MSA Ultima X Replacement Sensor Guide: A-ULTX-SENS Gas Codes and PCB Options
How to Decode Replacement Sensors, Match Monitor Construction, and Identify PCB Configurations
Upgrading or replacing your MSA Ultima X gas detector sensor requires matching the correct A-ULTX-SENS gas code with your transmitter’s PCB architecture. This guide decodes sensor part numbers, pin configurations, and board options to ensure seamless, reliable safety compliance.Most people landing on this page are in one of three situations. You may have an Ultima X displaying SENSOR WARNING or CHANGE SENSOR. You may be trying to decode a damaged sensor label. Or you may be repairing a transmitter that is reporting a main-board fault. This guide covers all three, with the A-ULTX-SENS ordering tables first and the PCB and fault-message references behind them.
MSA has discontinued the Ultima X Series and identifies the Ultima X5000 as the current alternative. That does not automatically make every working Ultima X point a migration project. If the transmitter is healthy and the exact approved sensor is available, a sensor replacement can still be the cleanest way to support a standardized installed base..
The practical rule: Use the gas code to narrow the search, but use the complete A-ULTX-SENS or A-ULTX-PCB label to place the order. Gas, range, construction, approval, wiring, and configured options all matter.
Table of Contents
- → 1. How Ultima X Replacement Part Numbers Work
- → 2. How to Read an A-ULTX-SENS Part Number
- → 3. Complete Gas and Range Code Tables
- → 4. Construction and Enclosure Codes
- → 5. Common A-ULTX-SENS Examples
- → 6. How to Identify an A-ULTX-PCB Board
- → 7. Sensor or PCB? Use the Fault Message
- → 8. Replacement and Calibration Notes
- → 9. Keep the Ultima X or Move to the X5000?
- → 10. What to Send TCI Supply for an Accurate Quote
- → 11. Source MSA Ultima X Parts From TCI Supply
- → Frequently Asked Questions
1. How Ultima X Replacement Part Numbers Work
The official product family is MSA Ultima X. You may see A-ULTIMAX in complete-monitor catalog numbers, while the replacement sensor and circuit-board families use the shorter A-ULTX prefix. The three numbers below solve different maintenance problems:
| Part-number family | What it identifies | What must match |
|---|---|---|
| A-ULTX-SENS | Configured smart sensor module | Gas, full-scale range, construction / approval, and final configured suffix |
| A-ULTX-PCB | Configured main printed circuit board | Monitor family, output / wiring, HART or Modbus, LEDs / relays, language, software, and custom options |
| A-ULTIMAX | Complete configured monitor assembly | The entire transmitter model and every selected option |
A body, guard, calibration cap, or flow cap is a separate physical accessory. This article focuses on the two most common sourcing questions: the replacement sensor module and the configured main PCB.
2. How to Read an A-ULTX-SENS Part Number
The supplied ordering-sheet material uses the format A-ULTX-SENS-[gas code]-[enclosure code]-[custom code]. Some catalog records compress the final characters, so copy the number exactly as printed.
| Position | Example | Meaning in A-ULTX-SENS-16-0-0 |
|---|---|---|
| Prefix | A-ULTX-SENS | Ultima X configured replacement sensor |
| Gas / range code | 16 | Hydrogen sulfide, 0-50 ppm |
| Construction code | 0 | General-purpose plastic construction |
| Custom / configured code | 0 | Standard option shown in this example |
A-ULTX-SENS-16-0-0 and A-ULTX-SENS-16-1-0 share the same gas and range. They are still different parts because the construction code changes from general-purpose plastic to explosion-proof stainless.
3. Complete Gas and Range Code Tables
The three tables below preserve the gas codes and ranges from the MSA A-ULTX-SENS ATO ordering sheet.
Toxic gas and oxygen sensor codes
| Code | Gas | Range | ATO ordering note |
|---|---|---|---|
| 11 | Carbon monoxide | 0-100 ppm | Standard |
| 12 | Carbon monoxide | 0-500 ppm | Standard |
| 57 | Carbon monoxide | 0-1000 ppm | Standard |
| 13 | Oxygen | 0-10% | Standard |
| 14 | Oxygen | 0-25% | Standard |
| 55 | Oxygen, solvent-tolerant | 0-25% | XP stainless only |
| 62 | Oxygen, solvent- and CO2-tolerant | 0-25% | XP stainless only |
| 63 | Oxygen, low range | Per specification | XP stainless only |
| 64 | Oxygen, low range and solvent-tolerant | Per specification | XP stainless only |
| 15 | Hydrogen sulfide | 0-10 ppm | Standard |
| 16 | Hydrogen sulfide | 0-50 ppm | Standard |
| 17 | Hydrogen sulfide | 0-100 ppm | Standard |
| 25 | Hydrogen sulfide | 0-500 ppm | Standard in the supplied ATO transcription |
| 18 | Chlorine | 0-5 ppm | GP stainless, plastic, ATEX, or UL Division 2 |
| 28 | Chlorine | 0-10 ppm | GP stainless, plastic, ATEX, or UL Division 2 |
| 61 | Chlorine | 0-20 ppm | GP stainless, plastic, ATEX, or UL Division 2 |
| 20 | Nitric oxide | 0-100 ppm | Standard |
| 22 | Hydrogen cyanide | 0-50 ppm | Standard |
| 23 | Hydrogen chloride | 0-50 ppm | GP stainless, plastic, ATEX, or UL Division 2 |
| 24 | Chlorine dioxide | 0-3 ppm | GP stainless, plastic, ATEX, or UL Division 2 |
| 26 | Hydrogen fluoride | 0-10 ppm | GP stainless, plastic, ATEX, or UL Division 2 |
| 70 | Sulfur dioxide | 0-25 ppm | IS barrier with ATEX, or UL Division 2 |
| 71 | Sulfur dioxide | 0-100 ppm | IS barrier with ATEX, or UL Division 2 |
| 72 | Nitrogen dioxide | 0-10 ppm | IS barrier with ATEX, or UL Division 2 |
Catalytic combustible and infrared sensor codes
| Code | Gas / technology | Range or calibration basis | ATO ordering note |
|---|---|---|---|
| 31 | Combustible, catalytic | 0-100% LEL, natural gas and hydrogen | Standard |
| 32 | Combustible, catalytic | 0-100% LEL, petroleum vapors | Standard |
| 33 | Combustible, catalytic | 0-100% LEL, solvents | Standard |
| 51 | Combustible, catalytic, ATEX | 0-100% LEL, natural gas and hydrogen | ATEX calibration basis |
| 52 | Combustible, catalytic, ATEX | 0-100% LEL, petroleum vapors | ATEX calibration basis |
| 53 | Combustible, catalytic, ATEX | 0-100% LEL, solvents | ATEX calibration basis |
| 38 | Combustible, infrared | 0-100% LEL, methane | Custom order in the supplied ATO transcription |
| 39 | Combustible, infrared | 0-100% LEL, propane / non-methane | Custom order in the supplied ATO transcription |
| 58 | Combustible, infrared, ATEX | 0-100% LEL, methane | Standard in the supplied ATO transcription |
| 59 | Combustible, infrared, ATEX | 0-100% LEL, non-methane | Standard in the supplied ATO transcription |
| 34 | Acetylene, infrared | 0-2.5% by volume | Custom order |
| 35 | Carbon dioxide, infrared | 0-5% by volume | Standard in the supplied ATO transcription |
| 36 | Carbon dioxide, infrared | 0-2% by volume | Standard in the supplied ATO transcription |
| 37 | Carbon dioxide, infrared | 0-5% by volume | A second 0-5% selection; the supplied material does not explain the distinction |
Specialty toxic gas sensor codes
| Code | Gas | Range | ATO ordering note |
|---|---|---|---|
| 41 | Phosphine | 0-2 ppm | Standard |
| 42 | Arsine | 0-2 ppm | Standard |
| 43 | Silane | 0-25 ppm | Standard |
| 45 | Diborane | 0-50 ppm | Standard |
| 46 | Bromine | 0-5 ppm | GP stainless, plastic, ATEX, or UL Division 2 |
| 47 | Fluorine | 0-10 ppm in the supplied ATO transcription | Manual cross-check differs; see revision note below |
| 48 | Ammonia | 0-100 ppm | GP stainless, plastic, ATEX, or UL Division 2 |
| 54 | Ammonia | 0-1000 ppm | GP stainless, plastic, ATEX, or UL Division 2 |
| 49 | Hydrogen | 0-1000 ppm | Standard |
| 50 | Ethylene oxide | 0-10 ppm | GP stainless, plastic, ATEX, or UL Division 2 |
What the instruction manual adds or changes
The ATO sheet is the better catalog decoder; the instruction manual is the better maintenance and troubleshooting source.
| Code / group | Manual entry | Why it matters when ordering |
|---|---|---|
| 44 | Germane, 0-3 ppm | Appears in the instruction-manual replacement table but was omitted from the supplied ATO transcription. |
| 47 | Fluorine | The supplied ATO transcription shows 0-10 ppm; the instruction-manual replacement table shows 0-5 ppm. Match the printed sensor label and document revision. |
| 63 | Low-range oxygen | The supplied ATO transcription says “per specification”; the instruction-manual replacement table shows 0-25% for the listed Model E construction. |
| 64 | Low-range, solvent-tolerant oxygen | The supplied ATO transcription says “per specification”; the instruction-manual replacement table shows 0-25% for the listed Model E construction. |
| ATO-only selections | Codes 25, 26, 28, 34-37, 51-53, and 58-59 | These appear in the supplied ATO transcription but are not all reproduced in the manual’s standard replacement table. That does not make them interchangeable or invalid; it makes the full label and approval essential. |
Revision rule: For a discontinued platform with multiple approval variants, the full installed label outranks a generic two-digit code table. When a range or suffix differs between documents, send the label to TCI Supply.
4. Match the Construction and Enclosure Code
| Code | Construction | Practical interpretation |
|---|---|---|
| 0 | General-purpose plastic | Commonly associated with Model A / Ultima XA configurations |
| 1 | 316 stainless explosion-proof | Commonly associated with Model E / Ultima XE; the manual also lists selected XIR sensors with this suffix |
| 6 | General-purpose stainless | Model G construction in the instruction-manual replacement table |
| 2 through 12 | IR, ATEX, and regional stainless constructions | The supplied ATO transcription groups these as special construction values. Copy the exact installed number; do not infer the suffix from the gas alone. |
The manual replacement table makes the common pattern easy to see: a gas code may be offered with -0 for general-purpose plastic, -1 for the Model E explosion-proof construction, or -6 for general-purpose stainless. Not every gas is available in every construction. Also check the approval label. A sensor that appears to thread into the housing is not automatically the correct replacement.
5. Common A-ULTX-SENS Examples
| Example part number | Gas and range | Construction |
|---|---|---|
| A-ULTX-SENS-11-0-0 | Carbon monoxide, 0-100 ppm | General-purpose plastic |
| A-ULTX-SENS-14-0-0 | Oxygen, 0-25% | General-purpose plastic |
| A-ULTX-SENS-16-0-0 | Hydrogen sulfide, 0-50 ppm | General-purpose plastic |
| A-ULTX-SENS-16-1-0 | Hydrogen sulfide, 0-50 ppm | Explosion-proof stainless |
| A-ULTX-SENS-31-0-0 | Catalytic combustible, natural gas / hydrogen basis | General-purpose plastic |
| A-ULTX-SENS-38-1-0 | Infrared combustible, methane basis | Listed Model E / XIR construction |
| A-ULTX-SENS-57-6-0 | Carbon monoxide, 0-1000 ppm | General-purpose stainless |
6. How to Identify an A-ULTX-PCB Replacement Board
A sensor is selected primarily by gas, range, and construction. A PCB is selected by the monitor family and its electrical and option configuration. The supplied catalog defines six positions after A-ULTX-PCB:
| Position | Options in the supplied ATO decoder |
|---|---|
| Model type | A = general-purpose plastic (XA); E = explosion-proof 316 stainless (XE); T = plastic, no display (XT); L = stainless, no display (XL) |
| Sensor output | 1 or B = 4-20 mA two-wire toxic / O2 with no LEDs or relays; 2 = two-wire HART; 3 or E = three-wire with LEDs or relays; 4 or H = three-wire HART; M = Modbus |
| LEDs and relays | 0 = none; 1 = LEDs only; 2 = relays only; 3 = LEDs and relays |
| Language | 0 = English; S = Spanish |
| Software | 0 = North American; 1 = European; 3 = custom horn |
| Custom features | 0 = none; 1 = internal 12 VDC supply; 2 = internal 24 VDC supply; C = custom |
The instruction manual provides a useful cross-check for the core A and E numeric board prefixes:
| Core board prefix | Monitor construction | Output configuration |
|---|---|---|
| A-ULTX-PCB-A-1 | XA / general-purpose plastic | 2-wire, 4-20 mA |
| A-ULTX-PCB-A-2 | XA / general-purpose plastic | 2-wire, 4-20 mA with HART |
| A-ULTX-PCB-A-3 | XA / general-purpose plastic | 3-wire, 4-20 mA |
| A-ULTX-PCB-A-4 | XA / general-purpose plastic | 3-wire, 4-20 mA with HART |
| A-ULTX-PCB-E-1 | XE / XIR explosion-proof | 2-wire, 4-20 mA |
| A-ULTX-PCB-E-2 | XE / XIR explosion-proof | 2-wire, 4-20 mA with HART |
| A-ULTX-PCB-E-3 | XE / XIR explosion-proof | 3-wire, 4-20 mA |
| A-ULTX-PCB-E-4 | XE / XIR explosion-proof | 3-wire, 4-20 mA with HART |
7. Sensor or PCB? Use the Ultima X Fault Message
The ATO sheet identifies parts; it is not a troubleshooting guide. For diagnosis, the MSA instruction manual gives a better starting point because its displayed messages distinguish sensor-module faults, main-PCBA faults, and calibration failures.
| Display message | What it indicates | Practical next step |
|---|---|---|
| SENSOR WARNING | The sensor is approaching end of life | Plan the correct A-ULTX-SENS replacement. The warning itself does not change the analog output. |
| CHANGE SENSOR | The sensor is at end of life | Replace the sensor module and perform the required calibration before returning the point to service. |
| SENSOR MISSING | The monitor has lost communication with the sensor | Check the sensor connection; connect or replace the correct sensor module. |
| SNSR FLASH FAULT | Sensor-module program memory is invalid | Replace the sensor module. |
| SNSR RAM FAULT | Sensor-module memory fault | Replace the sensor module. |
| SNSR DATA FAULT | The sensor data sheet is invalid | Reset through the controller; replace the sensor if the fault remains. |
| INVALID SENSOR | The attached sensor is not compatible with the instrument | Install the correct gas, range, construction, and configured sensor type. |
| MN FLASH / MN RAM / MN EEPROM FAULT | Main-PCBA memory fault | Replace the main PC board. |
| MN SUPPLY FAULT | Main-PCBA supply is outside its permitted range | Check the sensor wiring; replace the main PC board if the fault persists. |
| ZERO CAL FAULT | The monitor could not complete zero calibration | Check zero gas / clean air and the gas path, then repeat calibration before condemning hardware. |
| SPAN CAL FAULT | The monitor could not complete span calibration | Check the calibration gas, regulator, tubing, flow, and blockage before replacing a part. |
| LOW SIGNAL | Low infrared signal | Follow the XIR optics-cleaning procedure or replace the sensor as directed by the manual. |
| CHECK CAL | Calibration should be verified | Perform the site-required bump test or calibration. |
This is why “replace the sensor first whenever you are unsure” is not a sound universal rule. CHANGE SENSOR points directly to the sensor. MN EEPROM FAULT points to the main board. SPAN CAL FAULT can be caused by calibration gas, regulator, tubing, flow, blockage, or the sensor, so the gas path should be checked.
8. Sensor Replacement and Calibration Notes
The sensor is the routine maintenance item and has a finite life, but the source ordering sheet does not support one universal replacement interval for every gas and technology. A warranty period is also not the same thing as a guaranteed service life. Use the calibration program, the sensor’s ability to complete calibration, and SENSOR WARNING or CHANGE SENSOR to plan replacement.
- Identify the required sensor from the complete A-ULTX-SENS code on the interior label.
- Replacing a sensor with the same gas type preserves the existing alarm setpoints and relay functions.
- Replacing it with a different gas type loads the new sensor’s default alarm setpoints and upscale / downscale relay behavior. Review the PLC or DCS scaling, alarm logic, labels, and procedures before placing a different gas in service.
- Perform the calibration with the new sensor before returning the point to service.
- The manual describes a 60-second Sensor Swap Delay, but the permitted replacement procedure depends on the exact model, approval, hazardous-area classification, and site procedure. Do not treat SafeSwap as a blanket hot-swap instruction.
- Use genuine MSA replacement parts.
Life-safety equipment note: This article is a part-identification and sourcing guide. It does not replace the MSA instruction manual, approval drawings, or your site gas-detection, lockout, hazardous-area, calibration, and commissioning procedures.
9. Keep the Ultima X Running or Move to the X5000?
MSA identifies the Ultima X5000 as the current alternative. The practical decision depends on whether the problem is a normal sensor replacement or a broader transmitter-lifecycle issue.
| Keep supporting the Ultima X | Evaluate an Ultima X5000 retrofit |
|---|---|
| The transmitter is healthy and only its sensor is due for replacement. | Main-board faults are recurring or the exact configured board is difficult to source. |
| The site has a standardized fleet and already stocks matching Ultima X spares. | The project is already opening the loop, changing the enclosure, or modernizing the gas-detection system. |
| The complete A-ULTX-SENS number is known and the correct approved module is available. | You want a current MSA platform and can revalidate power, wire gauge, approvals, alarm logic, and commissioning. |
| Keeping the existing point avoids an unnecessary change to a proven installation. | The identical mounting holes and conduit-entry geometry can reduce mechanical retrofit work, subject to the current MSA installation requirements. |
MSA designed the X5000 for retrofit applications. Its mounting holes and conduit-entry locations match the Ultima X, and existing wiring can be reused when the wire gauge and available power meet the current X5000 requirements.
The X5000 uses the A-5K-SENS replacement-sensor family. Legacy A-ULTX-SENS spares do not transfer. A sensible installed-base plan can support healthy Ultima X points with exact replacement sensors while moving selected failed or strategic points to the X5000 during planned work.
10. What to Send TCI Supply for an Accurate Quote
The fastest quote is built from labels, not a typed description such as “MSA H2S sensor.” Send the following:
- A clear photo of the full transmitter model and approval label.
- A clear photo of the interior sensor label showing the complete A-ULTX-SENS code.
- The gas and printed full-scale range.
- Plastic or stainless construction, and whether the point is general-purpose, explosion-proof, ATEX, or another approved configuration.
- For a PCB: the full A-ULTX-PCB label, supply voltage, 2- or 3-wire output, HART or Modbus requirement, local display, and relay use.
- Quantity, ship-to country, required date, and any required approval or certificate.
If the label is damaged, send several close-up photos plus one wider photo showing the monitor body, sensor assembly, conduit entries, and installation. That usually provides more useful identification detail than a partial typed number.
11. Source MSA Ultima X Parts From TCI Supply
TCI Supply sources discontinued and hard-to-find industrial control and safety products, including configured Ultima X sensors and boards. Browse the MSA Safety catalog, review the verified A-ULTX-SENS-16-0-0 H2S sensor, or contact TCI Supply with label photos. You can also call 1-800-755-2883.
Reference documents: A-ULTX-SENS ATO ordering sheet and Ultima X Series instruction manual.