Ametek Process Instruments - THERMOX

Model 933 - H2S in Natural Gas



The Model 933 is a unique UV-based photometric analyzer system for H2S analysis when present in concentrations below 100 ppm in natural gas and other applications. The analyzer responds quickly to changes in H2S concentration and it quickly self-recovers from extreme upset conditions.

  • Extended, unattended operation for up to 6 months or longer
  • Self-recovery after high concentration H2S events
  • Fast response time to increasing or decreasing H2S concentrations
  • Concentration measurements of carbonyl sulfide (COS) and methyl mercaptan optionally available
  • No consumables, reagents, or disposables other than zero gas

This is a low maintenance, reliable H2S analyzer in that reduces the high expenses associated with frequent scheduled and unscheduled maintenance.

The Model 933 has a unique sample conditioning system that uses frontal elution chromatography to separate and eliminate interfering species. This ensures an accurate analysis of the gas via direct-UV absorption spectroscopy. Two self-regenerating columns are employed in the 933. While one column is conditioning the gas sample, the other is automatically regenerated.

  • Methodology: Proprietary frontal elution sampling; nondispersive ultraviolet analysis for hydrogen sulfide (H2S), carbonyl sulfide (COS) and methyl mercaptan (MeSH)
  • Full Scale Ranges: ppm ranges are standard; mg/Nm3 and other ranges are available
  • Standard Range: ±2% of full scale
  • H2S: 0 to 25 ppm up to to 1000 ppm max. (Secondary higher ranges available.)
  • COS option: 0 to 100 ppm min. to 0 to 500 ppm max.
  • MeSH option: 0 to 50 ppm min. to 0 to 250 ppm max.
  • Higher ranges are available upon request

Low Range

  • H2S: 0 to 3 ppm up to 50 ppm (Secondary higher ranges available.)
  • COS option: 0 to 25 ppm up to 250 ppm
  • MeSH option: 0 to 15 ppm up to 100 ppm


  • Standard range: ±2% of full scale
  • Low range: ±5% of full scale


  • Standard range: ±2% of full scale

Zero Drift

  • Standard range: less than ±2% of full scale in 24 hours
  • Low range: less than ±5% of full scale in 24 hours
  • Response Time, Excluding Sampling System
  • H2S: less than 30 seconds to 90% response
  • COS: less than 60 seconds to 90% response
  • MeSH:
    • less than 180 seconds to 90% response
    • Higher ranges for H2S, COS or MeSH are available upon request.
  • Sensitivity: 0.5% of full scale
  • Cross-interference: H2S concentration measurement—less than 2% of the sum of COS and MeSH
  • Sample Stream Requirements:
    • greater than 80% methane, less than 10% ethane, less than 3% propane, less than 1.25% total butanes, less than 0.5% C5+. Maximum water content is 0.6%.1.
    • Applications that exceed the composition limits possible, but must be reviewed by the factory as different specifications may apply.
  • Zero Gas:
    • Instrument zero purity carbon dioxide; UHP nitrogen, or UHP methane. Minimum auto-zero interval is once per 24 hours.
  • Process Pressure Requirement: 830 kPag to 13790 kPag (120 psig to 2000 psig)
  • Maximum Sample Cell Pressure: 100 psig (6.9 barg)
  • Maximum Sample Gas Temperature: 50°C (122°F)
  • Typical Flow: 2.5 L/min. (5 SCFH)
  • Outputs: Up to 4 isolated 4-to-20 mA, loop or self-powered; 4 non-isolated 1 to 5 VDC; 5 independent sets of SPDT, Form C, potential free alarm relay contacts, 2 A at 240 VAC
  • Digital Communication: RS485 Modbus port; RS232/RS485 service port
  • Power Consumption: <500 W (excluding sample and vent line)
  • Power:
    • 104 to 132 VAC, 47 to 63 Hz, <3A
    • 207 to 264 VAC, 47 to 63 Hz, <2A
  • Ambient Temperature: 0°C to 50°C (32°F to 122°F)
  • Dimensions: HxWxD 1185 x 780 x 254 mm (46.65 x 30.7 x 9.97 in.)
  • Weight: Approximately 100 kg (220.5 lbs)

  • CEC Class I, Division 1, ExD B,  Groups C & D; Ex d IIB T3
  • Class I, Division 1, Groups B, C&D / Class I, Zone 1, AEx d IIB+H2, T3
  • CEC/NEC Class I, Division 2, Groups B, C&D, ExP T3 (unit verification)
  • ATEX II 2 G Ex db IIB+H2 T3 Gb 
  • EMC
  • Russian Ex Proof Certification; 1ExdIIBT3 X
  • Russian Gosstandart Pattern Approval
  • Complies with all relevant European directives

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