Key Technical Specifications
| Parameter | Detail |
|---|---|
| Part Number | SPXCDALMHX |
| Manufacturer | Honeywell (Honeywell Analytics) |
| Product Type | Fixed Gas Detector / Transmitter |
| Series | Sensepoint XCD |
| Target Gas | Hydrogen Sulfide (H₂S) |
| Measurement Range | 0–100 ppm (standard; verify on unit nameplate) |
| Resolution | 0.1 ppm |
| Enclosure Material | Aluminum |
| Cable Entry | 2 × M20 (or ¾” NPT) field wiring entries, 1 × M25 sensor connector |
| 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), programmable alarm relays, programmable fault relay |
| Sensor Type | Plug-in / replaceable electrochemical sensor cartridge |
| Accuracy | ≤ ±5% full scale (H₂S) |
| Repeatability | ±1% full scale |
| Response Time | T50 ≤ 15 s, T90 ≤ 30 s (electrochemical) |
| Dimensions | 164 × 225 × 80 mm |
| Weight | Approx. 1.7 kg (aluminum enclosure) |
Product Introduction
H₂S will kill you before you smell it. That is not a dramatic opening — it is the reason this detector exists. At concentrations above 100 ppm, hydrogen sulfide paralyzes your olfactory nerve. You stop smelling it. Then you stop breathing. The Honeywell SPXCDALMHX is the H₂S-specific variant in the Sensepoint XCD family, built around an electrochemical sensor cartridge that plugs into a standard XCD transmitter body. It measures 0–100 ppm H₂S, runs on 16–32 VDC, and pushes a 4–20 mA signal plus programmable alarm and fault relays back to your DCS or PLC.What makes the XCD platform worth keeping on the shelf is the plug-in sensor architecture. When the H₂S cartridge reaches end of life — typically 2 to 3 years depending on exposure — you swap the cartridge without replacing the transmitter. The magnetic switch configuration lets one tech calibrate, set alarm points, and run diagnostics through the LCD without opening the enclosure in a classified area. The tri-color backlight (green normal, flashing yellow fault, flashing red alarm) is readable from a distance during a night shift walkthrough. I have seen these units survive years in sour gas service with nothing more than regular bump tests and a sensor swap when the time came.
Quality SOP & Tech Pitfalls
The Lab Report
Before I ship any XCD H₂S unit, here is what I check:
- Visual and marking verification: Confirm the part number SPXCDALMHX is legible on the label, the enclosure is aluminum, and the M20 entries are intact with no cross-threading.
- Sensor cartridge check: Verify the plug-in H₂S sensor is seated, the part number on the sensor matches the H₂S variant, and the sensor is within its shelf life.
- Power-up bench test: Apply 24 VDC, confirm the LCD boots, the tri-color backlight responds, and the status reads green (normal).
- Output verification: Measure the 4–20 mA loop with a calibrated multimeter. 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
The number one mistake I see with H₂S detectors is skipping the bump test. H₂S electrochemical sensors can drift silently. The LCD might show zero while the sensor is actually dead. I once walked onto a sour gas skid where the H₂S detector had been showing “0 ppm” for six months straight. A bump test with certified gas revealed the sensor had failed open-circuit. The detector was reading zero because it was reading nothing. If you are not bump-testing at least monthly with certified H₂S calibration gas, you are running a paper detector.The second mistake is confusing the transmitter body with the sensor cartridge. The SPXCDALMHX is the complete transmitter with the H₂S sensor. The replacement sensor cartridge alone is a different part number (SPXCDXSHXSS for the H₂S 0–50 ppm cartridge). Order the wrong one and you are back on the phone with the supplier while the unit sits unmonitored.
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 SPXCDALMHX
Confirm the part number, sensor cartridge type (H₂S), 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. H₂S has a molecular weight of 34 — heavier than air — so mount at breathing zone height or near the floor where gas accumulates, depending on the specific hazard assessment for your site. - 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 H₂S 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 SPXCDALMHX | ✅ Drop-in replacement if the application requires H₂S monitoring, aluminum enclosure, M20 entry, and the same relay/output configuration. |
| Honeywell SPXCDALMHXM | ⚠️ Closely related variant in the XCD H₂S family. The “M” suffix may indicate a specific configuration or regional variant — verify the exact difference with the OEM datasheet before substituting. |
| Other Sensepoint XCD models (e.g., SPXCDALMG1, SPXCDALMFX, SPXCDALMO1) | ❌ Same transmitter platform but different target gases (H₂, combustible, O₂). Not interchangeable unless you are also changing the monitored gas and reconfiguring the control system. |
Frequently Asked Questions
What gas does the SPXCDALMHX detect?
Hydrogen sulfide (H₂S) at a standard range of 0–100 ppm. It uses an electrochemical sensor cartridge, not a catalytic bead or infrared sensor.Can I swap just the sensor without replacing the whole transmitter?
Yes. The sensor cartridge plugs into the bottom of the transmitter and the unit auto-configures when the new cartridge is inserted. The H₂S replacement sensor cartridge is part number SPXCDXSHXSS.Where should I mount this detector for H₂S service?
H₂S is heavier than air (molecular weight 34 vs. 29 for air). It pools at low points, sumps, and floor level. Mount at breathing zone height or near the floor depending on your site-specific hazard assessment. This is the opposite of hydrogen service, where you mount high.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.How often does the sensor need replacement?
Electrochemical H₂S 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.Is this unit rated for hazardous areas?
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.
