Transportable H2S Analyzer – Model 331

331 Transportable H2S Analyzer

331 Transportable H2S Analyzer

This simple design by Envent Engineering utilizes a lead acetate based detection system that provides a linear and interference-free output of the H2S or optional Total Sulfur concentration.


  • Ranges from 0-50 ppb to 0-100%
  • 1.5% accuracy, 1% repeatability
  • Modbus Serial communications,
  • Displays up to 5 times full scale
  • Dual Isolated 4-20 mA outputs
  • Less than 20-second response time to alarm
  • Optional Total Sulfur and CO2
  • Interference-free Lead acetate sensor
  • 60-90 day tape (longest tape life in the market)
  • Zero sensor drift

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Log On Procedure

Accessing Factory View

Retrieving Archives

Changing Alarm

Latching Outputs

Adjusting Analog Output range

Polling and Synchronizing Clock

Firmware Upgrade

Application Page


H2S Analyzer Maintenance Tutorials

Frequently Asked Questions


  1. What Cal-Gas do I need?
  2. How often should I calibrate the H2S analyzer?
  3. How often should I change the sample system filter?
  4. How much is the analyzer venting?
  5. What should the sweep valve be set at?
  6. How long will the tape last?
  7. Does the Bypass time out?
  8. How often does the humidifier need to be filled?
  9. What are the analog output settings?
  10. What is the range of the H2S analyzer?
  11. Sensor Re-Zero Procedure
  12. What is the gain of the analyzer supposed to be?
  13. Troubleshooting Guide


1. What Cal-Gas do I need?

Ideal Cal-Gas concentration would at the value normally operating at or alarming at. Alternatively Cal-Gas to use is 3/4 concentration of the range of the analyzer.

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2. How often should I calibrate the H2S analyzer?

Calibration checks should be done every 3 to 6 months. Or as required by site preventive maintenance schedule.

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3. How often should I change the sample system filter?

The Filter should be changed as required. The frequency is dependent on the liquid content and particulates in the gas. A higher volume of a liquids and particulates will require more frequent changes. Filters should be checked when analyzer is calibrated.

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4. How much is the analyzer venting?

The standard H2S analyzer vents 4 standard cubic feet per hour.

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5. What should the sweep valve be set at?

The sweep valve should be just opened slightly until flow can be felt from the tubing. This will reduce lag time and allow liquids to be removed.

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6. How long will the tape last?

The analysis time is dynamic and changes with the concentration of the H2S during the sample.

Aperture H2S (ppm) Tape Life (Days)*
None 4.0 ppm 34.7
1/16th 16.0 ppm 30.4

*Tape life based on constant concentration sample over a averaged analysis time with a 300 ft tape.

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7. Does the Bypass time out?

By default the H2S analyzer does NOT time out when placed in bypass. A time out feature is available in the software, as well as Bypass Disable.

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8. How often does the Humidifier need to be filled?

As long as there is some fluid in the humidifier it will be able to maintain the humidity of the gas. The fluid should be topped up each time the analyzer is calibrated or the tape is changed.

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9. What are the Analog Output settings?

4-20mA wiring

The analog output of the H2S analyzer is a 4-20 mA signal calibrated at factory. The connections requires loop power from the PLC. If loop power is not able to be supplied, a powered AO board addition can be purchased.

Symptom Solutions
PLC Indicating 0 mA Wiring to analyzer is incorrect.
  Power to loop is not present. If loop power not supplied then Powered AO board option may be required.
PLC Indicating ~2 mA Analyzer in Bypass
PLC Indicating higher or lower signal than analyzer display Scaling of analyzer is different setting on PLC

Click here for typical Analog Output wiring diagrams

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10. What is the range of the H2S Analyzer?

Determining the range

The range of the analyzer is set at the time of purchase. The accuracy and tape life are optimized to the customers request. The optimized range can be determined by the calibration sheet found in the back of the manual.

The Envent H2S analyzer is capable of indicating 10x the optimized range of H2S concentration. Accuracy of the reading can not be guaranteed when indicating outside the optimized range.

Optimizing the range

Range (ppm) Aperture
0-10 None
0-20 * 1/16th
0-50 1/32nd
0-100 Consult Envent Eng.

* If the shut in alarm value is less then 10 ppm no aperture is ideal.

For dilution analyzer systems the range adjustment depends on the dilution tubing. Please consult Envent about dilution range adjustments

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11. Sensor Re-Zero Procedure.

The sensor Fault is a result of the sensor mV level being below 500 mV or above 1200 mV. Most often caused by an incorrect zero being achieved.

  1. Put the Analyzer in Bypass
  2. Turn off sample gas at flowmeter
  3. Press the Menu/Set button until MTR RUN displayed
  4. Press Left or Right on the Display to move the tape to a new white piece of tape
  5. Remove the cover the sensor block. Press the small pushbutton located on the lower left side next to the wire connector. The sensor block will enter a “re-zero” procedure indicated by a red led.
  6. When the “re-zero”procedure is complete the green led should illuminate.
  7. Initiate another motor run
  8. Press the Menu/Set button until zero is displayed. Value should be between 900 & 1100 mV.
  9. Turn on sample gas to flow of two
  10. Take analyzer out of Bypass.

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12. What is the gain of the analyzer supposed to be?

Aperture Gain*
None 2.5-3
1/16th 7.5-8.5
1/32nd 10-13

*Gain numbers based on standard H2S analyzers. Numbers do not apply to Total Sulfur and Dilution systems. Please consult factory cal-sheet for calibration numbers.

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13. Troubleshooting Guide

Problem Possible Reasons Solutions
Erratic H2S Readings Trigger slide not seated properly Ensure Trigger slid is seated in the groove of the sample chamber
Pressure in building moving up and down from fan, exhaust or wind Eductor may be plugged or bent blocked. Check that all vent tubing and fittings are 316 stainless steel, sized 3/8″ or larger.
Sample vent blocked or insufficient. Vent should be 1/2″ tubing on a downward slope. Ending at shepards hook. Possible heat trace required on tubing
Liquid carry over in sample conditioning system Sample conditioning system requires cleaning. Refer to Cleaning Procedure
Regulator not maintaining 15 psig Pre-regulation to 50 psig of sample at sample point. Possible regulator requires repair or replacement.

Hydrocarbon liquid carried over through the sample regulator. Heated regulator may be required.

Contaminants in sample system Sample system has been flooded or contaminated. Sample system requires cleaning. Refer to Cleaning Procedure.
Analog Input 2 jumper removed Re-install jumper in Analog Input across (+4-20 & -4-20).
Sensor Block Fault Re-zero Sensor block. Refer to Sensor Re-Zero Procedure. Check for green light on sensor block.
Contaminants in sample chamber Clean sample chamber. Replace aperture and window if required. Clear grease from window.

Envent no longer uses grease on chamber window. Applies to early H2S analyzers

Aperture Strip became dislodged Reattache aperture strip with RTV 102 silicone.
Tape Doesn’t Advance No tension on take up reel. Check setscrew in take up reel collars. Check to see if manual advance is possible on tape
Tape Breaking High liquid content in sample gas Genie probe and additional filtration may be required
Feed wheel not spinning freely Dust and refuse build up between feed wheel and chassis. Requires removal and cleaning of chassis
Tape cover wheels pressing against tape. Tape cover wheel became warped. Needs to be flattened to not contact tape when on feed wheel bolt.
Trigger slide not seated properly Ensure trigger slide is seated in groove of sample chamber.
Slow Response Incorrect Aperture for range Removal or change of aperture type required. Contact Envent Tech support for ideal aperture size.
Liquid contamination in sample tubing Sample conditioning system requires cleaning. Refer to Cleaning Procedure
Sensor Block Fault Re-Zero Sensor block. Refer to sensor re-zero procedure
Higher or Lower than expected reading Liquid contamination in sample tubing sample condition system requires cleaning. Refer to Cleaning Procedure.
Sample vent blockage Vent should be 3/8″ or larger 316 stainless steel tubing on a downard slope. Heat trace maybe required to prevent freezing
Contaminants in sample chamber Sample conditioning system requires cleaning. Cleaning Procedure
Fault light indication Sensor Low Fault Re-zero Sensor block. Refer to Sensor Re-zero Procedure
Sensor High fault Re-zero Sensor block. Refer to Sensor Re-zero Procedure
Low tape Sensing tape requires change. Possible low tape sensor failure
Low Pressure Pressure regulator outlet lower then setpoint of pressure switch. (factory set to 10 psi)

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An analyzer that measures H2S and/or Total Sulfur (optional) in natural gas, petrochemical streams, condensate, or LPG with optional integrated CO2 IR bench.


The 331 measures H2S in natural gas or other process gases. The unit has dual 4–20 mA outputs, dual measurement alarms, plus trouble and fault alarm relays. The 331 displays H2S concentration in user specified units (ppb, ppm, grams, %, m/g3) and as well will auto range to 5 times the lower calibrated range. Calibration is performed manually via the operator interface or automatically when connected to a calibration bottle and optional auto calibration solenoid. The 331 is available with a number of sample systems and accessories to handle various applications such as high range H2S, Total Sulfur or H2S in liquids.

Lead Acetate Sensors: Lead acetate is essentially interference-free from other sulfur compounds. This is why it is used by the vast majority of industry users when accurate measurement is required. The linear output enables the user to perform a single-point calibration.

Longest Tape Life: Envent Tape is the strongest in the market, with the longest tape life by using a proprietary tape advance algorithm to make complete use of the 300-foot tape. The unit will accept up to a 350-foot roll.


Power 12-24 vdc @ less than 3 watts or 100-240 VAC 50/60 HZ
Electrical classification Class 1, Div1 Grps A, B, C & D certified to CSA standards
Ambient Temperature 0-50 ˚C (std) consult factory for other requirements
Output Ranges H2S 0-50 ppb to 0-100 ppm are standard. For higher concentrations, consult factory.
Response time 20-sec alarm
Accuracy 1.5% of full scale (H2S only); 3% (TS and Dilution)
Inputs 24 bit A/D. Microprocessor-based sensor block. Low Tape Alarm. Remote calibrate, Stream switch, or Low pressure
Outputs Dual isolated 4-20 mA (loop power required). Modbus serial RS-232 & RS-485. Four 5 amp SPDT alarm relays (high reading, high high reading, low tape, failure). Four solid state solenoid drivers ( auto cal, total sulfur, stream switching, valve actuator)
Displays 2 x 16 character LCD with backlighting. Menu is scrolled by internal button

Optional Equipment

Total Sulfur Total sulfur furnace—converts all sulfur compounds to H2S which allows analyzer to measure Total Sulfur.
CO2 Dual-beam IR bench. CO2 concentration is displayed on second line of display and can be configured to output 4-20 mA. CO2 from 0-2% up to 0-20%
Configuration Software A custom GUI for customer configuration, archive retrieval, Modbus mapping
Auto Calibration Allows user to initiate a calibration based on time or external switch
Stream Switching Allows switching of two input streams or from H2S to Total Sulfur measurement
Dilution Allows measurements up to 100% H2S using permeable membrane dilution.
Liquid sampling Liquid sample system to measure H2S from hydrocarbon liquids
Custom systems Envent can design custom integrated systems to meet application requirements
Panels 331 is 13″W x 15″H x 8″L fiberglass enclosure. Standard system and 331 are mounted on a 24″W x 16″H x ¼” thick anodized panel