If you've just stepped out of university and into your first role involving industrial instrumentation, you'll quickly find that connection heads are one of those components that seem simple until you actually have to specify one. They're the protective enclosures that sit atop thermocouple and RTD probe assemblies, and getting the right one matters more than many engineers initially realize.
What a Connection Head Actually Does
At its core, a connection head provides three functions. It protects the wiring terminations from moisture, dust, and mechanical damage. It offers a convenient access point for wiring, recalibration, or sensor replacement. And increasingly, it serves as the housing for a head-mounted temperature transmitter that converts the raw millivolt or resistance signal into a more robust 4-20 mA or digital output.
Without a good connection head, you'd be trying to make reliable connections at process temperatures in environments that often include vibration, corrosive atmospheres, and washdown conditions. That doesn't end well for your measurement accuracy or your maintenance schedule.
The Standard Form Factors
Most connection heads you'll encounter are defined by DIN 43729, which specifies the physical dimensions of the head itself. You'll also see DIN EN 50446 referenced on datasheets, and it helps to know the distinction: EN 50446 covers the complete straight thermocouple assembly (protection tube, insert, and head together), while 43729 defines the head as a standalone component. Most manufacturers cite both.
The two most common form factors are Form B and Form A. Form B is the compact everyday workhorse, with a flat-face cap and just enough interior volume for a terminal block or a single head-mounted transmitter. Form A is taller with considerably more interior space, which matters when you need to fit a transmitter stacked on top of a terminal block or when field wiring is on the bulky side. You'll also encounter larger US-style heads outside the DIN family, typically specified by NEMA ratings rather than form letters.
American installations frequently use NEMA-style heads with 1/2-inch NPT conduit entries, while metric installations favor M20×1.5 threads. Verify this early in any project because adapters add failure points you don't need.
Materials and Environmental Ratings
Cast aluminum is the default for most general-purpose applications because it's light, machinable, and affordable. Stainless steel, usually 316, becomes necessary for pharmaceutical, food, marine, and aggressive chemical environments. Cast iron still appears in heavy industrial settings. Glass-filled polymer heads exist for specialized low-cost applications but are less common than the metal options.
Look at the IP rating carefully. IP65 handles dust and low-pressure water and is usually the minimum for outdoor installations. IP66 tolerates high-pressure water jets, and IP67 adds temporary submersion. For hazardous areas, you'll be dealing with ATEX, IECEx, or Class/Division ratings, and the head must match the entire loop's area classification. Explosion-proof (Ex d) flameproof heads are substantially heavier and have threaded covers with specific engagement requirements that must not be modified in the field.
Head-Mounted Transmitters
This is where modern installations have diverged from older practice. Rather than running long thermocouple extension wire or 3/4-wire RTD cable back to a control cabinet, you mount a transmitter directly inside a Form B or Form A head. The transmitter linearizes the signal, applies cold junction compensation for thermocouples, and outputs 4-20 mA with HART or a fieldbus protocol over ordinary two-conductor cable.
The practical benefits are significant: better noise immunity, lower wiring cost, improved accuracy, and the ability to do sensor matching for RTDs where the transmitter is programmed to the specific Callendar-Van Dusen coefficients of the connected element.
Selection Considerations
When specifying a head, think about ambient temperature at the installation location, which is often different from the process temperature. Most transmitters are rated to around 85°C ambient, so hot process lines with poorly thought-out thermowell extensions can cook your electronics. Consider conduit entry orientation, whether you need dual entries for conduit looping, and whether the head needs to accept a display module.
Connection heads are unglamorous hardware, but picking the right one upfront saves you from the kind of field retrofit that nobody wants to explain to their project manager.
