You spec a Class A RTD for a critical loop, and the accuracy numbers look right on paper. Three months later, readings drift and nobody trusts the loop. You swap the sensor, and the problem returns.
The connection head could cause a drift like this. Other parts of the assembly and the installation can cause it too, so check the head before you replace the sensor.
You often treat the connection head as an afterthought when you spec a sensor. You pick a housing that fits the mounting location and move on. That choice affects the accuracy of the readings the sensor produces. The effect does not show up on the calibration sheet, so it is easy to miss.
Check these factors before you order one.
- Ambient heat at the connection head location
- Terminal block material and cold junction stability
- RTD lead wire and contact quality
What a Temperature Sensor Connection Head Does Beyond Housing Wires
Your connection head sits between the sensing element and the lead wire running back to your control system. It houses a terminal block or a transmitter, and protects both from dust, moisture, and physical damage.
You might stop there. The head also controls the thermal environment around the electronics and connections doing the signal conversion. Get the environment wrong, and accuracy suffers regardless of sensor quality.
Ambient Heat Is a Variable You Might Skip
Many temperature loops now use a head-mounted transmitter. It converts the raw sensor signal to 4-20 mA or a digital protocol right at the connection point. This standardizes wiring and improves signal integrity across long cable runs.
The transmitter carries its own ambient temperature rating, commonly -40 F to 185 F for the electronics. If the head sits near a hot line, insulation, or heat-radiating equipment, the electronics exceed the limit.
The sensor calibration sheet will not show any of this, because the fault sits in the connection head. The result shows up instead as drift, output error, or premature transmitter failure.
Check the ambient temperature at the head location before you specify one, not the process temperature alone. Account for radiant heat and any surrounding insulation.
Cold Junction Stability Matters for Thermocouples
Thermocouples measure a voltage difference generated at the junction. The measurement only holds up if the reference, or cold, junction stays at a known and stable temperature. Uneven heating across a terminal block introduces error at the exact point you rarely check.
Terminal block material plays a direct role here. Ceramic terminal blocks hold up at higher temperatures than plastic ones. That stability keeps the cold junction more consistent when the head runs warm. A lower-grade terminal block lets ambient heat swing the reference point, and the loop reads a temperature it never saw.
If a thermocouple loop drifts inconsistently, particularly on hot days or during process upsets, check the terminal block material first. Do this before you second-guess the sensor.
RTD Circuits Depend on Lead Wire and Contact Quality
RTDs measure resistance. Any resistance in the lead wire adds to the reading unless the circuit compensates for it. A 3-wire circuit subtracts one leg of that lead resistance. A 4-wire circuit subtracts the whole thing.
The terminal block inside the head has to support the wiring configuration in use. A block with poor contact quality, corroded terminals, or loose brass contacts adds resistance of its own. That resistance stacks on top of whatever the lead wire contributes. It reads as temperature error and drifts as the contacts age.
Field-wiring quality inside the head matters as much as wire gauge or run length outside it.
When to Move the Head Away From the Process
Sometimes your fix is a different location for the head, not a better head.
Extensions raise the connection head above and away from a hot line, surrounding insulation, or tight equipment obstructions. Moving the head even a few inches from a radiant heat source drops the ambient temperature at the electronics. That drop often brings a struggling loop back into spec.
This matters most on high-temperature process lines, near furnace walls, or wherever the mounting location traps heat around the head. If a loop keeps drifting after you check the terminal block and the transmitter rating, look at head placement next. Ask whether the head sits too close to the heat source it measures.
Matching Connection Head Type to the Area Classification
Area classification decides which temperature sensor connection head style is allowed, and it changes cost, size, and maintenance access.
General-purpose heads work for indoor or outdoor locations without hazardous gas or dust risk. They come in large sizes for easy field access and small sizes where weight or space is limited.
Hazardous locations often need an explosion-proof head, but the exact requirement depends on the area classification and the application. Match the head to the class, division, and group present at the site. These heads carry heavier housings and certified sealing methods to contain any internal ignition. Getting this wrong creates a safety issue, and a permit audit will find it.
Sanitary applications, common in food, beverage, and pharmaceutical processes, need heads built for washdown and cleaning cycles. The design must avoid trapping contamination in seams or threads.
Screw-cover heads are the traditional design, but repeated field access wears the threads over time, especially on stainless housings. Flip-top designs remove the threaded connection between the cover and the body entirely, which solves that problem.
The table below maps each application to the SSi model designators and ratings it needs.
Application |
Models |
Certifications and Ratings |
Notes |
| Indoor or outdoor, no hazardous gas or dust | CA, CI | NEMA 4X (Cast Aluminum model) | Accepts DIN-size transmitters and terminal blocks up to 2 inches. Cast Aluminum or Cast Iron housing. |
| Washdown or caustic exposure, no hazardous gas or dust | CSS | Not hazardous-rated | Same design as the EPS explosion-proof head, without hazardous certifications. Cast Stainless Steel resists caustic washdown and costs less than aluminum where aluminum cannot be used. |
| Hazardous area, gas, vapor, or combustible dust | EPA, EPS | CSA, FM, ATEX, IECEx; Class I, Division 1, Groups B, C, D; Class II, Division 1, Groups E, F, G; NEMA 4X, IP68 | Standard-size explosion-proof heads in Aluminum (EPA) or Stainless Steel (EPS). |
| Hazardous area, large terminal block or tight field wiring | EHA, EHI | Same approvals as EPA and EPS | Oversized cavity in Aluminum (EHA) or Cast Iron (EHI). Accepts the TB200 compensated block to reduce wiring errors. |
| Sanitary washdown, no field-mounted transmitter needed | PPS, FTP | FTP is NEMA 4 rated and FDA approved | Polypropylene (PPS, screw-cover) or white Flip Top Poly (FTP). Plastic body loses the RFI shielding a metal head provides, so neither is recommended for a field-mounted transmitter. |
| Frequent field access, transmitter required | FTA | IP68 | Cast Aluminum flip-top head. No threads to gall, still accepts DIN-size transmitters and terminal blocks up to 2 inches. |
Full dimensions and ordering codes for each model are in the SSi Temperature Sensors catalog (Form 1667)(PDF).
A Quick Checklist Before You Order a Connection Head
- Confirm ambient temperature at the head location, not just the process temperature.
- Match the transmitter electronics rating to that ambient temperature, with margin.
- Choose a ceramic terminal block for thermocouple loops running in warm environments.
- Confirm the terminal block supports the RTD wiring configuration in use, 3-wire or 4-wire.
- Check contact material and terminal quality, not just wire gauge.
- Add an extension if the head sits close to a radiant heat source.
- Confirm the head rating matches the area classification at the installation site.
Where to Find the Right Temperature Sensor Connection Head
SSi Temperature Sensors Connection Heads cover general-purpose, hazardous, and sanitary applications. Large sizes give easy access, and small sizes cut weight. Each head houses a terminal block, transmitter, or display, with flip-top options built to eliminate thread galling in the field.
SSi Temperature Sensors Ceramic Terminal Blocks fit special-purpose and general-purpose heads alike. They ship field-configurable for single or dual-sensor setups and work with either thermocouples or RTDs. Brass contacts keep field wiring straightforward. EHA and EHI explosion-proof heads accept a compensated block with thermocouple contacts.
When a hot process line puts the head too close to the heat source, SSi Extensions solve it. Options include Nipple, Nipple/Union, Nipple/Union/Nipple, and Hex Nipple/Union/Nipple configurations, in standard galvanized carbon steel or stainless steel.
SSi Temperature Sensors In-Head Temperature Transmitters address the electronics rating question directly. All models run from -40 F to 185 F and hold long-term stability of 0.1% per year. The PT model adds input/output isolation to filter voltage spikes and eliminate ground loops. The HC model uses HART protocol and includes sensor error correction. The LCP model is programmable and accepts 2, 3, or 4-wire RTD input.
Frequently Asked Questions
Q. What is the difference between a general-purpose and an explosion-proof connection head?
A. General-purpose heads suit indoor or outdoor locations without hazardous gas or dust present. Explosion-proof heads match specific hazardous area classifications and carry certifications such as CSA, FM, ATEX, and IECEx. Ratings run as high as Class I, Division 1, Groups B, C, and D. They also cover Class II, Division 1, Groups E, F, and G.
Q. Does the same terminal block work for thermocouples and RTDs?
A. SSi Temperature Sensors Ceramic Terminal Blocks work with either sensor type and ship field-configurable for single or dual-sensor applications. The ceramic base protects against elevated temperature.
Q. How do I move a connection head away from a hot process line?
A. SSi Temperature Sensors Extensions raise the head above and away from high ambient heat, insulation, or obstructions. Configurations include Nipple, Nipple/Union, Nipple/Union/Nipple, and Hex Nipple/Union/Nipple, in galvanized carbon steel or stainless steel.
Q. What ingress and hazard ratings do SSi Temperature Sensors connection heads carry?
A. Ratings include NEMA 4X and IP68 ingress protection. Hazardous location approvals go up to Class I, Division 1, Groups B, C, and D. Some models also cover Class II, Division 1, Groups E, F, and G.
Q. Do you offer electronic temperature transmitters?
A. SSi Temperature Sensors In-Head Temperature Transmitters accept RTD and thermocouple inputs and are field configurable. All models operate from -40 F to 185 F with long-term stability of 0.1% per year. They also carry FM Class I, Division 1, Groups A-D approval. The PT model isolates input and output to filter voltage spikes and eliminate ground loops. The HC model runs HART protocol with sensor error correction. The LCP model is programmable for 2, 3, or 4-wire RTD input.
Q. Where can I order a connection head or terminal block for a temperature assembly?
A. Browse SSi Temperature Sensors Connection Heads and SSi Temperature Sensors Terminal Blocks and Extensions on the SOR Controls Group website. Request a quote directly from the SSi Temperature Sensors team.
