Probe End Types: V-Notch, Plain Cut, Integral NPT, and Custom Configurations

Probe End Types and Cut Configurations

The tip of the probe tube — the end that sits inside the process pipeline and interfaces directly with the flowing medium — has a profound effect on sampling performance. The end type determines how the process fluid enters the probe bore, how susceptible the probe is to particulate plugging, and whether the sample is biased toward certain phases or flow regimes.

This guide covers every standard probe end type, from the simplest plain cut to complex multi-side V-notch configurations, along with practical guidance on cut angle selection and downstream connection options at the probe tip.

Plain End (Flat Cut)

The plain end is the simplest probe tip configuration. The tube is cut perpendicular to its axis at a 90-degree angle, leaving a flat, open face. The entire bore cross-section is exposed to the process stream.

Characteristics

• The probe bore faces directly into (or away from) the flow depending on installation orientation
• No directional bias is introduced by the cut geometry
• Maximum bore area is exposed to the process, providing the highest potential flow rate
• Most susceptible to particulate entry in dirty or multiphase services

When to Use

Plain ends are appropriate for clean gas services where particulate ingress is not a concern, and where no directional sampling preference exists. They are also the default choice when the process engineer has no specific tip requirement and the service conditions are mild.

Plain End with Laser Markings

A variant of the plain end includes laser-etched orientation marks on the outer surface of the probe tube near the tip. These marks indicate the angular position of the probe relative to the flow direction (upstream, downstream, left, right) so the installer can correctly orient the probe during insertion.

Laser markings are critical when the probe must be oriented at a specific angle to the flow — for example, facing the bore opening upstream to capture a representative isokinetic sample, or facing downstream to avoid direct impingement of liquid droplets in a wet gas stream.

V-Notch Cut

The V-notch is the most widely specified end type for process sampling applications. Instead of a flat perpendicular cut, the tube end is cut at an angle, creating a pointed or beveled opening. This geometry offers several advantages over a plain end.

How V-Notch Works

The angled cut reduces the open area of the bore relative to a plain end while creating a tapered entry point. The notch geometry helps deflect particulates and liquid slugs, reducing the likelihood of probe plugging. In gas services, the V-notch creates a slight aerodynamic effect that can improve sample extraction consistency.

Cut Angle Selection

The cut angle (measured from the tube axis) can be specified anywhere from 15 degrees to 90 degrees, with 45 degrees being the most common default:

A smaller cut angle creates a more tapered, needle-like point that sheds particulates more effectively but reduces the effective bore opening. A larger angle approaches the behavior of a plain end with progressively less particulate rejection.

Probe End Angle (A) vs Cut Angle

It is important to distinguish between the probe end angle (referred to as the "A" dimension or angle A in many configurators) and the cut angle:

• Angle A (Probe End Angle): Describes the overall geometry of the probe tip. A 45-degree probe end angle means the tip is cut at 45 degrees from the tube axis, creating a pointed end. A 90-degree probe end angle means a flat (plain) cut perpendicular to the axis.
• Cut Angle: Refers to the specific angle of each individual cut face when multiple cuts are present (multi-side configurations). On a single V-notch, the cut angle and probe end angle are the same.

The distinction matters most in multi-side cut configurations, where each face may have a different angle.

Multi-Side V-Notch Cuts

For demanding applications, the probe tip can be cut on multiple sides to create specific flow entry patterns. The four possible cut positions are:

• Front (upstream) — The cut faces directly into the incoming flow. This is the standard single V-notch orientation and maximizes sample extraction from the flowing stream.
• Back (downstream) — The cut faces away from the flow. This configuration is used when the engineer wants to minimize direct impingement and sample primarily from the lower-velocity wake region behind the probe.
• Left — Cut on the left side (when viewed from above, looking downstream). Used in specific multi-phase or stratified flow applications.
• Right — Cut on the right side. Paired with left cuts for symmetrical entry geometry.

Common multi-side configurations include:

• Front only (single V) — Standard, most common
• Front + back (double V) — Creates two opposing openings, useful for bidirectional or oscillating flow
• Front + left + right (three-sided) — Provides 270-degree flow access, used in swirling or turbulent flow regimes
• All four sides — Maximum exposure, used in highly turbulent or multiphase flows where comprehensive phase capture is required

Each cut side can be specified with its own angle (15 to 90 degrees), allowing asymmetric configurations tailored to specific flow conditions.

Integral NPT End

An integral NPT end features a male NPT thread machined directly into the probe tube tip. This allows a downstream sampling line, filter, or fitting to be threaded directly onto the probe tip inside the pipe or at the pipe wall.

Characteristics

• The NPT thread is machined from the tube wall, so adequate wall thickness is required (typically 0.083" or greater)
• Available in sizes from 1/4" NPT to 1/2" NPT depending on tube OD
• Provides a threaded mechanical connection without a separate fitting
• Less common than external end connections but useful in specific configurations

When to Use

Integral NPT ends are specified when the probe tip must connect directly to a downstream component — for example, a sintered metal filter element threaded onto the tip to prevent particulate entry, or a small-bore sample line that must be rigidly attached to the probe inside the pipeline.

Tube End Fitting (NPT Compression)

A tube end fitting provides a compression-style connection at the probe tip. Rather than machining threads into the tube itself, a small compression fitting (such as a Swagelok-type connector) is attached to the tube end, providing an NPT female or male outlet.

Characteristics

• The compression fitting is installed on the tube end and can accommodate flexible downstream tubing
• Does not require machining the probe tube wall, so thinner-wall tubes can be used
• Provides a removable connection at the tip — the downstream component can be changed without replacing the probe
• Available in 1/4" and 3/8" NPT sizes

When to Use

Tube end fittings are used when the sampling system design requires a flexible or changeable connection at the probe tip. They are common in laboratory and pilot plant installations where the downstream sample conditioning equipment may be reconfigured periodically.

Selection Guide

Choosing the right end type depends on the process conditions and sampling objectives:

• Clean, dry gas with no particulate concern: Plain end or plain end with laser marking
• Gas with entrained liquids or particulates: V-notch at 30-45 degrees, front cut
• Heavy particulate or slurry service: V-notch at 15-30 degrees, single front cut
• Bidirectional or oscillating flow: Double V-notch (front + back)
• Turbulent multiphase flow: Multi-side V-notch (3 or 4 sides)
• Downstream filter or connection required at tip: Integral NPT or tube end fitting
• Orientation-critical installation: Plain end with laser markings

For a complete understanding of how the probe tip works within the full assembly component stack, and to select the right assembly type for your application, refer to our companion guides.

Impact on Sample Quality

The probe end type is not merely a mechanical detail — it directly affects the representativeness of the extracted sample. A poorly chosen end type can introduce phase bias (preferentially sampling liquid or gas in a multiphase stream), allow particulate plugging that interrupts analyzer operation, or create dead volume at the probe tip that slows sample transport time and smears compositional transients.

Process engineers should consider the end type as part of the overall sampling system design, alongside probe material selection, coating specification, immersion depth, and orientation. The goal is always the same: a representative, timely, uncontaminated sample that faithfully reflects the composition of the bulk process stream.