1. Home
2. Threads



Threads in Pressure Gauge Applications

Threads are one of the fundamental elements in mechanical connection systems. Their function goes beyond simply joining components, involving critical aspects such as sealing, safety and overall system reliability.

In the pressure gauge sector, the thread plays an even more crucial role: it is the interface between the measuring instrument and the process. Proper installation, mechanical stability and, above all, the absence of leaks in pressurized systems depend on it.

The choice of thread type, cylindrical or tapered, with sealing on a gasket or on the thread, must therefore be carefully evaluated, considering design requirements, ease of assembly and operating conditions.

A thorough understanding of the different thread types and the relevant standards helps avoid mismatches, ensures operational safety and guarantees long-term performance.

In Bourdon tube pressure gauges, EN 837-1 defines dimensions, metrological requirements, performance and process connections. .

1. General Overview

In pressure gauge applications, threads can be divided into two main families:

• Cylindrical threads (mechanical connection), where sealing is not achieved on the thread but on a gasket, flat face, O-ring or sealing surface;
• Tapered threads (thread sealing), where sealing is achieved directly through thread interference, often with sealant or PTFE.

A thread consists of: pitch (Passo), profile angle (angolo di profilo), inner diameter (diametro interno), Outside diameter (diametro esterno), medium diameter (diametro medio), root (fondo), crest (cresta) and flank (fianco).

2. Cylindrical threads BSPP

In technical terminology, 'cylindrical GAS' corresponds to G threads, i.e. BSPP (British Standard Pipe Parallel).

The reference standard is ISO 228-1, which defines parallel pipe threads intended for mechanical fastening and not suitable for sealing on the thread itself. Sealing must be achieved using external sealing surfaces and a gasket.

Main characteristics: Whitworth profile at 55°, cylindrical thread.

This is one of the most common solutions in Europe and is explicitly referenced in EN 837-1.

Typical designations: G 1/8, G 1/4, G 3/8, G 1/2.

3. Tapered threads BSPT

The term ‘conical thread’ generally refers to the BSPT family, now standardised by ISO 7-1 for pipe threads where the seal is formed on the thread. In the text of EN 837-1, conical threads are designated by the letter B.

ISO 7-1 defines:

• R = tapered external thread
• Rc = tapered internal thread
• Rp = parallel internal thread for sealing with a tapered male thread

Main characteristics: Whitworth profile at 55°, tapered thread, sealing on the thread using sealant or PTFE.

4. NPT

NPT (National Pipe Taper) is the American tapered thread system.

Reference standard: ASME B1.20.1.

Main characteristics: tapered thread, sealing on the thread, use of sealant or PTFE.

Unlike BSPT, it has a 60° profile and a different pitch (except for 1/2”).

For this reason, NPT and BSPT are not interchangeable.

Other Thread Types

5. SAE (UNF)

Here it is important to distinguish clearly between two things:

a) UNF as a thread form
UNF (Unified National Fine) belongs to the unified inch thread system, governed by ASME B1.1, which defines the thread form, series, classes and tolerances of UN / UNR / UNJ threads.

Therefore, UNF is not a ‘pipe’ thread like G, BSPT or NPT, but a mechanical thread in inches.

b) SAE as an instrumentation/hydraulic connection
In the hydraulic and instrumentation sectors, when referring to SAE, one often means a straight UNF thread used in conjunction with an elastomeric seal, typically an O-ring.

A typical example is the SAE-7/16-20 straight thread connection, used on some pressure gauges in the refrigerant gas sector.

In these SAE connections: the thread is primarily used for tightening, and the seal is provided by the O-ring or the dedicated seat. Compatibility depends not only on the pitch/thread, but also on the SAE sealing seat.

Metric Threads

These threads are not the ‘standard’ solution specified as the primary option in EN 837-1 for the standard process connection of Bourdon tube pressure gauges, but EN 837-1 itself acknowledges that other threads specific to certain sectors may be used.

This means that an M10x1, M12x1.5, M14x1.5, M20x1.5, etc., connection may be found on instruments or adaptors, but should be regarded as a product- or sector-specific solution.

ISO metric threads are governed primarily by:
ISO 261 for the general system of nominal sizes and pitches;
ISO 965-1 for the tolerance system;
with the basic profile specified in ISO 68-1 and basic dimensions in ISO 724.

Main characteristics: 60° profile; cylindrical thread; typically used as a general-purpose mechanical thread; sealing is not achieved via the thread.

FAQ

Difference between Tapered and Cylindrical BSP

Although they have the same number of threads (pitch), the CYLINDRICAL GAS (BSPP – British Standard Pipe Parallel) thread ensures a seal from the base of the thread, usually with a gasket. The thread manages the force that presses against this base. It is regulated worldwide by the ISO 228-1 standard.

The CONICAL GAS (BSPT British Standard Pipe Tapered) is regulated by standard EN 10226-1. In this case, the seal is ensured by the taper of the thread. Like a wedge, the male thread presses against the female thread. Here too, it is standard practice to use a sealant (Teflon, hemp, liquid sealants, etc.)

Difference between NPT and BSP

In both cases, the seal is ensured by the thread wedge, aided by sealants (Teflon, hemp, liquid sealants, etc.); the difference lies in the number of threads.

The difference stems from the relevant standards. GAS threads comply with standards most commonly used in Europe, whilst NPT threads comply with standards most commonly used in Anglo-Saxon countries.

May I interchange BSP with NPT??

The official answer is no. GAS and NPT threads have different thread pitches and seal configurations. Unofficially, a 1/2” NPT fitting can be fitted onto a 1/2” GAS fitting, and a 1/2” tapered male GAS fitting can be fitted onto an NPT fitting because the thread pitch is the same; in both cases, a sealant must be applied.