LT DIN Rail Analog Transmitters with Serial Data Communication and Analog Outputs for Thermocouple Temperature Applications P/N LT20KF

Thermocouple DIN Rail Transmitter Frequently Asked Questions

Why does Type T have two different documented conformity errors instead of one figure across its full range?

Documented specification specifically splits Type T's conformity error into two segments — ±0.03°C from 0°C to +400°C, and a wider ±0.2°C from -257°C to 0°C — reflecting that thermocouple conformity to its standard reference table isn't uniform across the full temperature span; the sub-zero portion of Type T's range is documented with a specifically wider error tolerance than its above-zero portion.

Does the 1 kΩ maximum lead resistance figure limit how far a thermocouple can physically be from the transmitter?

Indirectly, yes — documented specification ties rated accuracy specifically to lead resistance staying at or under 1 kΩ; since lead resistance increases with wire length and decreases with larger wire gauge, this figure functions as a practical constraint on the combination of distance and wire gauge used, rather than being expressed directly as a maximum distance figure itself.

Why is CJC performed with an external transistor mounted adjacent to the thermocouple input, rather than measuring temperature elsewhere in the transmitter?

Documented description specifically places this compensation transistor adjacent to the thermocouple input terminals — since cold junction compensation must correct for the actual temperature at the point where the thermocouple wire connects to the transmitter's copper terminals (the reference junction), the sensing element needs to be physically close to that specific junction rather than measuring an unrelated location inside the transmitter that might run at a different temperature.

Does selecting "0 mA" versus "greater than 20 mA" for open sensor indication change anything besides which direction the output goes on failure?

Not as documented — this is described as a simple selectable choice between two output states specifically triggered by an open sensor condition; the underlying detection of the open-sensor condition itself is documented as the same regardless of which of the two output responses (0 mA or above 20 mA) is selected to represent that failure state.

Why must relay modes be non-latching specifically because 4-20 mA temperature transmitters lack external reset?

Documented reasoning specifically connects these two facts: a latching relay, once tripped, would normally require an external reset input to clear it and resume normal operation — since this transmitter's documented interface doesn't provide that external reset capability, a latched relay would have no way to be cleared, which is why the documented relay modes are restricted to non-latching operation on this particular transmitter type.

Does the 17 ms relay response time apply to the transmitter's temperature reading itself, or only to the relay's physical switching action?

Documented description specifically frames the 17 ms figure as the "typical response time" of the dual solid state relay output option itself — this describes how quickly the relay switches once triggered, distinct from the transmitter's own signal conversion and measurement timing, which is governed separately by its documented Concurrent Slope™ conversion process.

Can the same physical transmitter be field-reconfigured from one thermocouple type to another, or does changing type require a different hardware model?

Documented note specifically states the same signal conditioner board can be user-configured for all listed thermocouple types and for °C or °F — this points to configuration-level flexibility on shared hardware rather than needing a separate physical transmitter model purchased for each individual thermocouple type.

Does the documented ±0.02 deg/deg reference junction tempco figure describe a large or small potential error source compared to the per-type conformity error?

It's a proportional figure rather than a fixed value, so its practical size depends on the ambient temperature swing at the transmitter's cold junction — a small ambient shift produces a correspondingly small reference-junction error, while a larger ambient swing scales that error up proportionally; this is documented as a separate error contributor from the fixed per-type conformity error, meaning the two figures address different sources of inaccuracy rather than one subsuming the other.

Does the selectable digital filter time constant (80 ms to 9.6 s) interact with the documented 80 dB NMR figure at 50/60 Hz?

Documented specification lists these together as complementary noise-rejection mechanisms — the 80 dB NMR figure describes a baseline rejection of 50/60 Hz normal-mode noise, while the separately selectable digital filter (with its own adjustable time constant) is documented as available in addition to that baseline figure, letting an installation add further smoothing for signals with noise beyond what the baseline NMR spec alone addresses.

Can the 125 Vac overvoltage protection rating be exceeded briefly without damaging the transmitter, or is it a hard limit?

The page documents 125 Vac specifically as the overvoltage protection rating for the thermocouple input, without further detail on transient or brief-excursion tolerance beyond that figure — treating it as the documented limit for the input protection circuitry, rather than assuming any specific margin above it, is the appropriate way to read this specification absent further documented detail.

Thermocouple Burnout & Open-Sensor Detection Questions From the Field

Why is an open (burned-out) thermocouple specifically problematic for high-input-impedance measuring instruments?

Documented explanation specifically identifies the core issue: with the circuit open, there's no complete path for the instrument's high-impedance input to reference against, which makes it susceptible to picking up electrical noise from nearby sources such as power lines, motors, and variable-frequency drives — that stray noise can then be falsely interpreted as a wildly varying, meaningless temperature reading rather than a clear failure indication.

What is the documented difference between "upscale" and "downscale" burnout protection modes?

Documented guidance specifically describes upscale burnout as forcing the output to a high, full-scale-like reading when the sensor fails open, while downscale burnout forces the output toward a low reading instead — the correct choice is documented as depending on the application's safety requirements, such as a heating process wanting an open sensor to read as dangerously hot (upscale) so the control system shuts the heat off, rather than misreading it as dangerously cold.

Why would a heating application specifically want burnout protection set to upscale rather than downscale?

Documented field example specifically explains that if a furnace's sensing thermocouple fails open without upscale burnout protection, the lost signal could be misread as a very low measured temperature — this would fool a controller into increasing heat input to compensate for what it thinks is a cold process, when the process may actually already be at or above a safe operating temperature, risking a dangerous upset; upscale burnout instead drives the reading high, causing the controller to shut the heat off.

Is a burned-out thermocouple the only documented cause of the same failure symptom a burnout mode is designed to catch?

No — documented guidance specifically notes that something as simple as a wire in the instrument loop becoming disconnected produces the identical effect as an actual thermocouple burnout, from the instrument's perspective; this is documented as a reason burnout protection is considered good practice for all thermocouple applications generally, not just ones where thermocouple failure itself is the primary concern.

Are there documented typical current values used to represent upscale versus downscale burnout on a 4-20 mA transmitter output?

Yes — documented industry reference specifically cites typical values of 3.2 mA or below for downscale burnout indication, and 21.6 mA or above for upscale burnout indication on a 4-20 mA transmitter loop — both figures documented as deliberately placed outside the normal 4-20 mA measurement range so the burnout condition is unambiguous to downstream equipment.

How does a basic thermocouple burnout detection circuit typically create a defined signal state when the sensor circuit opens?

Documented circuit description specifically explains that a resistor is placed to provide an alternate current path specifically for the open-circuit condition — this resistor is documented as sized in the mega-ohm range specifically to minimize its effect on the signal during normal operation when the thermocouple circuit is complete, while still providing a defined path that drives the output to a known state once the circuit opens.

Does burnout/open-sensor protection matter equally for RTD-based temperature transmitters, or is it specific to thermocouples?

Documented guidance specifically extends this concern to RTDs as well — one documented example describes an RTD failing open (resistance becoming very high), which without burnout protection could similarly be misread by the instrument, illustrating that the underlying burnout-function concept is a general good practice across different sensor technologies used with temperature transmitters, not a thermocouple-specific consideration alone.

Is periodically verifying that a burnout/open-sensor detection scheme is correctly configured considered good ongoing practice, or is it a one-time setup task?

Documented field guidance frames burnout mode selection as a configuration choice that should be deliberately verified for each specific application's safety requirements (heating versus cooling, for example) — one documented forum discussion specifically recommends confirming this setting is one of the first things checked when reviewing an existing installation, rather than assuming a default configuration is automatically appropriate for the application at hand.