Engineers who ask "transmitter vs Pt100" are usually trying to decide what to put on the requisition line. The honest answer is that they sit at different points in the same measurement chain: the Pt100 is the 100 Ω platinum resistance sensor that changes resistance with temperature, while the temperature transmitter is the conditioning device that turns that resistance change into a 4–20 mA current, a 0–10 V voltage, or a digital HART / PROFIBUS PA / FOUNDATION Fieldbus signal that the DCS or PLC can read.
The clean separation matters because most modern loops do not run a raw Pt100 lead run back to the controller. PR electronics lists its 7501 as a "field-mounted HART temperature transmitter" and its 5437A, 5437B and 5437D family as 2-wire HART 7 devices, with the 5450A/B/D PROFIBUS PA line sitting alongside as digital options [S4]. Even where the transmitter is integrated into the probe head, the two functions are still physically distinct: sensor and conditioner. For background on the transmitter side of that pair, see the [temperature transmitter selection criteria: sensor, output, safety, diagnostics](/news/temperature-transmitter-selection-criteria-sensor-output-safety-diagnostics.html) reference, and for the sensor side the RTD Pt100 encyclopedia entry.
What the Pt100 actually does in the loop
A Pt100 is a platinum resistance thermometer with a nominal 100 Ω resistance at 0 °C and a defined temperature coefficient of approximately 0.385 Ω/°C (the IEC 60751 characteristic). A 2-wire, 3-wire or 4-wire Pt100 is the input stage only: it has no 4–20 mA output, no display, and no HART stack. The 2-wire, 4–20 mA, IP65-rated ZHYQ WR-203 product page is explicit that the Pt100 is the "input type" and the 4–20 mA is the "output signal" [S1]. DATEXEL's DIN-rail DAT2065 carries the same split, listing "RTD, Pt100" under input type and a programmable current output under output signal [S2]. Treat the Pt100 as the transducer and the transmitter as the signal conditioner, and the rest of the specification work gets easier.
What the temperature transmitter actually does
A temperature transmitter accepts a low-level RTD or thermocouple input, linearises the non-linear resistance-versus-temperature curve, applies cold-junction compensation for thermocouples, and drives an industrial current, voltage or digital output. PR electronics' product page defines the function plainly: "A temperature transmitter converts the small signal from a temperature sensor, typically a thermocouple or RTD, into a more robust 4–20 mA or digital signal" [S3]. The bjhxrs temperature transmitter module demonstrates the same idea at module level, accepting 2/3-wire Pt100 inputs and outputting linear 4–20 mA or 0–10 V. The transmitter is also where HART 7, PROFIBUS PA and FOUNDATION Fieldbus sit when the loop needs more than analogue, per the PR electronics 7501/5437/5450 families [S4].
Selection criteria: 2-wire, 3-wire and 4-wire Pt100 lead configurations
Lead resistance is the single biggest source of error on a Pt100, and the choice between 2-wire, 3-wire and 4-wire is made at the sensor, not the transmitter. A 2-wire hookup is the cheapest and the most error-prone, since the lead resistance adds directly to the 100 Ω element. A 3-wire hookup is the industrial workhorse and the most common configuration on Chinese-supplied probes such as the 3.5 mm threaded PT100 transmitter assembly at US$2.1–5 [S6]. A 4-wire hookup cancels lead resistance almost completely and is reserved for laboratory-grade or high-accuracy loops. Transmitter input must match: the bjhxrs module explicitly supports 2/3-wire Pt100 inputs, the ZHYQ WR-203 is specified for 2-wire Pt100 [S1], and most modern DIN-rail and head-mount transmitters such as the DAT2065 and PR electronics 5437 family accept 2-, 3- and 4-wire automatically [S2][S4].
Output-side decision: 4–20 mA, HART, PROFIBUS PA, FOUNDATION Fieldbus
Output is decided on the transmitter. For a single-loop analogue DCS or PLC input, a 2-wire 4–20 mA device such as the ZHYQ WR-203 or the bjhxrs module is enough [S1]. When the loop needs remote configuration, dual-input redundancy or device-level diagnostics, HART 7 transmitters in the PR 5437 and 7501 families are the typical answer [S4]. PROFIBUS PA, such as the PR 5450 series, and FOUNDATION Fieldbus are chosen where the brown-field bus is already installed, not because of any sensor-side advantage. ITM's catalogue page shows the breadth of the installed base, with duct-mount, wall-mount and head-mount 4–20 mA and RTD-probe assemblies from multiple brands listed on a single results page [S5].
Comparison: Pt100 sensor, head-mounted transmitter, DIN-rail transmitter
Decision criteria line up cleanly across the three options. Cost: a bare Pt100 probe such as the 3.5 mm threaded version starts at US$2.1 [S6], a head-mounted 4–20 mA transmitter adds roughly the cost of the electronics, and a DIN-rail multi-channel module is the most expensive per channel but the cheapest per point on a marshalling cabinet. Accuracy: a 4-wire Pt100 plus a high-accuracy head transmitter is the most accurate; a 2-wire Pt100 with a 2-wire transmitter is the least. Mounting: a probe plus head transmitter lives in the thermowell; a DIN-rail transmitter such as the DAT2065 lives remote from the sensor and is used when the field environment is too hot, too vibrating, or too inaccessible for an in-head device [S2]. Cabling: a head-mounted transmitter lets the field wiring be a standard 2-wire 4–20 mA loop, while a remote DIN-rail transmitter means running the RTD leads all the way back to the cabinet. Process fit: hygienic, sanitary or high-vibration processes usually need the probe-and-head combination; dense marshalling rooms with hundreds of points usually prefer DIN-rail.
Where the two-pair fails: 2-wire sensors, long runs, and shared cable trays
Three failure modes come up repeatedly. First, a 2-wire Pt100 with lead runs over about 10 m will drift visibly with ambient temperature changes in the cable tray; specify 3-wire or 4-wire for longer runs. Second, a head-mounted transmitter mounted directly on the thermowell in a process above about 85 °C will cook its ambient rating; use a remote DIN-rail or remote-mounted head in those services. Third, sharing a multi-core cable with VFDs or high-current power cables will inject common-mode noise into a low-level Pt100 signal; route the RTD lead on its own, or accept the noise and pay for a transmitter with better common-mode rejection. None of these constraints are fixed by buying a "better" Pt100 or a "better" transmitter; they are fixed by matching the sensor, lead configuration and transmitter location to the service. [S1]
Who each option is for
A bare Pt100 probe with no transmitter is the right answer for OEMs building their own signal chain on a PLC analogue input card, for laboratory and test stands, and for retrofitting into an existing transmitter. A head-mounted 2-wire 4–20 mA transmitter with an integrated Pt100 — the typical ZHYQ WR-203 [S1] or the bjhxrs module form factor — is the right answer for new greenfield process loops that need a single field device on a thermowell, a single 4–20 mA pair back to the DCS, and minimal commissioning. A DIN-rail programmable transmitter such as the DAT2065 [S2] or a HART / PROFIBUS PA / FOUNDATION Fieldbus device from the PR electronics families [S4] is the right answer when the loop is one of hundreds on a marshalling panel, when configuration changes by software are useful, or when the field environment is too harsh for in-head electronics. A temperature controller sits further up the chain; it uses the transmitter's signal but is a different device.
Standards and sourcing notes
The Pt100 characteristic itself is defined by IEC 60751, which fixes the 100 Ω nominal resistance at 0 °C and the platinum curve. Output-side standards are the usual industrial ones: 4–20 mA on a 2-wire loop is the HART FSK overlay, PROFIBUS PA and FOUNDATION Fieldbus are digital fieldbuses that replace the analogue current — they are not compatible with HART — and ATEX / IECEx apply when the loop enters a hazardous area. A separate temperature monitor or alarm relay is typically used for trip functions rather than a control-loop transmitter. Trackable signals for the next update: HART 7 device revisions in the PR 5437 family [S4], new Chinese-supplied 3.5 mm threaded Pt100 probe pricing on the 2.1–5 USD range [S6], and ITM's catalogue continuing to list more than 200 temperature transmitter products [S5].