www. martechcontrols.comCalibration and Repair services - iFacility Services www.iFacilityServices.com

Standard 2 ElectrodeElectrochemical Sensor – working electrode and anode

3 Electrode EC Sensor

Temperature Sensor

3. Counter Electrode

Electrolyte.Typically Sulph. acid in water or a salt solution, or an organic electrolytewithout water

1. Working Electrodew/noble metal catalyst Replaceable Dust or

Selective Filter

Porous Teflon Housing

2. Reference Electrode (anode)

Pressure Release

Porous WickEEPROM

Connection Header

2 Electrode vs 3 Electrode EC SensorOxygen example

• Consumptive 2 Electrode Sensor• Two electrode standard electrochemical sensor - 12-18 month life• Oxygen is reduced to hydroxyl ions at the cathode: • Cathode reaction O2 + 2H 2 → O + 4e- 4OHHydroxyl • Ions oxidize the (lead) anode: 2Pb + 4OH → 2PbO + 2H2 O + 4e¬• Overall cell reaction: 2Pb + O2 → 2PbO

• Non-consumptive 3 Electrode Sensor• 3-electrode technology – 60-84 month life• O2 + 4H+ + 4e– → 2H2O at the working electrode, and • 2H2O → O2 + 4H+ + 4e– at the counter electrode

EC issues

Reasons EC sensors fail or don’t perform to spec:• All of Pb anode converted to PbO (in 2 electrode)• Electrolyte poisoned by exposure to contaminants:• High concentrations of acid gases - H2S and CO2• Solvents – cross contamination• Electrolyte leakage• Desiccation – electrolyte dries out• Excessive heat and humidity – pay attention to specifications• Blockage of capillary pore• Calibration isn’t done or isn’t done correctly

Typical degradation curve for an EC sensor

EC sensors – what to look for• Sensor Size – larger size = more electrolyte = longer life.

Many fixed sensors are portable sensors in a housing (i.e. same sensor used in both portable and fixed instruments)

• Accuracy and Stability• Cross Sensitivity – data on interference from other gases• Detection limits• Temperature range• Diagnostics. How do you know when to replace?• Calibration Interval• Warranty length• Warranty provisions – is it pro-rated, or a true

replacement warranty?

Smart sensors – predictive maint.With the addition of a micro chip in the sensor, a series of self tests are possible such as:• Electrolyte Level • Logging of Gas Concentrations over time• Temperature Real Time for compensation as well as over time• Sensitivity • Resistance, etc..• Any parameter out of check will signal a calibration, warning or faultTrue predictive maintenance sensors have the ability to start sending a warning signal 30-60 days before a sensor fails:• Lower cost of ownership• Sensors are replaced when they are nearing end of life, not proactively• Costs associated with down time are avoided• Sensor replacement can be scheduled, not done as an emergency

Sensor specifications – details matter! Measuring range limit 100 ppm Detection limit 1 ppm Alarm response / on gas exposure with 5x alarm threshold t 0-20 5 sec / on gas exposure with 1.6x alarm threshold t 0-63 30 sec Calibration interval - default 6 months, range min./max. 1 day/12 months

Warm up time

Ready for use after max. 30 minutes, Ready for calibration after max. 120 minutes

Measurement accuracy

Measurement uncertainly (of meas. value) < ±5%

Minimum < ±1.5 ppm

Loss of sensitivity, per year <-15%

Expected service life, in ambient air >24 months

Ambient conditions

Temperature, min./max. -40/65 °C

Rel. humidity, min./max. 15/95%

Ambient pressure ±3%

Cross sensitivities exist. Contact manufacturer

Ammonia monitoringPersonnel protection, workplace monitoringFor protecting workers in the plant and or visitors in buildings portable and fixed instrumentation can be used. The measuring range should match the TLVrequirements of the applied regulation.Compressor and machinery room and ammonia tanks leak detectionThe machinery room houses the refrigeration system. Containing compressor, valves, pumps surge tanks and many joints it is prone to leaks. Small leaks might cause a constant background concentration. The instrumentation should monitor the range beyond the TLV levels to alert the control room and trigger ventilation and switch counter measures.Cold storage room leak detection and tainting protectionA tiny Ammonia leak can have a damaging effect on stored goods. Small concentrations can taint food or other refrigerants or causes corrosion. Concentrations to be monitored are in the range up to TLV for getting early warnings. De-icing is an important process, where fast and reliable monitoring is requested to alarm emerging leaks.Local requirements regarding performance, alarm-thresholds, approvals result from different standards for Ammonia handling, cooling and refrigeration, transportation of dangerous goods, pressurized gases, workplace monitoring.

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Gas Sensors

– Electrochemical - ppm• Electrochemical sensors are

particularly suited to the detection of toxic gases in the lower ppm range

– Catalytic & IR - % LEL• IR and catalytic methods are

used to detect combustible substances in concentrations below the lower explosive limit

Two methods of measurement have firmly established themselves over many years

Ammonia exposure limits

• TWA (Time Weighted Avg)– Niosh 25 ppm 8 hr exposure level – OSHA 50 ppm 8 hr exposure level35 ppm

• STEL (Short Term Expsosure Limit)15 minute exposure limit) – NIOSH 35 ppm

• IDLH (Immediately Dangerous to Life and Health)300 ppm (NIOSH)

• 500 ppm IDLH (OSHA)• 1720 ppm Convulsive Cough – Fatal <30min.• 5000 ppm Respiratory Spasm – Rapidly fatal.

Installation Considerations

• In cooling areas, locate sensors near control / piping end of evaporators and valve stations. Do not mount in front, back or on top of evaporators.

• When installing sensors near evaporators, keep sensor out of direct airflow from and to the evaporator and away from any moisture created during defrost.

• Make drip loops for cables on the transmitter. • Do not mount the transmitter over door in refrigerated area, because the sensor

will become a chunk of ice.• Seal all conduit connections • In compressor rooms the transmitter should be installed at the ceiling. Ammonia is

lighter than air and will rise first. So even if you don’t smell Ammonia at ground levels the concentration at the ceiling can be high.

Catalytic Sensors – measuring for combustion or flammability

• Lower explosive limit (lel) - Minimum concentration of a combustible gas or vapor in air which will ignite if a source of ignition is present

• Upper explosive limi (uel) - most but not all combustible gases have an upper explosive limit– Maximum concentration in air which will support

combustion – Concentrations which are above the uel are too “rich” to

burn

• Flammability range – between LEL and UEL

Catalytic Bead Sensor

Catalytic Bead rules of thumb

Catalytic Bead rules of thumbRule 2: The more molecules react the higher the reaction rate, the higher the temperature increaseRule 2a: Anything forcing the molecules to move slower will reduce the reaction rate – sensor sensitivity decreases. Example: diffusion rates through sintered discs, filters, contaminated sinter, etc.Rule 3: Oxygen concentration must be higher than 10-12 %/volumeNeed enough oxygen for catalyzing of combustible gas – can’t support combustion on the active pellistor without enough O2.Rule 4: Catalyst poisoning reduces the reaction rate - Activated oxygen cannot form on poisoned surfaces.- Too few reactions produces too little heatLess reactions per time = less power = less temperature increase.

Issues with single bead catalytic sensors

• Using two different pellistors creates a zero drift/noise issue. Each pellistor degrades at a different rate, changing difference between the two.

• Require frequent calibration due to above (drift)

• Sintered disk is susceptible to poison and reactive gases.

• Sintered disk delays the response time since molecules have to travel a longer distance to reach the pellistor.

• Sensors typically cannot be used in applications where the physical orientation of the sensor may be variable (i.e. airplanes, ships, etc.)

New two active bead catalytic sensor

Two active bead advantages

• Optimum compensation – both of the sensor’s pellistors, and measuring and compensating elements, are part of a Wheatstone bridge and are completely uniform

• Both are active, both degrade at about the same rate• Faster response times - using a wire mesh disc as the gas

entrance and flame arrestor will make the diffusion paths noticeably shorter so that the response times increase

• Independent of orientation • Improved poison resistance – new ceramics and catalyst

Catalytic advantages and disadvantagesAdvantages:• Low up front cost• Can measure %LEL H2• Small physical footprint

Disadvantages:• Long term costs• Finite lifetime• High frequency of calibration (for standard single active pellistor sensors)• Should calibrate with measurement gas• No smarts built into sensor (no calibration curves)

What to look for:• Long term stability (zero drift)• Calibration interval requirement• Position requirement• Speed of response

Infrared (IR) sensors for combustibles

• Infrared technology is based on the scientific principle that specific gases absorb infrared light at specific wavelengths.

• Identify and quantify gas volume by measuring infrared light absorption.

• Specific gases can be selectively targeted by narrowly filtering the wavelength of the infrared light introduced, thereby avoiding any infrared absorption by other non-targeted gases that may be present in the sample.

• Higher concentrations of gas will absorb more IR light energy.

Infrared (IR) sensors

Single source 2 beam IR sensor

Dual Source 4 beam IR Sensor

4 Beam Advantages

IR vs. Catalytic

Infrared advantages and disadvantagesAdvantages:• Extremely stable measurement• Non consumptive• Calibration usually not necessary or can be extended.• Low long term costs• Can measure %LEL, PPM, %Vol depending on instrument used• Gas Library – depending on instrument used• No false zero vs catalytic – i.e. zero reading is all ir light getting to the receiver (100% of light)

Disadvantages:• Higher initial cost than a catalytic sensor• Cannot measure H2

What to look for:• Gas Library, Measure one gas, calibrate with another.• Water proof• Ease of configuring• Accuracy• Temp. coefficients• Dust effects

Toxic vs Explosive – protection vs combustion100 LEL % = 1,000,000 ppm

PPM = LEL % by Vol x % LEL . Toluene 1.1% LEL x 100% LEL = 11,000 ppm

GAS LEL % by vol % LEL* PPM** OSHA exposure limits***

Toluene 1.1% 100% LEL = 11,000 ppm IDLH 500 ppm

10% LEL = 1,100 ppm TWA 100 ppm

Xylene 0.9% 100% LEL = 9,000 ppm IDLH 900 ppm

10% LEL = 900 ppm TWA 100 ppm

MEK 1.4% 100% LEL = 14,000 ppm IDLH 3000

10% LEL = 1400 ppm TWA 300 ppm

Ethylene Glycol 1.8% 100% LEL = 18,000 ppm IDLH not specified

10% LEL = 1,800 ppm TWA 50 ppm

NIOSH pocket guide

https://www.cdc.gov/niosh/npg/ - Toluene• Synonyms & Trade Names Methyl benzene, Methyl benzol, Phenyl methane, Toluol• CAS No. 108-88-3 RTECS No. XS5250000 DOT ID & Guide 1294 130• Formula C6H5CH3 • Conversion 1 ppm = 3.77 mg/m3• IDLH 500 ppm• NIOSH REL TWA 100 ppm (375 mg/m3) ST 150 ppm (560 mg/m3) • OSHA PEL TWA 200 ppm C 300 ppm 500 ppm (10-minute maximum peak) See Appendix G• Physical Description - Colorless liquid with a sweet, pungent, benzene-like odor.• Molecular Weight 92.1• Boiling Point 232°F• Freezing Point -139°F • Solubility (74°F): 0.07%• Vapor Pressure :21 mmHg• Ionization Potential - 8.82 eV• Specific Gravity - 0.87• Flash Point - 40°F• Upper Exposive Limit - 7.1%• Lower Explosive Limit- 1.1%

Combustible curves vs multipliers

Carbon Dioxide – CO2

Ambient background in air – 350-450 ppm

IDLH - 40,000 ppm (4%)

Exposure LimitsNIOSH TWA 5000 ppm , ST 30,000 ppm

CO2 detection with IRMeasuring ranges 0 to 10 vol. % (Standard)

0 to 2,000 ppm … 30 vol. % (configurable)Display Backlit graphic LCD; 3 Status LEDs (green/yellow/red)Normal operation 4 to 20 mAMaintenance Constant 3.4 mA or 4 mA ±1 mA 1 Hz modulation; (adjustable)Signal output analog Fault < 1.2 mAPower supply 10 to 30 V DC, 3-wirePower consumption (max.) w/o relay, non-remote 300 mA at 24 V

w/ relay, remote 350 mA at 24 VElectrical dataRelay specification (option) 2 alarm relays and 1 fault relay, single-pole two-way contact 5 A @230 VAC, 5 A @ 30 VDC, resistance-boundTemperature -40 to 77°C (-40 to 170°F) without relay-40 to 70°C (-40 to 158°F) with relayPressure 20.7 to 38.4 inch Hg / 700 to 1,300 mbarEnvironmental conditionsHumidity 0 to 100% r. h., non-condensingTransmitter housing Epoxy coated copper-free aluminum or stainless steel SS316

Photo Ionization Detector (PID)

• PIDs use ultraviolet light as source of energy to remove an electron from neutrally charged target molecules creating electrically charged fragments (ions)

• This produces a flow of electrical current proportional to the concentration of contaminant

• The amount of energy needed (in electron volts eV) to remove an electron from a particular molecule is the ionization potential (or IP)

• The energy in eV must be greater than the IP in order for an ionization detector to be able to detect a particular substance

PID• IP determines if the lamp can detect the gas

• If the IP of the gas is less than the eV output of the lamp the PIDcan detect the gas

• IP is a measure of the bond strength of a gas and does not correlate with the Correction Factor

• Ionization Potentials are found in the NIOSH Pocket Guide

• PIDs cannot distinguish between different contaminants they are able to detect anything present with an IP below the eV rating of the lamp will be ionized.

• Single total reading for all detectable substances present

What PIDs can and can’t measureCommon Gases Detectable by a PID1. All hydrocarbons, whose chemical names end in –ane, -ene or –yneexcept: Methane & Ethane2. All alcohols, whose chemical names end in –ol3. All aldehydes, whose names end in aldehyde4. All ketones, whose names end in –one5. All esters, whose names end in -ate

What PID’s do not measure…1) Radiation2) Air (N2, O2, CO2 & H20)3) Common Toxics (CO, HCN & SO2)4) Natural Gas (Methane and Ethane)5) Acid Gases (HCL, HF & HNO3)6) Others: Freons, Ozone (O3), Hydrogen Peroxide7) Non-Volatiles: PCBs & Greases

PID sensor

Ionization potentials

Quantified in electron volts (ev) – IP is the amount of uv light required to ionize a chemical

IP in evAcetaldehyde 10.23Acetamide 9.77Acetic acid 10.37Acetic anhydride 10Acetone 9.67Acetonitrile 12.22Acetophenone 9.27Acetyl bromide 10.55Acetyl chloride 11.02Acetylene 11.41Acrolein 10.1

Correction factors – lamps are 11.7, 10.6, & 9.8 eV Typically calibrated to Isobutylene – response 1:1

PID advantages and disadvantagesAdvantages:- Flexible - can measure hundreds of gases - Detection limits of ppm, and sometimes ppb, levels- Stable, easy, inexpensive calibration with isobutylene- Multiple lamps – can limit detection of some gases by choice of lamp

Disadvantages:- Non-specific – any gas present that has an ionzation potential below the power of the lamp will be ionized, reading is cumulative- Multiple lamps – need to choose correct lamp - Correction factors – different for every chemical, and for each type of lamp

Halocarbon refrigerants

Halocarbon refrigerants are colorless, odorless and, in general, boil at or below room temperature. They are high molecular weight compounds with vapor densities three to five times that of air.When refrigerants escape from closed systems, they can be expected to settle into low lying areas (in the absence of air movement).

Halocarbon refrigerantsIn the past, notification of refrigerant loss has been by pressure, temperature, or liquid level sensor alarms caused either by inability of the system to maintain temperature or by mechanical malfunction of the equipment. Perhaps 20 to 40% of the total refrigerant in a system can be lost before temperature, pressure, or liquid level sensors alarm. This can amount to hundreds and sometimes thousands of pounds of refrigerant being lost.

Halocarbon refrigerantsDetection through leak monitoring has the ability to detect the presence of minute concentrations (parts-per-million) in the air of enclosed spaces thereby providing early and rapid notification that a leak exists.In most circumstances, refrigerant loss can be reduced significantly by monitoring. For example, in a 10,000 cu. ft. 90°F machine room, the loss of less than one pound of R 12 would trip a 300 ppm alarm. If the leak rate was 1/2 lb/min., an alarm would be given in less than 2 minutes.

Sensor technology optionsSensor technology selection criteria include the requirements for gas selectivity, sensitivity, and the speed of response. There are tradeoffs between all three of these issues and instrument cost-effectiveness.

The two most widely applied technologies for refrigerant leak monitoring are IR (infrared) and CMOS (ceramic metal oxide semiconductor).

Infrared technology is based on the absorption of a specific wavelength of IR light by the refrigerant molecule.

CMOS technology is based on the change in conductivityof sensitized metal oxides upon exposure of refrigerant gases.

Ceramic Metal-Oxide Semiconductor (CMOS)

• CMOS is an older but still useful technology for refrigerant leak detection.

• CMOS technology is based on the change in conductivity of sensitized metal oxides upon exposure to refrigerant gases.

• While CMOS systems perform adequately to satisfy the safety requirements of all refrigerants, IR technology is recommended for toxic applications, refrigerant conservation efforts, or in areas where cross contamination by extraneous chemicals is a factor.

IR options – NDIR & Photoacoustic IRNon-dispersive (NDIR) or absorptive infrared systems determine gas concentration by comparing an air sample to a sample of inert gas (usually nitrogen) stored in the monitor. The room sample and reference gases are individually irradiated with infrared light. The light absorption measured in each gas sample is compared, and the difference between the two measurements determines the concentration.Inaccuracies with this technology can occur when the assumed zero ·baseline measurement of the reference gas drifts due to changes in room temperature and atmospheric pressure, the aging of the infrared light source, or contamination of the reference cell.Systems must be pulled of/line to recalibrate the instruments, and such downtime can be costly.

Photoacoustic IRPhotoacoustic infrared avoids the problems associated with zero drift by eliminating the need to compare results to a reference sample, and the associated downtime required for recalibration.PIR devices operate on the principle of sound. IR absorption causes the molecules of the gas to be excited to a higher vibration energy state and increases the pressure inside the instrument. This pressure creates an acoustic wave, which can be detected by an extremely sensitive microphone.Photoacoustic infrared devices direct measure the changes in pressure that occur when infrared light is absorbed by the refrigerant present in the sample. The greater the pressure, the greater the concentration. Conversely, when no pressure pulse occurs, then no gas is present.

Photoacoustic IR

CMOS vs Photoacoustic IRApplication Factors To Consider for Refrigerant Monitors:

Sensitivity- Different Refrigerant Safety Groups- Different Purpose of Installation

Selectivity- Different Refrigerant Safety Groups- Different Refrigerants- Different Types of Equipment Present

Speed of Response- High Pressure/low Pressure- Big Rooms/small Rooms

CMOS vs Photoacoustic IRP Infrared CMOS

Sensitivity 1-100ppm 20-30ppm HCFC’s30-40ppm HFC’s 40-100ppm CFC’s

Selectivity Selective Non-selective

Cost Higher ($3-10K) Lower (<$3K)

Calibration reference only every 3-6 months

Sensor life long short

QUESTIONS??

Joel Myerson – President

Safety Inc – ESafetyInc.comETA Process Instrumentation – etapii.comMartech Controls – MartechControls.comiFacilityServices – iFacilityServices.com

joel@ESafetyInc.com