Troubleshooting ULTRAMAT 23 Flow Too Low Faults: Complete Guide

Problem Overview

The Siemens ULTRAMAT 23 is a multi-channel NDIR gas analyzer used in continuous emissions monitoring (CEMS) and process gas analysis. The instrument can measure up to four gas components simultaneously, including up to three infrared-active gases such as CO, CO₂, NO, SO₂, CH₄, and N₂O via NDIR, plus oxygen through an external OXYMAT 6/61 module. Two of the most disruptive field faults reported on the ULTRAMAT 23 are:

• "Flow too low during measuring" – raised in the MEAS window while sample gas is flowing through the analyzer.
• "Flow too low during AUTOCAL" – raised when the analyzer's automatic calibration cycle is interrupted (typically stalls at approximately 02:21 remaining time).

These faults are not analyzer electronics failures. They are caused by a loss of sample-gas flow at the internal pressure switch, even when an external flowmeter on the gas conditioning train continues to indicate nominal flow (typically 1.2 to 2.0 L/min). The diagnostic path covers the differential pressure switch, internal tubing integrity, gas conditioning, and the calibration gas manifold.

Affected Product Range and Firmware

The fault pattern applies to all ULTRAMAT 23 variants in the 7MB2335 / 7MB2337 / 7MB2338 families configured for 1, 2, or 3 IR channels with optional electrochemical or paramagnetic O₂ sensor. There is no firmware revision that suppresses the fault: it is generated by the on-board pressure switch hardware input and is not bypassable from the operator menu.

For full hardware specifications, refer to the Siemens ULTRAMAT 23 Operating Manual (A5E00276912 / 84233616) and the EPA-published ULTRAMAT 23 NDIR Gas Analyzer Specifications Sheet.

Symptoms and Fault Display

The "Flow too low" fault is presented in two distinct contexts on the ELAN/MEAS operator interface:

1. MEAS mode – the analyzer continuously polls the differential pressure switch input. If the contact opens for longer than the configured debounce time (default ~5 s), the analyzer latches a flow alarm and the bell (fault) icon appears in the MEAS window header.
2. AUTOCAL mode – during the calibration cycle, the analyzer purges the sample cell with zero gas (typically N₂) and span gas. The internal timer normally counts down from approximately 05:00 (depending on the number of components and configured purge time). The cycle is interrupted at 02:21 when the pressure switch contact opens, leaving the analyzer in a calibration-failed state with the calibration gas still being delivered.

The 02:21 stop point is a common symptom reported in field service: it corresponds to the switch-over from the second calibration gas purge phase to the measurement-zero phase. A pressure-switch drop-out at that point halts the cycle rather than completing it.

Root Cause Analysis

Four root causes account for more than 95% of "Flow too low" field incidents on the ULTRAMAT 23:

1. Defective Differential Pressure Switch

The internal pressure switch monitors the pressure drop across the sample cell capillary. With correct flow (1.2 – 2.0 L/min) the contact should be closed. Field data show the most common failure mode is a stuck-open contact or a contaminated diaphragm, especially after 3–5 years of service in humid or particulate-laden streams. The original Siemens replacement switch carries order number C79302-Z1210-A2. A generic SPDT pneumatic switch with a trip point of approximately 3 mbar (relative to ambient) is electrically compatible, but Siemens strongly recommends the OEM part to retain FM/CSA approvals on certified analyzers.

2. Leaking or Disconnected Internal Tubing

The ULTRAMAT 23 uses flexible tubing between the gas inlet fitting, the pressure switch, the sample cell, and the gas outlet. The factory-installed material is Viton (FKM) for most ranges. Viton can crack, harden, and become brittle when exposed to:

• Concentrated NO₂ or SO₃
• High ozone concentrations from electrostatic precipitators
• Sustained temperatures above 80 °C in the analyzer cabinet
• Long-term exposure to hydrocarbon condensates

Cracks develop at the barbed fitting transitions, particularly at the pressure-switch inlet and the cell inlet. A cracked tube allows sample gas to bypass the cell and the pressure switch never sees the differential pressure, so the contact opens and the fault triggers even though the external flowmeter still shows flow.

3. Blocked Sample Inlet / Conditioning

A partially blocked inlet filter or coalescing filter element reduces flow to the analyzer. The external flowmeter is typically installed downstream of the conditioning but upstream of (or in parallel with) the analyzer; in some installations the flowmeter does not reflect what the analyzer is actually receiving because the analyzer's own internal capillary is partially restricted.

4. Calibration Gas Pressure Drop

During AUTOCAL, the calibration gas cylinder may be nearly empty, causing pressure to fall below the regulator setpoint during the long purge. The cylinder should be replaced when the high-pressure gauge drops below ~30 bar (435 psi) for N₂ zero gas.

Diagnostic Procedure

Follow this sequence to isolate the root cause. The procedure assumes the analyzer is in a safe state and that the operator is qualified to work on pressurized gas systems.

1. Confirm external flow. Read the flowmeter on the gas conditioning panel. Target: 1.2 – 2.0 L/min. If below 1.2 L/min, address the conditioning train first (filter replacement, pump capacity, bypass valve adjustment). If at or above 1.2 L/min but the fault persists, proceed to step 2.
2. Inspect the calibration gas train. Verify regulator setpoints, line pressure, and cylinder volume. Replace any cylinder with < 30 bar remaining. Confirm solenoid valves (internal or external) energize and de-energize during AUTOCAL.
3. Power down and open the top cover. Depressurize the sample line, isolate the calibration gas, and remove the top cover of the analyzer (4 captive screws on standard cabinets).
4. Visual inspection of internal tubing. With a flashlight, examine every flexible tube from the inlet bulkhead fitting to the outlet. Look for cracks, hardened sections, or dislodged barbs. Pay particular attention to the tubing between the inlet block and the pressure switch, and between the pressure switch and the sample cell head.
5. Manually check the pressure switch. With the analyzer still powered, gently apply a low-pressure air source (~50 mbar) to the analyzer's gas inlet while observing the ELAN/MEAS screen. The flow alarm should clear within 5–10 s. If it does not, the pressure switch is mechanically stuck and must be replaced.
6. Measure the pressure switch contact. Disconnect the two leads from the pressure switch. With sample flow at 1.5 L/min, the contact should measure closed (0 Ω). Without flow it should measure open (> 1 MΩ). Replace if the contact is stuck open under flow.

Pressure Switch Replacement (C79302-Z1210-A2)

The differential pressure switch is mounted on a bracket near the gas inlet manifold. Replacement procedure:

1. Isolate the analyzer from the process and depressurize the sample line.
2. Open the top cover and locate the pressure switch (typically a small rectangular plastic-bodied component with two pneumatic ports and a 2-pin electrical connector).
3. Disconnect the electrical leads. Note the polarity; the switch is polarity-insensitive but maintain the existing wiring layout.
4. Pull the pneumatic tubes off the two barbed ports. Note which is the high-pressure (sample inlet) side and which is the low-pressure (cell) side.
5. Remove the two screws securing the switch bracket to the chassis.
6. Install the replacement switch C79302-Z1210-A2, re-attach the bracket, and reconnect the pneumatic tubes with new clamps if the originals are fatigued.
7. Reconnect the electrical leads, restore sample flow at 1.5 L/min, and verify the flow alarm clears.

Tubing Inspection and Material Selection

The factory default is Viton (FKM) for analyzer internal pneumatic lines. Viton offers good chemical resistance to most stack gases but is known to harden and crack in high-ozone or high-NO₂ service. Several alternative materials are used in the field:

When replacing tubing, cut the new section to the exact length of the original. Avoid running tubing across sharp edges or over the optical bench. Use new hose clamps (Oetiker or ear-type) at every barbed fitting. Re-tighten the analyzer top cover and re-pressurize slowly to detect any residual leak at the fittings using a soap-and-water solution or a handheld electronic leak detector (e.g., Agilent HLD or equivalent).

Sample Gas Flow Requirements and Verification

The ULTRAMAT 23 specification for sample gas flow is 1.2 to 2.0 L/min for all configurations. This is measured at the analyzer gas inlet, downstream of the conditioning train. Below 1.2 L/min, the analyzer's pressure switch may not latch closed even if the gas path is intact. Above 2.0 L/min, the cell pressure rises and the analyzer's linearization may shift, producing calibration drift and a slow response time.

Use a calibrated rotameter or mass flow meter with an accuracy of ±2% of reading. The flow should be measured with all calibration gases connected and the calibration valves energized to reflect the worst-case flow path that the analyzer will see during AUTOCAL. A common error is to set the flow with only the sample gas connected; this over-states the available flow because calibration gas line pressure drops when its solenoid is open.

AUTOCAL Procedure Verification

After replacing the pressure switch or tubing, perform the following verification:

1. From the ELAN operator panel, navigate to MAINT > AUTOCAL and start a single calibration cycle.
2. Observe the countdown. The cycle should complete without stalling at 02:21.
3. On completion, read the diagnostic menu (MAINT > DIAG) and confirm the zero and span drifts are within ±2% of the certified range.
4. Return the analyzer to MEAS and confirm the bell (fault) icon does not reappear.
5. Allow the analyzer to run for at least 30 minutes and re-check the drift to confirm there is no thermal warm-up drift introduced by the new components.

Per Siemens, after replacement of certain internal components (including the IR source, detector, or cell), a temperature compensation cycle must be re-run. The procedure is documented in chapter 5 of the ULTRAMAT 23 Operating Manual. Skipping this step can leave residual gain errors of up to ±5% at extremes of the ambient temperature range.

Related Issue: Calibration Successful but No Reading Variation in MEAS

A separate but related field issue is the case where the analyzer calibrates successfully (zero on N₂ and span on a calibration gas), but shows no variation in MEAS when exposed to the process gas. This pattern typically points to one of the following:

• Sample conditioning bypass. The sample line may be connected but the conditioning pump may not be drawing from the correct sample point, or a 3-way valve in the conditioning train may be in the wrong position.
• Cross-port leak in calibration block. The internal calibration block has separate ports for zero gas, span gas, and sample. A failed O-ring between the calibration block and the cell head can leak calibration gas into the sample path during AUTOCAL (causing the calibration to appear successful) while also leaking sample gas to exhaust during MEAS (causing no variation).
• Plugged sample capillary. A partially plugged cell inlet capillary produces a flow rate that satisfies the pressure switch but starves the cell of fresh gas. The flowmeter shows nominal flow because it is measuring downstream of the restriction; the cell response is slowed by an order of magnitude.

To diagnose, perform a step test: introduce a known span gas (e.g., 10% CO₂ in N₂) directly to the analyzer's sample inlet and observe the response. If the analyzer tracks the step in 30–60 s, the sample conditioning train is the issue. If the response is sluggish (> 5 minutes) or absent, the cell capillary or calibration block O-rings are the likely cause.

Spare Parts and Order Numbers

Verify all order numbers against the latest Siemens PI (Process Instrumentation) catalog before ordering, as Siemens revises part numbers periodically.

Field Service Escalation

If the analyzer continues to flag "Flow too low" after the steps above, escalate through Siemens Industry Online Support:

1. Open a support request at the Siemens Industry Online Support portal (registered users only).
2. Attach the analyzer's diagnostic log (export from the ELAN menu: MAINT > LOG > EXPORT) covering at least 24 hours of operation including the AUTOCAL cycle.
3. Note the analyzer serial number, firmware version (visible in the INFO menu), and the exact text of the fault messages.
4. Include the calibration gas certificates, sample conditioning schematic, and the analyzer's installation orientation if not horizontal.

Siemens technical support can also dispatch a field service engineer with calibrated reference gases and an OXYMAT 6/61 simulator for end-to-end loop checks. Note that warranty service requires proof that the gas conditioning meets the specifications in the analyzer's installation manual, including sample temperature, dew point, and particulate loading.

Preventive Maintenance Schedule

To minimize repeat "Flow too low" faults, integrate the following PM items into the site maintenance plan: