LT DIN Rail Analog Transmitter with Serial Data Communication and Analog Outputs for Thermocouple Temperature Applications P/N LT21NC

Thermocouple Temperature Transmitter Frequently Asked Questions

Why does the transmitter need cold junction compensation?

A thermocouple only produces a voltage proportional to the temperature difference between its measuring junction and its connection point at the transmitter, not an absolute temperature. Cold junction compensation uses a separate temperature-sensing component at the connection point to measure that reference temperature, allowing the transmitter to calculate true absolute temperature rather than just a relative difference.

Why does conformity error differ between thermocouple types?

Each thermocouple type has its own characteristic voltage-versus-temperature curve, and the transmitter's linearization must approximate that curve across its full range; some types have smoother, more linear curves than others, which is why Type T (±0.03°C over 0-400°C) achieves tighter conformity than a wider-range type like Type S (±0.12°C).

Can I use the same transmitter for a different thermocouple type later?

The thermocouple type and range are user-selectable via the setup software rather than fixed at the factory for one type only, so switching types is a configuration change on the same signal conditioner board rather than requiring different hardware, provided the new type is among those the board supports.

What does the transmitter do if the thermocouple wire breaks?

Open sensor detection can be configured to drive the analog output to either 0 mA or above 20 mA when the sensor circuit opens, so a broken thermocouple produces a distinctly abnormal signal rather than being mistaken for a valid low or high temperature reading.

How fast can this transmitter respond in an ON/OFF control application?

With the optional dual solid state relay output, typical relay response time is 17 ms, fast enough for closed-loop ON/OFF temperature control applications as well as supervisory alarm/shutoff functions.

What's the difference between the relay's band deviation mode and its standard setpoint mode?

Standard setpoint mode triggers the relay when the reading crosses a single threshold. Band deviation mode instead triggers an alarm whenever the reading moves a selected number of counts above or below the setpoint in either direction, useful for detecting a process that has drifted out of an acceptable range rather than just exceeding one limit.

Why must the relay modes be non-latching on this transmitter?

Since 4-20 mA temperature transmitters don't provide an external reset input the way a full digital panel meter might, relay modes are kept non-latching so the relay clears automatically once the condition that triggered it resolves, rather than requiring a manual reset that isn't available on this hardware.

What's the maximum lead resistance this transmitter tolerates for rated accuracy?

Up to 1 kΩ of lead resistance is supported while maintaining rated accuracy, which accommodates reasonably long thermocouple wire runs without needing additional compensation.

Does this transmitter include an excitation output like the DC or load cell versions?

No — thermocouple and AC RMS signal conditioners are the noted exceptions among LT models that don't include the isolated transducer excitation output, since thermocouples are self-generating sensors that don't require external excitation.

Can multiple thermocouple 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.

Thermocouple Temperature Transmitter Questions From the Field

My temperature reading is consistently off by a fixed amount across the whole range — what's the likely cause?

A consistent fixed offset across the full range often points to a cold junction compensation issue rather than a thermocouple fault, since a CJC error shifts every reading by roughly the same amount; verifying the CJC sensor and its connection at the transmitter's input terminals is a common first step.

Why does my reading drift when the ambient temperature around the control panel changes?

This is consistent with a cold junction compensation problem, since CJC is specifically meant to track and compensate for changes in the reference junction's temperature; if the compensation isn't tracking correctly, ambient panel temperature swings will show up as spurious drift in the displayed reading.

My open-sensor alarm is triggering even though the thermocouple seems intact — what should I check?

A loose or corroded terminal connection can present electrically similar to an open sensor even when the thermocouple wire itself is unbroken; checking the screw-clamp connections at the transmitter terminals before assuming the thermocouple itself has failed is the standard first step.

Can using the wrong thermocouple type configured in software cause a plausible-looking but wrong reading?

Yes — since each thermocouple type has a different voltage-to-temperature curve, connecting one type of thermocouple while the transmitter is configured for a different type produces a reading that can look plausible while still being significantly wrong; confirming the configured type matches the physically connected thermocouple is an important troubleshooting step.

My fast ON/OFF relay control seems to chatter rapidly near the setpoint — how is that addressed?

Rapid relay chatter near a setpoint is typically addressed with programmable hysteresis and time delays, which prevent the relay from re-triggering on small, normal fluctuations right at the threshold; increasing the hysteresis band is the usual first adjustment.

Why would my reading be noisy specifically near welding equipment or large motors?

Thermocouple signals are very low-level and susceptible to electrical noise from welders, motors, and variable-frequency drives; checking cable shielding, grounding, and physical separation from the noise source is the standard remedy, alongside using the transmitter's digital filtering options for persistently noisy environments.

Can two transmitters configured for the same thermocouple type read slightly differently for the same sensor?

Small unit-to-unit differences can occur since each unit's factory calibration is specific to its own signal conditioner board and CJC sensor; if the difference exceeds the specified conformity error for that thermocouple type, verifying both units against a known reference temperature source is the standard field step.