Net Safety Millennium II M21 Single Channel and M22 Dual Channel Gas Transmitter, Emerson Universal Gas Detection Transmitter for Toxic and Combustible Gas Sensors

What is the Net Safety Millennium II M21 and M22 gas transmitter?

The Net Safety Millennium II M21 and M22 are Emerson’s universal gas transmitters for fixed gas detection points. The M21 single channel version takes one plug-in sensor and the M22 dual channel version takes two; each sensor (infrared, catalytic bead or electrochemical) communicates digitally with the transmitter, which turns the gas reading into a standard Analog 4-20 mA output, with optional relays, Modbus RTU and, on the M21, HART. Emerson lists both models as Active.

Sensors leave the factory calibrated; the transmitter identifies a newly fitted sensor and loads its configuration profile on its own, so one transmitter type serves toxic and combustible gas points.

Maintenance note: Both data sheets list the same service features: current loop test jacks for each channel on the faceplate, non-intrusive one-man calibration with a magnetic switch, menu access by push buttons as a backup, a displayed log of 980 events and read-back of the last calibration data stored in the sensor. Menus run in English, French, Spanish or Portuguese.

What are the Net Safety Millennium II M21 and M22 specifications?

Figures from the Emerson M21 and M22 data sheets, side by side.

Specification	M21 single channel	M22 dual channel
Channels	Single Channel	Dual Channel (two sensor inputs, two separate output signals)
Compatible sensors	Combustible Gases: SC3 Series Infrared and Catalytic Bead; Toxic Gases: ST3 Series Electrochemical	Same as M21
Power consumption	2.4 W @ 24 VDC (average, varies by sensor types/quantities)	Same as M21
Voltage range	10.5-32.0 VDC (HART: 18-32 VDC)	10.5-32.0 VDC
Outputs	Standard: Analog 4-20 mA; Optional: Four 5 A Form C Relays, and RS 485 Modbus RTU, and HART	Standard: Analog 4-20 mA; Optional: Four 5 A Form C Relays, and RS 485 Modbus RTU
Display	Organic LED (OLED), Power/Status LED’s; English, French, Spanish, Portuguese menus	Same as M21
Operating temperature	-67 °F to +185 °F (-55 °C to +85 °C), Stainless steel; -58 °F to +185 °F (-50 °C to +85 °C), Stainless steel, North American explosion-proof certification; -40 °F to +185 °F (-40 °C to +85 °C), Aluminum	Same as M21
Relative humidity	0-95 % RH non condensing	Same as M21
Ingress protection	IP66 (Aluminum), IP67 (Stainless Steel), IP64 (with sensors) Type 4X	Same as M21
Enclosure material	Cast Aluminum (A359) or Stainless Steel (SS316)	Same as M21
Conduit opening	¾” NPT (3X)	Same as M21
Mounting	Surface Mount/Pipe Mount/Duct Mount	Same as M21
Weight	Aluminum: 5.5 lbs (2.5 kg), Stainless steel: 7.0 lbs (3.2 kg)	Same as M21
Sensor separation	+2,000 ft/600 m	+2,000 ft
Event logging	980 events	980 events
EMC compliance	EN 50270:2006 per EMC directive 2004/108/EC	Same as M21
Warranty	3 years	3 years

Selection note: The two data sheets print identical power (2.4 W @ 24 VDC average), temperature, enclosure and weight figures, so the choice is channel count and protocol. The M22 accepts two sensor inputs and gives two separate output signals; HART appears only in the M21 ordering table (M21-AH, M21-AHR), where the HART versions need an 18-32 VDC supply against 10.5-32.0 VDC for the others.

Where is the Net Safety Millennium II transmitter used, and when is the M21 or the M22 the better fit?

The Millennium II serves fixed toxic or combustible gas points in classified areas (its North American marking covers Class I, Division 1, Groups BCD and Class I, Zone 1 IIB+H2). It mounts on a surface, pipe or duct, and the sensor can sit up to 2,000 ft (600 m) from the transmitter.

• Pick the M21 for a single gas point, when the control system reads HART, or for oxygen deficiency: the ST3 data sheet allows the O2 sensor only on the M21-AH-A/S and M21-AHR-A/S transmitters.
• Pick the M22 to cover two detection points with one transmitter, for example an ST3 H2S sensor next to an SC311 infrared LEL sensor; its ordering table has analog, relay and Modbus versions but no HART version.

For the wider picture of point, open path and wireless detection, see the PE Energy guide to gas detection systems.

Selection note: The sensor decides the gas, the transmitter the signal. You can fit the SC311 infrared sensor for hydrocarbons such as methane and propane (Emerson states it is immune to poisoning), the SC310 catalytic bead sensor for hydrocarbon and non-hydrocarbon combustibles including hydrogen, or the ST3 XChem for H2S, CO or O2 deficiency. Warranty: five years on the combustible sensors, two on the ST3. Calibration: six months (ST3 H2S), three months (CO, O2), annual (SC311).

Which approvals, outputs and documents apply to the Net Safety Millennium II M21 and M22?

The M21 and M22 data sheets print the same approval set:

North American	Class I, Division 1, Groups BCD T5; Class I, Zone 1, AEX/Ex d IIB+H2 T5; -50 °C ≤ Ta ≤ + 85 °C; NEMA Type 4X/IP67; FM6320, ANSI/ISA 12.13.01, CSA 22.2 No. 152:2006
IECEx	Ex d IIB+H2 T5 Gb; IECEx DNV 12.0014 (Aluminum) IP66, controller only; IECEx FMG 12.0006X (Stainless Steel) IP 67, controller only; IEC 60079-0: 2007-10 & IEC 60079-1: 2007-04
Inmetro	Ex d IIB+H2 T5 Gb IP67
EMC	EN 50270:2006 per EMC directive 2004/108/EC

Outputs are listed in the specification table above. Documents: M21 data sheet, FGD-PDS-M21-Transmitter (July 2016) and M22 data sheet, FGD-PDS-M22-Transmitter (July 2016). Emerson model pages: M21 single channel and M22 dual channel.

Installation note: The IECEx entries are marked “controller only”, and the ingress row drops to IP64 once sensors are fitted, so rate the complete assembly by the sensor, not by the bare stainless steel housing (IP67). The SC310 and SC311 sensors also carry an INMETRO T4 marking, against T5 on the transmitter.

How do I request a quote for the Net Safety Millennium II M21 or M22?

Send the full model code built from Emerson’s ordering matrix (transmitter, output, enclosure material), the sensor and gas range for each channel, any accessories and the quantity. PE Energy confirms price, availability and lead time with the quote. Request a Millennium II transmitter quote.

M21-A	Single channel, Analog
M21-AR	Single channel, Analog & Relay
M21-AD	Single channel, Analog & Modbus
M21-ARD	Single channel, Analog & Relay & Modbus
M21-AH	Single channel, Analog & HART
M21-AHR	Single channel, Analog & HART & Relay
M22-A	Dual channel, Analog
M22-AR	Dual channel, Analog & Relay
M22-AD	Dual channel, Analog & Modbus
M22-ARD	Dual channel, Analog & Relay & Modbus
Enclosure suffix	-A aluminum, -S stainless steel

Model code example: M21-AH-S = M21 single channel, Analog & HART output, stainless steel enclosure; M22-AD-S = M22 dual channel, Analog & Modbus output, stainless steel (both Emerson’s own matrix examples).

Accessory note: from the M21 and M22 data sheets, by mounting condition:

• Duct sampling: UDM-003 universal duct mount assembly; UDM-001 and UDM-002 add a 1 m or 1.5 m inlet sampling pipe (extensions UDM-EX1, UDM-EX2).
• Pipe mounting: UN-MK-31, UN-MK-32, UN-MK-33 for 1”, 2” and 3” pipe.
• Sun and rain: SSK-51, SSK-52, SSK-53 stainless steel sun shade and rain guard kits, each with the matching UN-MK-3x kit.
• HART access: HPT-001 HART communicator port (M21 data sheet).