LT DIN Rail Analog Transmitters with Serial Data Communication and Analog outputs for RTD Temperature Applications P/N LT20P385F

RTD Temperature DIN Rail Transmitter Frequently Asked Questions

Why does the Cu10 RTD use a 5 mA excitation current while Pt100 and Ni120 use only 196 µA?

Documented specification specifically lists Cu10's nominal resistance as only 9.035Ω at 0°C, far lower than Pt100's or Ni120's 100Ω/120Ω — since a lower-resistance sensor produces a proportionally smaller voltage signal for a given excitation current, the documented higher 5 mA excitation current for Cu10 is what's needed to develop a comparably measurable voltage drop across that much smaller resistance.

Why does Pt100 DIN (alpha 0.003850) have a documented tighter max error (±0.03°C) than Pt100 ANSI (alpha 0.003902) at ±0.04°C, given both are Pt100 sensors?

These are documented as genuinely distinct calibration standards, not simply two names for the same curve — Pt100 DIN is calibrated per IEC 751 (IPTS-68) while Pt100 ANSI is calibrated per NIST Monograph 126, and the specific documented max error figure for each reflects how closely the transmitter's linearization can track that particular standard's defined resistance-versus-temperature curve, rather than being inherently identical for any "Pt100" sensor regardless of standard.

Why does the documented lead-resistance tempco allow up to 100Ω for 3- and 4-wire connections, but only up to 10Ω for 2-wire?

Documented specification ties this specifically to how much of the lead resistance error each wiring method can actually compensate — since 3- and 4-wire configurations are documented as actively compensating for lead resistance (either by subtracting it directly or by using separate excitation and sense leads), they can tolerate a much higher lead resistance before that compensation's own limits are reached, whereas 2-wire's more limited compensation approach is documented as reliable only up to a lower 10Ω lead resistance.

Does the 2-wire hookup's method of subtracting lead-wire resistance (measured by shorting the RTD during setup) provide the same ongoing accuracy as 3- or 4-wire methods?

No — documented description specifically notes that while the initial lead-wire voltage drop can be measured and subtracted during 2-wire setup, subsequent changes in lead-wire resistance due to ambient temperature changes are not compensated by this method; 3- and 4-wire hookups are documented as actively compensating for lead resistance on an ongoing basis, which 2-wire's one-time subtraction at setup does not replicate.

Does the "provision for user calibration" (multiplier plus offset) apply automatically, or does it need to be deliberately configured for an RTD that's out of nominal tolerance?

It's documented as a deliberate configuration option specifically intended for RTDs whose resistance differs from nominal at 0°C — rather than the transmitter automatically detecting and correcting for such deviation, this user calibration provision (a multiplier applied to RTD resistance plus a degree offset) is a feature the user applies specifically when a particular sensor's actual characteristics diverge from the standard nominal curve.

Does open sensor indication behave the same way regardless of which RTD type (Pt100, Ni120, Cu10) is configured?

Documented specification lists open sensor indication (0 mA or greater than 20 mA, selectable) as a general specification under RTD Input, without separately distinguishing behavior by RTD type — this points to the open-sensor detection and output response being a shared mechanism across the supported RTD types rather than one specifically tailored per sensor type.

Can the temperature span be set narrower than the RTD's full documented range, and does doing so affect accuracy?

Yes, span can be set as narrow as 150 counts (such as 15.0°) — documented guidance specifically notes this narrow-span limit is governed by considerations of electrical noise and digital filtering time constants, meaning an appropriately narrow span is supported by design, though very narrow spans push closer to the practical noise floor these considerations describe.

Does changing which analog output level (4-20 mA versus 0-10V, for example) is selected affect the transmitter's RTD measurement accuracy?

No — documented specification lists RTD input accuracy figures (max error, span tempco, zero tempco) separately from the analog output section's own documented output accuracy (±0.02% of output span); the analog output stage converts whatever accurately-measured temperature value the RTD input stage has already determined, so selecting a different output level type doesn't change the underlying input measurement accuracy.

Does the transmitter's step response time of 50 ms apply to the RTD input measurement itself, or specifically to the analog output?

Documented specification lists the 50 ms step response time specifically under Analog Output — this describes how quickly the analog output signal settles to a new value following a step change in the underlying measurement, rather than describing the RTD input conversion timing itself, which is governed separately by the transmitter's documented Concurrent Slope™ conversion process.

Can the same physical transmitter be field-reconfigured between Pt100, Ni120, and Cu10 RTD types, or does changing metal type require different hardware?

Documented note specifically states the same signal conditioner board can be user-configured for all RTD types listed, °C or °F, and resistance measurement — this indicates configuration-level flexibility on shared hardware rather than requiring a separate physical transmitter model purchased for each individual RTD metal type.

Does field-replacing the signal conditioner board require recalibrating the transmitter afterward?

No — documented description specifically states calibration factors are stored in EEPROM on the signal conditioner board itself, which allows boards and ranges to be changed in the field with no need for recalibration; the calibration data travels with the board rather than being separately stored in the main transmitter unit requiring re-entry after a board swap.

Does the 0.2 mA figure listed under "Excitation current" in the RTD Input table apply to all four RTD types, or is it a general figure distinct from the per-type table?

These are documented in two separate places with different figures — the per-type table specifically lists 196 µA for Pt100 and Ni120, and 5.0 mA for Cu10, while the general RTD Input specification section separately lists "Excitation current: 0.2 mA," which is consistent with the rounded 196 µA figure used for the platinum and nickel types rather than applying uniformly to Cu10's documented 5 mA excitation.

RTD Self-Heating Error Questions From the Field

What specifically causes "self-heating error" in an RTD measurement?

Documented explanation specifically describes this as a direct consequence of passing excitation current through the RTD's own resistive element — since current flowing through any resistor generates heat (proportional to current squared times resistance), that self-generated heat raises the RTD's own temperature slightly above the actual temperature of whatever it's measuring, causing the sensor to read artificially high.

Is there a documented standard limit for how much self-heating error is considered acceptable?

Yes — documented guidance specifically cites IEC 60751 as stating that self-heating should not exceed 25% of the sensor's own tolerance specification, giving a concrete, standards-based benchmark for how much of a sensor's overall accuracy budget self-heating is permitted to consume, rather than leaving the acceptable error level undefined.

Why do smaller, thin-film RTD elements tend to be more susceptible to self-heating error than larger, wire-wound elements?

Documented comparison specifically cites typical self-heating coefficients around 2.5 mW/°C for small thin-film elements versus around 65 mW/°C for larger wire-wound elements — since a lower self-heating coefficient means a given amount of dissipated power produces a larger temperature rise, smaller thin-film elements are documented as generally more sensitive to a given excitation current's self-heating effect than bulkier wire-wound construction.

Does simply lowering excitation current always improve measurement accuracy overall, or is there a genuine tradeoff?

There's a genuine documented tradeoff — while lower excitation current reduces self-heating error, documented guidance specifically warns that setting current too low increases the risk that small voltage changes across the RTD translate into disproportionately large apparent temperature swings, since the signal-to-noise ratio of the measurement degrades; excitation current must be chosen to balance both self-heating and adequate signal resolution.

Can a documented worked example show roughly how much temperature error a specific self-heating scenario produces?

Yes — one documented reference design specifically calculates that with 1 mA excitation at maximum RTD resistance, power dissipation stays under 0.4 mW, keeping self-heating measurement error to less than 0.01°C for a typical thin-film element in that scenario — illustrating that appropriately chosen excitation current can keep self-heating error to a genuinely small, well-characterized fraction of a degree.

Does the RTD's physical mounting and surrounding environment affect how much self-heating error actually results from a given excitation current?

Yes — documented analysis specifically notes that the magnitude of self-heating error depends on how well the generated heat can be dissipated, which in turn depends on how the RTD is constructed and mounted; the same excitation current can produce different amounts of actual self-heating error depending on installation-specific thermal dissipation conditions, not the excitation current alone.

Are there documented alternative techniques beyond simply using low excitation current to reduce self-heating error?

Yes — documented approaches specifically include using narrow current pulses rather than continuous excitation, timing the measurement to complete faster than the RTD element's thermal rise time, which limits cumulative heat buildup during the brief measurement window even if the pulsed current magnitude itself isn't minimized to the same degree a continuous low-current approach would require.

Does 4-wire ratiometric measurement design specifically help address self-heating error, or only lead-resistance error?

Documented reference designs specifically address both concerns together in the same 4-wire ratiometric circuit — while the 4-wire topology itself is primarily aimed at eliminating lead-resistance error, documented design guidance for such circuits separately and specifically selects an excitation current magnitude chosen to keep self-heating error within a defined small figure, showing that a well-designed 4-wire measurement circuit deliberately manages both error sources together rather than the wiring topology alone solving the self-heating concern.