Choosing a SENECA temperature converter: T121 head-mount, K121, K109PT or Z109REG2-1

1. The four places a converter can live

Location	SENECA model	Key specs	Choose it when
In the sensor head	T121	Pt100 (−200 to +650 °C), Pt500, Pt1000, Ni100, thermocouples J K R S T B E N, mV, potentiometer; 2-wire 4–20 mA, 7–30 V loop; 1,500 Vac isolation; 0.1 %; ambient −40 to +85 °C; Ø43.7 × 20 mm; configured with Easy-LP software	The sensor is far from the panel: two wires carry a current signal that does not care about cable resistance, and the mV or ohm signal never leaves the head
On the DIN rail, powered by the loop	K121	Same input families plus Cu50/Cu100, Ni120/Ni1000, ±30 V, ±24 mA and 0–400 Ω; passive 4–20 mA out; 7–30 Vdc from the loop; 1.5 kVac 2-way; 16-bit, 0.1 %; 6.2 mm; ATEX II 3G / II 3D (Zone 2 / 22)	The PLC or DCS card supplies loop power, there is no 24 V in the field cabinet, or the cabinet sits in a Zone 2 area
On the DIN rail, 3-way isolated with a voltage option	K109PT	Pt100 in; 0–10 V, 0–5 V or 4–20 mA out; three-way isolation; 6.2 mm	The receiver wants 0–10 V (many drives, chillers and HVAC controllers), or you need input, output and supply isolated from each other
On the DIN rail, universal with a limit relay	Z109REG2-1	TC J K R S T B E N, Pt100/500/1000, Ni100, mA, 75 mV–20 V, potentiometer; isolated analog out; one SPST alarm relay; 10–40 Vdc or 19–28 Vac; 0.1 %; microUSB configuration	Mixed sensor types on one site and you want one spare, or you need a local high-temperature trip without PLC logic
Straight to the network	Z-4RTD2	4 RTD inputs, Modbus RTU, 14-bit, 17.5 mm	Many temperatures going to SCADA or a data logger; skip the 4–20 mA stage entirely

2. Specification details that cause call-backs

• 3-wire vs 4-wire Pt100. Two-wire RTD connections add the cable resistance to the reading: 10 m of 1.5 mm² cable adds about 0.24 Ω, which is 0.6 °C. Use 3-wire as a minimum; the T121 and K121 accept 2-, 3- and 4-wire.
• Cold-junction compensation. Thermocouple converters compensate at their own terminals. A T121 in the head does this at the sensor; a DIN converter does it in the cabinet, so the thermocouple extension cable must run all the way to the cabinet in the correct compensating wire. Mixing copper cable into a thermocouple run is the most common cause of a 5–10 °C error.
• Burn-out direction. Configure the output to go upscale or downscale on sensor break and make sure the PLC alarm matches it. The Z109REG2-1 and K121 both offer selectable burn-out.
• Ambient temperature. A transmitter in a sensor head on a steam line sees far more than 65 °C in summer. The T121 is rated to +85 °C; standard DIN converters are rated to +65 °C.
• Response time. Head-mount and loop-powered converters respond in under 620 ms for RTD and thermocouple inputs; the sensor sheath itself is usually slower, so this rarely matters, but it does for thin-wire thermocouples in control loops.

3. Three worked examples

Boiler feed-water header, 60 m from the panel, Pt100 in a thermowell. T121 in the head with 3-wire Pt100, 4–20 mA over the existing two-core instrument cable to the PLC. Nothing to add in the cabinet.

Paint-shop oven in a Zone 2 area, eight type K thermocouples. Eight K121 on the rail of the Zone 2 cabinet, powered by the DCS loop supply, 6.2 mm each, 50 mm of rail in total, ATEX certificate in the project file.

Cold-storage warehouse, 24 Pt100 sensors to a SCADA. Six Z-4RTD2 modules on one RS-485 line to a Z-KEY gateway, no transmitters, no analog input cards; one Z109REG2-1 on the compressor discharge with the relay wired to the alarm beacon.

4. Ordering information to include

• Sensor type and wiring (Pt100 3-wire, type K, etc.) and the measuring range you want to map to 4–20 mA.
• Where the converter will be mounted and the ambient temperature there.
• Output required by the receiver: 4–20 mA, 0–10 V, Modbus.
• Whether the loop is powered by the receiver (then choose K121 or T121) or by a cabinet supply.
• Hazardous-area classification, if any.

We configure the converters before shipping, so the range and burn-out direction are already set when your technician wires them.

Quick answers

Can a T121 be mounted on a DIN rail instead of in the head?

Yes, with the DIN rail adapter; but at that point the K121 is usually the better buy because it has the same inputs and a wider range of ranges.

Which converter gives 0–10 V from a Pt100?

The K109PT: Pt100 in, 0–10 V or 0–5 V or 4–20 mA out, three-way isolated.

Do I need an isolator as well as a converter?

No. All four converters are galvanically isolated (1.5 kVac); the K109PT and Z109REG2-1 additionally isolate the supply from both input and output.

How are the ranges set?

T121 with Easy-LP, K121 with EASY SETUP over USB, Z109REG2-1 with EASY SETUP over microUSB or the Android app. We can set them for you before shipping.