Resolving ULTRAMAT 23 SO2 ****** Fault After Autocalibration

Resolving ULTRAMAT 23 SO2 ****** Fault After Autocalibration

Overview

The Siemens ULTRAMAT 23 is a 19-inch rack-mount NDIR (non-dispersive infrared) gas analyzer used in continuous emission monitoring systems (CEMS) and process analytical applications. Each unit measures up to three IR-active components, including CO, CO2, NO, SO2, CH4, and N2O, plus a fourth O2 channel using an electrochemical or paramagnetic sensor. The SO2 IR detector operates in an absorption band centered near 7.4 µm; if the detector signal, reference signal, or calibration drift exceeds the configured tolerance, the display masks the measured value with asterisks (typically ********) and writes a fault entry to the internal logbook.

This article addresses a specific, frequently reported symptom: the SO2 channel masks its reading with asterisks immediately after the second warm-up autocalibration cycle (approximately 23 minutes after power-on), while the O2 channel continues to read the live process value. The differential diagnosis between SO2 and O2 is the most important diagnostic clue: both channels share the same power supply, motherboard, display, and most of the sample-handling path, so a global fault would not selectively disable only SO2.

Problem Description and Symptom Timeline

The standard warm-up autocal sequence on a ULTRAMAT 23 runs two calibration cycles within the first hour after power-on, then settles into the configured scheduled autocal interval (typically 24 hours or 7 days). A typical fault-event sequence observed on the SO2 channel is:

1. Power applied; analyzer enters WARM-UP state, display shows current configuration.
2. Internal zero-gas solenoid energizes; first autocal cycle begins at approximately 10–15 minutes.
3. First autocal completes; both SO2 and O2 channels return to live process values.
4. Second warm-up trim cycle begins automatically at approximately 23 minutes from power-on.
5. SO2 channel output switches to ********; measured value is masked because drift exceeds ±limit.
6. O2 channel continues to display the live process value correctly.
7. Internal logbook writes a fault entry with timestamp and code.

The fact that O2 continues to read while SO2 masks out after the second autocal is the key finding. It rules out common-mode faults such as main power drop, motherboard failure, ambient temperature excursion, or complete loss of test gas. The root cause is local to the SO2 channel: its optical path, detector, IR source, or its calibration-gas delivery.

Reading the Logbook — ULTRAMAT 23 Fault Codes

The internal logbook is the primary diagnostic source for any ULTRAMAT 23 channel fault. Access it from the front-panel keypad:

1. Press MENU.
2. Navigate to DIAGNOSIS → LOG BOOK.
3. Record the most recent fault entries: full text, timestamp, and occurrence counter.

The most common logbook fault codes that produce the SO2 asterisk display are:

After recording the logbook entry, navigate to MENU → DIAGNOSIS → CAL DATA → DRIFT to read the actual zero and span drift percentages stored from the last successful calibration. Typical acceptance limit is ±2% of full scale; values of +3% zero with −4% span are a classic cell-contamination signature.

Root Cause Analysis

The fault pattern — SO2 masks out after the second warm-up autocal while O2 remains healthy — has a defined set of causes, ranked by likelihood based on field experience in CEMS and process-analyzer service:

1. Sample-cell window contamination (most likely)

Sulphur dioxide is hygroscopic and reactive with acid mist and particulates carried over from the stack. In stack monitoring, residual moisture and SO2-rich condensate progressively deposit on the cooler sample-cell windows and absorb IR strongly at 7.4 µm. The first autocal at 10–15 minutes still operates within tolerance because the contamination is partial and the cell temperature is still climbing. The second autocal at 23 minutes occurs after the cell reaches full thermal equilibrium, exposing the full contamination profile, and the span-drift limit trips.

2. IR source aging or contamination

The IR emitter is a heated filament whose emissivity declines with operating hours. After 23 minutes of warm-up, the filament reaches thermal equilibrium; if its output has degraded below the acceptance band, the detector signal collapses during the span-gas check and trips a span-drift or detector-signal-low fault.

3. Detector aging

SO2 detectors in the ULTRAMAT 23 family are typically micro-flow or pyroelectric devices whose responsivity drops and noise-equivalent absorbance rises with age. This presents as span-drift or detector-signal-low faults, often worsening with cell temperature.

4. Zero-gas contamination

If the test-gas cylinder used for zero calibration is depleted, contaminated, or contains residual SO2 (industrial-grade N2 is the typical zero gas), the analyzer calibrates to a non-zero baseline. The first autocal may pass because of residual carryover from the previous process-gas fill; the second autocal, with a longer purge, reveals the true non-zero baseline and trips the zero-drift limit.

5. Pneumatic leak in the zero-gas path

A leak past the zero-gas solenoid, a cracked tube, or a loose fitting draws ambient air (which contains trace SO2 in stack environments) into the cell during calibration. The apparent zero point shifts positively and the zero-drift limit trips.

6. Optical-path obstruction

Particulate carryover from the sample probe or chiller into the cell body deposits on the windows. This is mechanically the same as cell contamination but more severe — it typically triggers detector signal low in addition to span drift.

7. Electronics drift

Less common but seen on older units: preamplifier gain drift, ADC reference drift, or chopper motor speed out of tolerance. These typically produce reference signal out of range or calibration aborted log entries.

Diagnostic Procedure

Perform the following checks in order and record each result before proceeding. This sequence isolates the root cause with minimal disassembly:

1. Verify supply power at the analyzer terminals: 100–240 VAC or 24 VDC depending on configuration, within ±10% of nominal. Brown-outs during the chopper-driven measurement window produce intermittent faults.
2. Inspect sample conditioning: confirm sample probe filter condition, chiller temperature setpoint (typically 2–5 °C), and condensate drain operation. A blocked drain or failed chiller is the single most common upstream cause of SO2 cell contamination.
3. Test the zero-gas cylinder: confirm pressure > 30 bar (typical), regulator set to 1.0–1.5 bar gauge, and all connections tight. Verify zero-gas identity with an independent SO2 analyzer if available.
4. Test the span-gas cylinder: confirm pressure, regulator setting, and that the certified SO2 concentration is within the configured span range (typical span-gas concentration 50–80% of full scale).
5. Check gas flows: sample flow 30–60 L/h typical, zero-gas flow 30–60 L/h during calibration. Deviations greater than ±20% indicate a pneumatic issue.
6. Read logbook entries per the table above and identify which drift limit was exceeded.
7. Exercise the internal pneumatics via MENU → DIAGNOSIS → TEST → PNEUMATICS. Actuate each solenoid manually; verify the click and confirm flow at the sample outlet.
8. Read cell temperature and pressure via the service menu: typical cell temperature 60 °C ± 2 °C; out-of-band temperature produces faults.
9. Read drift values via MENU → DIAGNOSIS → CAL DATA → DRIFT and compare with the limits configured for the SO2 channel.

Sample Cell Cleaning Procedure

If diagnostics point to sample-cell window contamination — the most common cause for the fault pattern described — cleaning is the appropriate corrective action. The cell does not need to be replaced unless physical damage is observed.

1. Power down the analyzer and disconnect from mains.
2. Close the sample inlet and outlet ball valves to isolate the analyzer from the process.
3. Vent any residual sample gas through the bypass vent to a safe area.
4. Remove the front bezel and locate the sample-cell block (typically the largest anodized aluminum block on the front of the optical bench).
5. Disconnect the pneumatic fittings at the cell inlet and outlet ports; mark the orientation of each tube.
6. Remove the mounting screws holding the cell block to the optical bench and lift the block straight off the IR source and detector housings. Do not touch the source or detector windows.
7. Inspect the cell windows on both ends for haze, film, droplets, or particulate deposits.
8. Clean the windows as follows:
   • Use lint-free optical wipes (e.g., Kimwipes). Do not use cotton swabs, paper towels, or tissues; they leave fibers that scatter IR.
   • Wet the wipe with reagent-grade or electronic-grade isopropyl alcohol (IPA, ≥ 99%). IPA is the manufacturer-approved solvent for ULTRAMAT 23 cell windows (typically CaF2 or BaF2 with anti-reflective coating).
   • Wipe in a single circular motion with light pressure. Do not scrub — the anti-reflective coating is thin and can be damaged by aggressive abrasion.
   • Repeat with a fresh wipe if haze remains.
   • Allow the alcohol to evaporate fully (≈ 2 minutes) before reassembly.
9. If the interior bore of the cell shows deposits, flush with IPA through both ports, then dry with oil-free instrument air or nitrogen.
10. Reinstall the cell block, ensuring the IR source and detector O-rings seat cleanly. Torque the mounting screws evenly to the specified value (typically 2.5 Nm).
11. Reconnect the pneumatic lines.
12. Leak-check the assembly: pressurize to 1.5 bar gauge with N2 and verify < 1% pressure drop over 5 minutes.
13. Power the analyzer, allow full warm-up (≥ 30 minutes), and start a manual calibration via MENU → CALIBRATION → START.

Autocalibration Configuration and Spare Parts

The two-cycle warm-up sequence is configurable in the analyzer's service menu. Default timing is:

• Cycle 1 at 10–15 minutes: coarse zero and span adjustment.
• Cycle 2 at ≈ 23 minutes: trim adjustment after full thermal stabilization.

After Cycle 2 the analyzer enters normal measurement mode and runs scheduled autocal at the configured interval (typically 24 hours or 7 days). If the analyzer repeatedly fails Cycle 2 but passes Cycle 1, the root cause is almost always a thermal-stabilization-driven issue: contamination film fully desorbs/absorbs at the operating cell temperature, or the IR source output drops below bandpass at thermal equilibrium. After any maintenance action, perform an additional manual calibration and confirm drift values within ±2% before returning the analyzer to service.

Always confirm part numbers against the analyzer's serial-number-specific spare parts list shipped with the operating manual. The ULTRAMAT 23 is offered with several IR channel combinations and measurement ranges, and the spare parts catalog varies accordingly.

Verification After Maintenance

Complete the following verification checklist before returning the analyzer to CEMS reporting service:

1. Logbook cleared; no faults reappear after two consecutive automatic calibrations.
2. Zero drift < ±2% of full scale; span drift < ±2% of full scale.
3. Response time verified: T90 should match the configured value (typical ≤ 5 s for SO2).
4. Linearity check: introduce three certified span-gas concentrations (low/mid/high) and verify reading within ±2% of nominal.
5. Sample flow and pressure within configured ranges.
6. Analog output (0/4–20 mA) verified at zero, mid, and full scale.
7. If the unit is integrated with a Plant Information System, DCS, or PLC, verify SO2 value transmission and alarm threshold mapping.

For CEMS applications subject to EPA reporting under 40 CFR Part 60 or Part 75, perform a daily calibration assessment (DAC) and CEMS relative accuracy test audit (RATA) per the applicable standard. Reference the EPA ULTRAMAT 23 specifications document for the analyzer's expected performance envelope and the source data used in the analyzer's performance specification.

Troubleshooting Matrix

Integration with Plant Control Systems

In a typical SAP, cement, power, or metals plant CEMS network, the ULTRAMAT 23 communicates SO2 and O2 readings to the plant control system through one or more of the following:

• Analog 0/4–20 mA outputs (one per channel).
• Optional Profibus DP or Profinet interface per factory configuration.
• Modbus RTU over RS-485 via optional gateway module.
• Ethernet with embedded web server on later firmware revisions.

When the SO2 channel displays asterisks, the analog output switches to the configured fault current, typically 0 mA or 22 mA depending on the NAMUR NE43 recommendation. Configure via MENU → OUTPUT → FAULT CURRENT. The PLC should treat this as a sensor fault, not a valid low reading. NAMUR NE43 recommends fault low ≤ 3.6 mA and fault high ≥ 21.0 mA. If the SO2 value is used in regulatory reporting, the data acquisition system must mark the data point as invalid rather than substituting the fault current as a measurement; data-quality indicators matter more than raw mA values for compliance.

Long-Term Reliability Recommendations

• Replace the sample filter element at the configured interval; do not extend the interval.
• Verify chiller condensate drain operation weekly and during every site walk-through.
• Log drift values from every autocal; a rising trend predicts failure before the asterisk display appears.
• Perform an annual linearity check with three certified span-gas concentrations.
• Replace IR source preventively after 5 years in continuous-duty applications, even if no fault has appeared.
• Keep at least one spare IR source, one spare sample-cell window set, and one spare zero-gas solenoid valve on site for critical CEMS service.

Frequently Asked Questions