Key Technical Specifications
| Parameter | Detail |
|---|---|
| Part Number | SPXCDALMG1 |
| Manufacturer | Honeywell (Honeywell Analytics) |
| Product Type | Fixed Gas Detector / Transmitter |
| Series | Sensepoint XCD |
| Target Gas | Hydrogen (H₂) |
| Measurement Range | 0–1000 ppm |
| Enclosure Material | Aluminum |
| Cable Entry | M20 (2 × M20 or ¾” NPT conduit entries) |
| Input Voltage | 16–32 VDC (nominal 24 VDC) |
| Communication | RS485, MODBUS RTU |
| Certified Temperature Range | -40°C to +65°C (-40°F to +149°F) |
| Ingress Protection | IP66 |
| Output | 4–20 mA (sink or source selectable), 2 programmable alarm relays, 1 programmable fault relay |
| Sensor Type | Plug-in / replaceable sensor cartridge |
| Certifications | UL, ATEX, IECEx, CSA, INMETRO, KTL, PA, GB, CCCF (verify specific certification markings on unit) |
Product Introduction
I’ve pulled more than a few dead gas detectors off pipe racks and process skids where someone wired up a transmitter and forgot to bump-test it for two years. The Sensepoint XCD platform was built to make that kind of neglect survivable — at least to a point. The Honeywell SPXCDALMG1 is the hydrogen-specific variant in the XCD family, packing a 0–1000 ppm H₂ sensor into an aluminum IP66 enclosure with M20 entries. It talks MODBUS RTU over RS485, runs on 16–32 VDC, and gives you two alarm relays, one fault relay, and a selectable sink/source 4–20 mA output.What makes this unit practical in the field is the plug-in sensor cartridge. When the H₂ sensor reaches end of life, you swap the cartridge without replacing the entire transmitter body. The magnetic switch configuration means one tech can calibrate, set alarm points, and run diagnostics through the LCD without cracking the enclosure open in a classified area. That single feature alone has saved me hours of hot-work permitting on live process units. The three-color backlight — green for normal, flashing yellow for fault, flashing red for alarm — is visible from a distance, which matters when you’re doing a walkthrough at 2 AM.
Quality SOP & Tech Pitfalls
The Lab Report
Before I ship any XCD unit, here is what I check:
- Visual and marking verification: Confirm the part number SPXCDALMG1 is legible on the label, the enclosure is aluminum (not stainless — that would be a different SKU), and the M20 entries are intact with no cross-threading.
- Sensor cartridge check: Verify the plug-in H₂ sensor is seated, the part number on the sensor matches the H₂ variant, and the sensor is within its shelf life.
- Power-up bench test: Apply 24 VDC, confirm the LCD boots, the three-color backlight responds, and the status reads green (normal).
- Output verification: Measure the 4–20 mA loop with a Fluke 115 or equivalent. Confirm the output sits at 4 mA in clean air and responds to a span gas challenge.
- Relay and MODBUS check: Trigger alarm thresholds and verify relay actuation. Poll the unit over RS485 to confirm MODBUS register responses.
- ESD-safe packaging: Seal in anti-static material with the part number, sensor type, and test status labeled externally.
The Engineer’s Warning
Two mistakes will burn you on this unit.First, confusing the transmitter body with the sensor cartridge. The SPXCDALMG1 is the complete transmitter with the H₂ sensor. The replacement sensor cartridge alone is a different part number (SPXCDXSG1SS for the H₂ 0–1000 ppm cartridge). If you order the wrong one, you either get a bare transmitter with no sensor or a sensor with nowhere to plug it in.Second, forgetting that hydrogen is lighter than air. I once walked into a compressor building where someone mounted the XCD detector at ankle height. Hydrogen rises. If your target gas is H₂, mount the detector high — near the ceiling or above the likely leak source. Honeywell even sells a conical gas collection hood (SPXCDCC) specifically for lighter-than-air gases like hydrogen. Skip that hood and you’re hoping the gas drifts down to your sensor before it finds an ignition source.
Installation & Configuration Guide
- De-energize and verify safe conditions
⚠️ Lock out / tag out the 24 VDC supply. Confirm zero energy with a rated multimeter before opening any conduit or wiring compartment. In classified areas, verify the hot-work permit status before breaking any enclosure seal. - Document the existing installation
Photograph the old detector from multiple angles. Note the cable entry positions, wiring terminal assignments, mounting orientation, and any DIP switch or magnetic switch settings. - Remove the old unit
Disconnect the 4–20 mA loop, relay wiring, and power supply. Label every wire. Unbolt the mounting plate and remove the transmitter. - Inspect the replacement SPXCDALMG1
Confirm the part number, sensor cartridge type (H₂ 0–1000 ppm), enclosure material, and M20 entry positions match the original. Verify the sensor cartridge is fully seated and locked. - Mount the new transmitter
Bolt the integrated mounting plate to the surface, pipe, or wall. For hydrogen service, mount high and consider adding the SPXCDCC conical collection hood. - Wire the terminals
Connect the 24 VDC power supply, 4–20 mA output loop, relay circuits, and RS485 / MODBUS wiring to the plug-in terminal block. Confirm sink or source configuration matches the existing control system input. - Power-on and verify boot
Restore 24 VDC. Watch the LCD boot sequence. Confirm the backlight shows green (normal). If it shows flashing yellow or red, do not proceed — diagnose the fault or alarm condition first. - Configure and bump-test
Use the magnetic switch through the glass window to set alarm setpoints, relay assignments, and MODBUS address. Perform a bump test with certified H₂ calibration gas. Verify the reading, alarm relay, and 4–20 mA output all respond correctly before returning the unit to service.
Compatible Replacement Models
| Model | Compatibility Notes |
|---|---|
| Honeywell SPXCDALMG1 | ✅ Drop-in replacement if the application requires H₂ 0–1000 ppm, aluminum enclosure, M20 entry, and the same relay/output configuration. |
| Honeywell SPXCDALMG1M | ⚠️ Closely related variant in the XCD H₂ family. Verify the exact suffix difference with the OEM datasheet before substituting — the “M” suffix may indicate a specific configuration or regional variant. |
| Other Sensepoint XCD models (e.g., SPXCDALMFX, SPXCDALMHX) | ❌ Same transmitter platform but different target gases (methane, H₂S, CO, O₂). Not interchangeable unless you are also changing the monitored gas and reconfiguring the control system. |
Frequently Asked Questions
What gas does the SPXCDALMG1 detect?
Hydrogen (H₂) at a range of 0–1000 ppm. It is not a combustible gas LEL detector — it measures hydrogen concentration in parts per million using an electrochemical-style sensor cartridge.Can I swap just the sensor without replacing the whole transmitter?
Yes. That is one of the main design advantages of the XCD platform. The sensor cartridge plugs into the bottom of the transmitter and the unit auto-configures when the new cartridge is inserted. The H₂ replacement sensor cartridge is part number SPXCDXSG1SS.Where should I mount this detector for hydrogen service?
High. Hydrogen is significantly lighter than air and will rise quickly. Mount the detector near the ceiling or above the most likely leak source. Use the SPXCDCC conical collection hood to improve gas capture. Mounting it at floor level for H₂ service is a mistake I’ve seen more than once.What power supply does it need?
16 to 32 VDC, nominal 24 VDC. It draws from a standard industrial 24 VDC loop supply. Verify your power supply can handle the total current draw of all devices on the same circuit.Does it support MODBUS?
Yes. RS485 communication with MODBUS RTU protocol. You can poll the unit remotely for gas concentration, alarm status, diagnostics, and configuration changes.Is this unit explosion-proof?
The Sensepoint XCD platform carries global hazardous area certifications including ATEX, IECEx, UL, and CSA. However, the specific certification markings on your unit depend on the exact build and regional approval. Check the nameplate and the OEM certification document before installing in a classified area.How often does the sensor need replacement?
Electrochemical H₂ sensors have a finite life — typically 2 to 3 years depending on exposure conditions, temperature, and humidity. Run regular bump tests. If the sensor fails to respond to a known concentration of calibration gas, replace the cartridge. The transmitter body stays; only the sensor changes.
