Rosemount 935 Open Path Combustible Gas Detector, Emerson Open-Path Infrared Hydrocarbon Gas Detector

What is the Rosemount 935 open path combustible gas detector?

The Rosemount 935 is Emerson’s open path, line of sight detector for combustible hydrocarbon gas. A source, which Emerson also calls the transmitter, sends infrared flashes to a detector (the receiver) mounted up to 660 ft. (200 m) away, and the receiver reports the gas concentration along that path in LEL.m. It is selective for C1-C8 gases and factory calibrated, with the receiver set by default to methane at 5 LEL.m full scale.

Both units have heated optics that keep condensation and ice off the window, stainless steel 316L housings, and field configuration over HART or RS-485 Modbus. Emerson lists the 935 as active, with a three-year warranty on detector and source.

Selection note: Emerson states that automatic gain control keeps detection accurate with up to 95 percent signal obscuration, and its product page names dust, fog, rain, snow and vibration as the causes. The displacement and misalignment tolerance is ±0.5 degrees, and the output separates a blocked beam (2 mA) from a fault (0 mA) and from the 4-20 mA gas reading, so the control system can tell an obscured path from gas.

What are the Rosemount 935 specifications?

Detection technology	Infrared flash, open path (source and receiver)
Detected gas	C1-C8 selective gases
Sensitivity range	0 to 5 LEL.m methane and propane; 0 to 8 LEL.m ethylene
Minimum detectable level	0.15 LEL.m
Accuracy	±7.5 % of the reading or ±4 % of the full scale (whichever is greater)
Response time	< 2 seconds
Installation distance	1 to 200 m (3.3 to 660 ft.) depending on source range and adapter, see below
Misalignment tolerance	±0.5 degrees
Temperature range	-67 to +149 °F (-55 to +65 °C)
Humidity	Up to 95 % non-condensing (withstands up to 100 % relative humidity for short periods)
Power supply	24 Vdc nominal (18 to 32 Vdc)
Typical power consumption	Detector: 220 mA; source: 240 mA
Warm-up time	30 seconds for transmitter and receiver
Current output	0-20 mA, sink (source option), maximum load of 500 ohm at 18 to 32 Vdc
Communication	HART on the 0-20 mA loop (FSK); RS-485 Modbus compatible
Status LEDs (receiver)	Green: power on; amber: fault; red: alarm
Electrical connection	Two ¾-in.-14 NPT conduits or 2 x M25 x 1.5 mm ISO
Enclosure	Stainless steel 316L with electro polish finish; conformal coated circuit boards
Dimensions / weight	10.5 x 5.1 x 5.1 in. (267 x 130 x 130 mm), 11 lb. (5 kg) each unit; tilt mount 4.2 lb. (1.9 kg)
Water and dust	IP66 and IP68; NEMA 250 6P
Environmental	Per DNVGL-CG-0339
Event recorder / MTBF	RTC event recorder, last 375 events; MTBF above 100,000 hours
Warranty	Three years for detector and source

Installation note: The source is chosen by distance, and short paths need an adapter. T1 covers 23 to 66 ft. (7 to 20 m), T2 50 to 132 ft. (15 to 40 m), T3 115 to 330 ft. (35 to 100 m) and T4 265 to 660 ft. (80 to 200 m). Below 23 ft. (7 m), a T2 source works with a range adapter: 888635-1 covers 1 to 2.5 m (3.3 to 8.2 ft.) and suits ducted installations only, 888635-2 covers 2.3 to 5 m (7.5 to 16.4 ft.) and 888635-3 covers 4.5 to 7 m (14.7 to 23 ft.).

Where is the Rosemount 935 used, and when is another Emerson model the better fit?

Emerson’s 935 applications center on places where a combustible cloud can spread over an area or along a boundary: refineries, oil platforms, FPSOs and fixed rigs, pipelines, LNG and LPG systems, compressor and pumping stations, fuel storage and refueling stations, and petrochemical, pharmaceutical and other chemical production and storage. It also names engine rooms, test cells, flammable chemical storage, hazardous waste disposal, and hazardous loading docks, transport depots and shipping warehouses.

When the job is different, another Emerson detector fits better:

• Toxic gas instead of combustible: the Rosemount 936 open path toxic gas detector uses ultraviolet technology for hydrogen sulfide or ammonia.
• A known leak point: the Rosemount 625IR point IR detector or the Rosemount 925FGD with its 625ND sensor read % LEL where they are mounted.
• Pressurized releases before a cloud builds: the Rosemount Incus ultrasonic gas leak detector reacts to the sound of the leak.
• A Spectrex specification: Emerson’s Spectrex SafEye Quasar 900 is also an open path combustible gas detector rated up to 660 ft. (200 m).

Selection note: The two Rosemount open path systems on Emerson’s figures. 935: infrared, installation distances up to 660 ft. (200 m), response < 2 seconds, full scale 5 LEL.m methane, minimum detectable level 0.15 LEL.m, typical draw 220 mA detector and 240 mA source. 936: ultraviolet, up to 200 ft. (60 m), response < 3 sec, full scale 500 ppm/m, minimum detectable 50 ppm/m, 135mA detector and 150mA source with heated optics. Both run on 24 Vdc nominal (18 to 32 Vdc) and are rated -67 to 149 °F (-55 to 65 °C).

Which approvals, outputs and documents apply to the Rosemount 935?

Each 935 unit, source and receiver alike, is ordered with one of these approval codes:

A1	ATEX and IECEx
A2	FM/FMC
E2	INMETRO
EM	TR CU (EAC)

The data sheet gives the reliability rating as SIL2 per IEC61508 (TÜV), water and dust protection as IP66 and IP68 and NEMA 250 6P, and environmental testing per DNVGL-CG-0339. Electrical input protection and EMI/RFI protection are per EN50270. The certification markings are in the Rosemount 935 Quick Start Guide, which the data sheet points to, so name the marking you need with the quote.

Outputs: a 0-20 mA current output (sink, with a source option) that carries the 4-20 mA gas reading and status levels, HART on that loop for maintenance and asset management, RS-485 Modbus, and three-color status LEDs on the receiver.

Documents: Rosemount 935 data sheet, July 2026, 00813-0100-4035 Rev. AF and the Rosemount 935 page on Emerson.com. Background: PE Energy gas detection systems guide.

Maintenance note: The current output doubles as a status line: 4 mA is a normal zero reading, 3 mA a maintenance call, 2 mA obscuration or beam block, 1 mA zero calibration mode and 0 mA a fault, with gas on 4-20 mA. Both units need a 30 second warm-up, and the RTC event recorder keeps the last 375 events for review.

How do I request a quote for the Rosemount 935?

Emerson sells the 935 as separate parts: a source (PN 935TXFXXXXX), a detector or receiver (PN 935R1F00XXXX), and accessories. Send PE Energy both model codes, or these choices plus the distance between source and receiver:

Source range	T1, T2, T3 or T4 (distances above)
Source gas calibration	F00 Transmitter
Receiver	R1 Receiver; F01 receiver for combustible gases, methane full scale 5 LEL.m (default)
Housing style / conduit, each unit	2S stainless steel, ¾-in. NPT; 4S stainless steel, M25
Approvals, each unit	A1, A2, E2 or EM

Useful accessories from Emerson’s list:

888270 / 799255	Tilt mount / wall mount
799225 / 888140	Pole mount, U-bolt 4 to 5 in. / 2 to 3 in.
888355-1 / 888920	Duct mount / duct converter kit (300 to 900)
888931 / 888263	Air shield / protective cover
888820 / 888810 / 794079	Auxiliary harness IS/RS 485 and HART / HART handheld diagnostic kit / USB/RS-485 harness converter kit
888635-1, 888635-2, 888635-3	Range adapters, T2 transmitter only

PE Energy confirms price, availability and lead time with the quote. Request a Rosemount 935 quote from PE Energy.

Model code example: Emerson’s sample codes decode field by field. Source 935T1F002SA1 = 935 source, T1 range 23 to 66 ft. (7 to 20 m), F00 transmitter, 2S stainless steel with ¾-in. NPT, A1 ATEX and IECEx. Receiver 935R1F012SA1 = 935 receiver, R1, F01 combustible gases with methane 5 LEL.m full scale, 2S, A1. A working system is one source plus one receiver, so quantities are counted in pairs.