Why sampling system design matters more than you think

Breaking silos in analytical system design

In our previous article, we looked at how sampling system problems, not instrument faults, account for around 80% of analyser unreliability. The analyser is often working exactly as intended, it's simply working with a sample that no longer represents the process.

We also introduced three questions as a starting diagnostic: 

• Is the sample timely?

• Is it representative?

• Is it being delivered under stable, compatible conditions?

This article now goes a level deeper, into the design decisions that determine whether those questions can ever be answered with confidence. Because before you can troubleshoot a sampling system, you have to understand where the problems are most likely to have been built in.

The design gap nobody talks about

Most analytical systems involve two distinct engineering disciplines: process engineering and instrumentation engineering. Process engineers own the tap selection and upstream conditions. Instrumentation engineers take over downstream, designing the conditioning and measurement loop.

The problem? The space between them (sample transport lines, tubing, pressure management, heat tracing, and physical installation) often falls into a grey area of shared, or absent, responsibility. Neither team fully owns it, and neither fully designs it.

This siloed approach can be one of the most common root causes of poor sampling system performance. Design decisions made in isolation, without a shared view of the full system, create weaknesses that only reveal themselves during commissioning or operation. At exactly the wrong moment.

Our Analytical System Integrity Checklist addresses this directly: a system should be reviewed as one interacting whole; tap, transport line, conditioning enclosure, analyser, and disposal. Not signed off section by section. That whole-system view must start at the design stage.

Common design mistakes and why they matter

1. Wrong sample extraction point

Representativeness starts at the tap. A sample drawn from the wrong location (too close to a bend, in a zone of turbulence, or where the stream is stratified or multi-phase) may never be representative, regardless of how well the rest of the system is designed.

This is one of the most consequential decisions in sampling system design, and one of the most commonly made on the basis of pipe accessibility rather than process behaviour. The checklist is clear on this: tap location should be chosen for a homogeneous, representative draw, not for convenience.

The risk: You're measuring the right process in the wrong place. And your data reflects that from the start.

2. Excessive transport line length and volume

Every sampling system introduces lag between the process extraction point and the analyser response. The question is whether that lag is short enough for the measurement to remain useful. Transport lines that are too long, or too large in diameter, add internal volume that the sample must displace before a process change reaches the analyser.

Poorly routed systems can also introduce dead legs – sections of tubing or component cavities with little or no continuous flow. Samples stagnate in these volumes and can later mix back into the flowing stream, contributing to delay, carryover, and contamination.

The checklist addresses this under ‘Timely’. Sample line length, diameter, pressure, and flow rate should all be specified to minimise dead time and lag. And that calculation should be done, not assumed.

The risk: You're measuring yesterday's process, not today's.

3. Incorrect material selection

Material compatibility is non-negotiable. Tubing, fittings, valves, and instrument bodies that react with the process fluid, adsorb components from the sample, or corrode under operating conditions will compromise sample integrity before it reaches the analyser.

The checklist specifies that materials of construction should be compatible with the sample's full range of composition, not just the nominal case. And that potential impurities should be considered. The problem arises when material selection is made on cost or availability rather than process chemistry; a shortcut that typically costs far more to correct later.

The risk: The sample you're analysing is no longer representative of the process.

4. Inadequate pressure and flow control

Pressure and flow influence transport time, phase behaviour, and whether the sample reaching the analyser remains in a state the instrument can handle. An inappropriate pressure reduction in a liquid sample can remove dissolved gases or lighter components, changing the composition of the remaining liquid. In gas systems, pressure and temperature must be managed together to ensure the sample remains in the required phase throughout.

Insufficient flow increases transport delay and, in samples containing suspended solids, contributes to deposition within the sample line. The checklist covers this under both ‘Timely’ (flow rate and pressure specified to minimise dead time) and ‘Compatible’ (pressure and temperature at the analyser inlet within the operating envelope).

The risk: The sample changes composition in transit, and the analyser reports the change, not the process.

5. Heat tracing gaps and cold spots

Heat tracing is frequently specified but inconsistently executed. Inadequate coverage leaves cold spots where condensation or freezing can occur. Incorrect temperature setpoints, or tracing that is installed but not commissioned correctly, can allow a gas sample to reach its dew point before it reaches the analyser, with components condensing out and the measured composition shifting as a result.

If a gas sample needs to remain heated to prevent condensation, the entire sample path needs to be considered. A well-specified heated section with an untraced valve body or fitting junction can undermine the whole system.

The risk: Sample composition changes between the tap and the analyser, silently and progressively.

6. Poor access and maintainability

A sampling system that cannot be easily inspected, adjusted, and serviced will degrade over time. Not through any dramatic failure, but through the accumulation of small problems that never quite get fixed. Partially blocked filters, regulator deterioration, fouled separation equipment: these tend to go unnoticed until they start affecting analyser performance.

The checklist identifies this under ‘Reliable’. Maintenance intervals and consumable replacement needs should be documented and resourced. But that's only possible if the system is designed for access in the first place, with sufficient clearance for tools, gauges that can be read without contortion, and valves that can be isolated safely.

Poor access also increases the time required to diagnose and repair faults, turning what should be routine maintenance into unplanned corrective work.

The risk: Gradual, undetected degradation, until it becomes a problem that can't be ignored.

7. Separate design ownership

The fragmentation of responsibility between process and instrumentation teams isn’t just an organisational issue. It has direct technical consequences. Decisions about tap location, transport line routing, and material specification are deeply interconnected. When they're made separately, without a shared view of the full sample path, optimisation in one area create problems in another.

The checklist's ‘Root-Cause Prevention’ section is explicit – the design should be reviewed as one interacting system. Tap location and sampling line routing should be engineer-reviewed during new construction, and installation should be supervised by someone with sampling system expertise. That level of joined-up oversight requires someone to own the full picture.

The risk: A system that looks correct on paper, signed off in sections, that behaves poorly in practice.

What the checklist tells us to look for

The Analytical System Integrity Checklist organises its criteria around three questions:

• Is the sample timely?

Transport volume and lag should be calculated, not assumed. Line length, diameter, pressure, and flow rate should all be specified with response time in mind. Where the process demands fast response, fast-loop arrangements should be considered.

• Is it representative?

Tap location should be chosen for process representativeness, not pipe accessibility. No conditioning step should alter composition in a way that invalidates the measurement. And where compromises are unavoidable, their impact should be documented and accepted, not assumed away.

• Is it compatible and stable?

All wetted materials should be reviewed against the full range of process composition, including excursions and impurities. Pressure and temperature at the analyser inlet must be within the operating envelope. Phase behaviour must be understood and managed throughout the sample path.

These three questions, applied at the design stage rather than during troubleshooting, are what distinguish sampling systems that perform reliably from those that require constant intervention.

Ready to review your own system?

The Analytical System Integrity Checklist gives you a structured framework to assess your sampling systems against established best practice, covering representativeness, timeliness, material compatibility, reliability, and more.

Use it as a design review tool, a commissioning check, or a basis for auditing existing installations. Whether you're specifying a new system or troubleshooting an existing one, it will help you identify gaps before they become problems.