LT DIN Rail Analog Transmitter with Serial Data Communication and Analog outputs for RTD Temperature Applications

RTD Temperature Transmitter Frequently Asked Questions

What's the difference between a DIN and ANSI Pt100 RTD, and does it matter which I connect?

DIN (alpha 0.00385) and ANSI (alpha 0.00392) Pt100 RTDs have slightly different resistance-versus-temperature curves, so the transmitter must be configured for the matching type; connecting a DIN RTD while configured for ANSI (or vice versa) will produce a plausible-looking but measurably wrong reading, since the two curves diverge as temperature rises.

Why does a 4-wire RTD connection eliminate lead resistance error entirely, while 3-wire only compensates for it?

In a 4-wire hookup, the excitation current and voltage-sensing paths use physically separate lead pairs, so no lead resistance appears in the voltage measurement at all. In a 3-wire hookup, lead resistance is still present in the measurement but is mathematically subtracted out, which works well only if both excitation leads have identical resistance and temperature behavior.

When is a 2-wire RTD connection an acceptable choice?

2-wire connections are reasonable for short cable runs in stable-temperature environments, since lead resistance is measured once during setup and subtracted as a fixed value; it becomes less accurate on long runs or where ambient temperature causes the lead resistance itself to change after calibration.

Why does the copper RTD use a much higher excitation current (5 mA) than the platinum and nickel types (196 µA)?

Copper RTDs have a much lower nominal resistance (about 9 ohms at 0°C compared to 100 or 120 ohms for platinum and nickel), so a higher excitation current is needed to produce a measurable voltage signal from that lower resistance while still keeping self-heating of the sensor within acceptable limits.

Can this transmitter be configured for an RTD with a non-standard resistance at 0°C?

Yes — Instrument Setup Software provides for user calibration via a resistance multiplier plus a degree offset, allowing the transmitter to be matched to an individual RTD whose actual resistance at 0°C differs from the nominal value for its type.

What does open sensor indication actually do, and why is it configurable?

It drives the analog output to either 0 mA or above 20 mA if the RTD circuit opens, making a broken sensor clearly distinguishable from a valid low or high temperature reading. Which direction is configurable so it can be matched to how the downstream control system is set up to interpret a fault condition.

Does switching RTD type require different transmitter hardware?

No — the same signal conditioner board can be user-configured for any of the supported RTD types (Pt100 DIN, Pt100 ANSI, Ni120, Cu10) as well as °C or °F, so switching types is a configuration change through the setup software rather than a hardware swap.

How narrow can the temperature span be set for a given RTD type?

The range can be as wide as the entire span rated for the RTD type or as narrow as 150 counts (such as 15.0°), limited mainly by electrical noise and the digital filtering time constant selected.

Does this transmitter include an excitation output for other sensors, like the DC or load cell variants?

No — temperature signal conditioners (both RTD and thermocouple) are among the models that don't include the general-purpose isolated transducer excitation output, since RTD excitation is a fixed, purpose-built current source specific to the RTD measurement itself.

Can multiple RTD transmitters be networked together?

Yes — up to 30 LT Transmitters and/or Digital Panel Meters can be daisy-chained on RS485 for LAN integration, or an LTE series Ethernet transmitter can be used instead for a direct Ethernet connection.

RTD Temperature Transmitter Questions From the Field

My RTD reading is off by a consistent amount across the whole range — what should I check first?

A consistent offset across the full range often points to a lead resistance or wiring configuration mismatch — for example, using a 2-wire connection where the calibration assumed 3- or 4-wire — rather than a sensor fault; confirming the actual wiring matches what's configured in setup software is the standard first step.

My reading drifts when the ambient temperature around the cabinet or cable run changes, even though the process itself is stable — why?

This is a classic symptom of uncompensated lead resistance, most often seen with 2-wire connections on longer cable runs; switching to a 3- or 4-wire hookup, which actively compensates for lead resistance changes, typically resolves this kind of drift.

My open-sensor alarm triggers intermittently rather than staying on or off — what does that suggest?

Intermittent open-sensor faults often point to a marginal or partially corroded connection at the RTD terminal block rather than a fully broken sensor; checking and reseating the screw-clamp connections is the standard first troubleshooting step before replacing the RTD itself.

Two RTDs of the same nominal type read slightly differently on the same transmitter setting — is that normal?

Some unit-to-unit variation is expected since individual RTDs have manufacturing tolerances around their nominal resistance curve; if the difference exceeds the transmitter's specified accuracy for that RTD type, using the user calibration feature to apply a resistance multiplier and offset for the specific RTD can correct for it.

Why would my reading be noisy specifically near VFDs, contactors, or other switching equipment?

RTD signals are relatively low-level and can pick up noise from nearby switching equipment; checking cable shielding, grounding, and physical separation from the noise source is the standard remedy, along with using the transmitter's digital filtering options in persistently noisy environments.

My reading seems accurate at moderate temperatures but diverges at the high end of the range — what's going on?

This can happen if the RTD alpha type (DIN vs. ANSI) configured in the transmitter doesn't match the physically connected RTD, since the two calibration curves diverge more as temperature increases; verifying the configured alpha type against the RTD's actual specification is the standard diagnostic step.

Can a damaged RTD produce a plausible but wrong reading instead of triggering the open-sensor alarm?

Yes — a partially degraded RTD element or a high-resistance connection short of a full open circuit can shift the reading without crossing the threshold that triggers open-sensor detection; comparing the reading against a known reference temperature is the way to catch this kind of gradual sensor degradation.