Rosemount 625ND Gas Sensor, Emerson NDIR Combustible Gas Sensor for the Rosemount 925 Transmitter

What is the Rosemount 625ND gas sensor?

The Rosemount 625ND is Emerson’s NDIR combustible gas sensor for the Rosemount 925 fixed gas detection transmitter. It measures hydrocarbon gases from 0 to 100% LEL and talks to the transmitter digitally all the time, which Emerson says keeps readings stable and fast even when sensor and transmitter are far apart. A 925 transmitter fitted with a 625ND is what Emerson sells as the Rosemount 925FGD fixed gas detector; with a Rosemount 905 junction box, the sensor can be mounted remote from the transmitter.

As a smart sensor module, the 625ND keeps its own configuration settings and calibration data. Besides field calibration, Emerson notes that sensors can be calibrated off-site and fitted later, and the 925 recognizes a newly fitted sensor and loads its configuration profile by itself. Emerson lists the 625ND as active, with a three-year limited warranty.

Selection note: Two Emerson products start with 625, and they are not interchangeable. In Emerson’s ordering codes, ND is a non-dispersive infrared sensor that works with the 925 transmitter, which provides the outputs, while the Rosemount 625IR (code IR) is a standalone optical infrared sensor with its own 4-20 mA, HART and Modbus outputs. The 625ND can be calibrated in the field through the 925; the 625IR is calibrated at the factory and cannot be calibrated in the field.

What are the Rosemount 625ND specifications?

Emerson publishes the 625ND figures in the Rosemount 925FGD data sheet, so the performance values below are for the sensor working with a 925 transmitter.

Sensor type	Rosemount 625 Non-Dispersive Infrared Sensor
Gas and range	Hydrocarbon gases 0-100% LEL, default methane (code F14)
Factory calibration gas options	G01 Ethane, G02 Propane, G03 Butane, G04 Ethylene
Response time (925FGD)	T50 < 7 sec, T90 < 10 sec for methane, propane, ethylene, n-butane, and ethane
Gas accuracy (925FGD)	±3 percent LEL < 50 percent LEL; ±5 percent LEL > 50 percent LEL
Repeatability (625ND)	±0.75 percent from 0-50 percent LEL; ±1.25 percent from 50-100 percent LEL
Humidity limits	0-95 percent relative humidity, non-condensing
Operating temperature (925FGD)	-40 to +158 °F (-40 to +70 °C)
Housing options	A1 aluminum or S1 stainless steel, ¾-in. NPT male thread connection; M1 aluminum NDIR sensor module only
Weight	Aluminum sensor 1.3 lb (0.6 kg); stainless steel sensor 2.7 lb (1.2 kg); replacement module 0.5 lb (0.2 kg)
Enclosure rating (925FGD)	NEMA 4X and IP66
Compatible transmitter	Rosemount 925 Fixed Gas Detection Transmitter
Warranty	Three-year limited

Maintenance note: Emerson warns against condensation inside the sensor. Above 95 percent relative humidity, temperature changes can form condensation in the sensor and cause errors; if that happens, the sensor may need a conditioning period before use so the moisture can clear. The rated humidity limit is 0-95 percent RH, non-condensing.

Where is the Rosemount 625ND used, and when is another Emerson model the better fit?

The 625ND belongs wherever a wired Rosemount 925 transmitter watches for combustible gas. Emerson describes the 925 platform as built for fixed combustible gas duty: NDIR sensing resists poisoning and long term drift and does not depend on oxygen, which Emerson says suits harsh and inert environments, and the low-maintenance design targets points that are hard to reach. Because each sensor holds its own calibration, a site can calibrate sensors off-site and fit them later, which Emerson says cuts field maintenance time.

Other Emerson products in this range fit other needs:

• One-piece detector, no field calibration: the Rosemount 625IR is a standalone stainless steel IR detector with its own outputs and a 5-year proof test interval.
• No cable at all: the Rosemount 928 wireless gas monitor with a Rosemount 628 MEMS combustible sensor reports 0-100% LEL over WirelessHART.
• A gas cloud across a line: the Rosemount 935 open path detector covers installation distances up to 660 ft. (200 m).

Selection note: Combustible gas figures from Emerson’s data sheets, side by side. 625ND in a 925FGD: T90 < 10 sec, accuracy ±3 percent LEL below 50 percent LEL. 625IR: T90 = 2 seconds with the environment shield, accuracy ±2% full scale. 628 MEMS sensor in a 928: T90 < 24 seconds, accuracy ±3 % LEL on methane. Of the three, the 625ND is the one that combines field calibration with a local operator interface on the transmitter.

Which approvals, outputs and documents apply to the Rosemount 625ND?

When the 625 is ordered as a standalone sensor, its ordering table lists these certification codes:

E1	ATEX/UKEX flameproof
ND	ATEX/UKEX dust
K1	ATEX/UKEX flameproof and dust
E7	IECEx flameproof and dust
E5	US explosion-proof and dust ignition-proof
E6	Canada explosion-proof and dust ignition-proof
RS	NDIR sensor module only

Emerson certifies the 925FGD detector to SIL2 functional safety standards, and the SIL2 certificate (option QT, IEC 61508) is an option on the 925FGD code, not in the standalone 625 table; if you need it, say so when you ask for a quote. The 625ND has no outputs of its own: the 925 transmitter provides 4-20 mA with HART 7 and relays.

Documents: the 625 ordering table and specifications are in the Rosemount 925FGD data sheet, July 2024, 00813-0100-4925 Rev AD. Emerson page: Rosemount 625ND Gas Sensor on Emerson.com.

Replacement note: For a like-for-like spare, the 625 table lists a housing code M1 (aluminum NDIR sensor module only) and a certification code RS (NDIR sensor module only). Emerson’s weight table puts the 625ND sensor replacement module at 0.5 lb (0.2 kg), against 1.3 lb (0.6 kg) for a complete aluminum sensor and 2.7 lb (1.2 kg) in stainless steel.

How do I request a quote for the Rosemount 625ND?

Send PE Energy the full Emerson model code if you have it, or these choices from the 625 ordering table:

Model	625 Gas detection sensor
Sensor technology	ND Non-dispersive infrared
Gas type and range	F14 Hydrocarbon gases 0-100% LEL, default methane
Housing material	A1 aluminum or S1 stainless steel, ¾-in. NPT male thread; M1 aluminum NDIR sensor module only
Product certification	E1, ND, K1, E7, E5, E6, or RS (module only)
Factory calibration gas	G01 Ethane, G02 Propane, G03 Butane, G04 Ethylene (methane if none)
Other options	IEC calibration standard; C1 custom software configuration for transmitter; Q4 calibration certificate

Say whether the sensor goes on an existing Rosemount 925 or a new 925FGD detector, and add quantity. PE Energy confirms price, availability and lead time with the quote. Request a Rosemount 625ND quote from PE Energy.

Model code structure: Emerson prints no sample code for the standalone 625, so read the table top to bottom: 625 (gas detection sensor), ND (non-dispersive infrared), F14 (hydrocarbon gases 0-100% LEL, default methane), a housing code such as S1, a certification code such as E5, then options such as G02 (propane calibration) and Q4 (calibration certificate). Emerson’s response figures cover methane, propane, ethylene, n-butane and ethane, so name the target gas clearly.