Temperature Measurement
Temperature Transmitter Configuration: 4-20 mA Ranging and HART Setup
A temperature transmitter configuration must cover means setting the sensor type, LRV, URV, damping, failure mode, and HART address.
Getting any one of these wrong produces a transmitter that passes loop checks but gives incorrect readings in service.
This guide covers the complete configuration sequence for a 2-wire HART temperature transmitter, from sensor wiring to range entry and output verification.
LRV and URV
Sensor Type Selection
HART 250 Ohm Resistor
Sensor and Output Trim
A temperature transmitter outputs 4 mA at the lower range value (LRV) and 20 mA at the upper range value (URV). Re-ranging sets new LRV and URV values. Calibration corrects actual measurement errors. These two tasks are not the same.
Temperature Transmitter Configuration Parameters Explained
Hello! Today we are going through the complete configuration sequence for a temperature transmitter with HART output with 4-20 mA output. Whether you are commissioning a new instrument on the bench or re-ranging a site transmitter through a HART communicator, this guide covers every parameter you need to set and why each one matters.
Before a temperature transmitter will give correct readings, it must know the sensor type, the temperature range for 4 mA and 20 mA, and the sensor failure behaviour.
These temperature transmitter parameters are stored in non-volatile memory and changed via a HART communicator or configuration software.
Three concepts are critical to understand before starting configuration: the difference between re-ranging and calibration, what the HART 250-ohm resistor is for, and the meaning of sensor trim vs output trim. Click any term to expand.
Re-ranging vs Calibration: Re-ranging changes the LRV and URV values stored in the transmitter memory. It tells the transmitter which temperature maps to 4 mA and which maps to 20 mA. It does not correct any measurement error. Calibration (trim) corrects actual errors between the physical sensor input and the transmitter output. A re-ranged but uncalibrated transmitter can be perfectly linear yet still read incorrectly if the sensor has resistance errors. Both are needed for an accurate installation.
HART 250-Ohm Resistor: The HART protocol communicates by superimposing a 1200 Hz FSK signal onto the 4-20 mA loop. To read this digital signal, a minimum loop resistance of 250 ohms is required between the communicator connection points and the power supply. Most DCS input cards already include a 250-ohm burden resistor. If connecting a HART communicator in the field (at the field junction box or across the transmitter terminals), check whether 250 ohms is already present. If not, a 250-ohm resistor must be added in series with the loop before communication is possible.
Sensor Trim vs Output Trim: Sensor trim corrects the input side of the transmitter. It adjusts the internal conversion of sensor resistance (for RTD) or millivolt signal (for thermocouple) to a temperature value. Output trim corrects the output DAC so the 4 mA and 20 mA endpoints match the actual current. See the full guide on sensor trim and output trim for the step-by-step procedure.
4 mA
Output at LRV (Lower Range Value) -- the minimum temperature of the configured range
20 mA
Output at URV (Upper Range Value) -- the maximum temperature of the configured range
250 Ω
Minimum loop resistance required for HART communication to work
3.6 mA
NAMUR NE43 low fault signal -- transmitter output on sensor failure (burnout down)
Step-by-Step Temperature Transmitter Configuration
1
Connect the HART communicator. Connect the communicator in parallel with the transmitter terminals (or at any point on the 4-20 mA loop). Verify at least 250 ohms of loop resistance exists between the connection points and the power supply. Power the loop with 24 V DC. The communicator will poll for a HART device and display the transmitter tag, model, and current configuration once communication is established.
2
Set the sensor type. In the temperature transmitter sensor configuration menu. Select the correct sensor type: RTD Pt100 (3-wire or 4-wire), thermocouple type K, J, T, E, N, S, R, or B, or millivolt input. Also select the number of wires for RTD (2, 3, or 4). An incorrect sensor selection causes the transmitter to linearise the input against the wrong characteristic curve, producing readings that are wrong across the full range. See the thermocouples and RTD guide for sensor type characteristics.
3
Enter the Lower Range Value (LRV). The LRV is the temperature that corresponds to 4 mA output. Enter this value in the transmitter's range configuration menu. For a reactor temperature measurement spanning 0 to 300°C, the LRV would be 0°C. The LRV must be within the sensor type's allowable input range for the selected sensor.
4
Enter the Upper Range Value (URV). The URV is the temperature that corresponds to 20 mA output. For the same 0 to 300°C reactor example, the URV would be 300°C. The span (URV minus LRV) must be at least the transmitter's minimum span for the selected sensor type. Most Pt100 transmitters require a minimum span of 10°C to 25°C.
5
Set the damping time constant. Damping filters out electrical noise and rapid fluctuations in the temperature reading. The default is typically 0 to 2 seconds. Increase damping (4 to 8 seconds) for noisy signals or slow processes. Reduce damping for fast-responding applications such as steam pipe temperature monitoring. See the zero and span adjustments guide for how damping interacts with loop tuning.
6
Configure the sensor failure mode (burnout direction). Set whether the output drives to 3.6 mA (burnout down, NAMUR NE43 low fault) or 21.5 mA (burnout up, NAMUR NE43 high fault) on sensor failure. This tells the DCS or controller what to do when the transmitter detects an open or short-circuited sensor. See the burnout function guide and the NAMUR NE43 guide for how to choose the correct direction for your process.
7
Write and verify. Send the temperature transmitter configuration to memory. Apply a known temperature (or a resistance simulator for Pt100) at the sensor input. Verify the output current matches the expected value using the formula below. Record the as-left configuration in the instrument data sheet.
4-20 mA Output Calculation for a Given Temperature
Temperature to mA Conversion
I (mA) = 4 + 16 × (T minus LRV) / (URV minus LRV)
I: output current in mA
T: actual temperature at the sensor
LRV: configured lower range value (temperature at 4 mA)
URV: configured upper range value (temperature at 20 mA)
Example: LRV = 0°C, URV = 300°C, T = 150°C
I = 4 + 16 × (150 minus 0) / (300 minus 0) = 4 + 16 × 0.5 = 12 mA
T: actual temperature at the sensor
LRV: configured lower range value (temperature at 4 mA)
URV: configured upper range value (temperature at 20 mA)
Example: LRV = 0°C, URV = 300°C, T = 150°C
I = 4 + 16 × (150 minus 0) / (300 minus 0) = 4 + 16 × 0.5 = 12 mA
This temperature transmitter output formula also works in reverse: if you measure the loop current, you can calculate the temperature the transmitter is reporting. See the 4-20 mA signal guide for why this current signal is preferred over voltage signals in industrial settings.
Temperature Transmitter 4-20 mA Calculator
4-20 mA Output and Temperature Conversion Calculator
Convert temperature to mA or mA reading to temperature
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Key Parameters in Temperature Transmitter Configuration
| Parameter | What to Set | Notes |
|---|---|---|
| Sensor type | RTD Pt100, Pt1000, thermocouple J, K, T, E, N, S, R, B, or mV | Must match the physical sensor. Wrong selection gives incorrect linearisation. |
| Sensor wiring | 2-wire, 3-wire, or 4-wire (RTD only) | 3-wire compensates for lead resistance. 4-wire gives highest accuracy. 2-wire RTD introduces lead resistance error. |
| LRV (Lower Range Value) | Temperature at 4 mA output | Must be within the sensor type limits. Can be negative for below-zero applications. |
| URV (Upper Range Value) | Temperature at 20 mA output | Span = URV minus LRV. Must exceed minimum span (typically 10 to 25°C for Pt100). |
| Damping | Filter time constant in seconds (0 to 32 seconds typical) | Higher damping reduces noise but slows response. Default 0 to 2 seconds for most applications. |
| Burnout direction | Up (21.5 mA) or down (3.6 mA) on sensor failure | Set based on the safe failure action required by the process. Drives the controller to the safe side. |
| Tag number | Instrument tag from the P&ID (e.g., TT-101) | Stored in HART memory. Displayed by the communicator for identification. |
| HART poll address | 0 for point-to-point (fixed 4-20 mA), 1 to 63 for multidrop | Multidrop mode fixes output at 4 mA. Use only where digital HART data replaces the analog signal. |
| Units | °C, °F, or K | Must match the DCS scaling. Mixing units causes a systematic scaling error across the full range. |
Re-ranging changes the LRV and URV but does not correct measurement errors. If the transmitter reads 2°C when the actual temperature is 0°C, changing the LRV to minus 2°C is the wrong fix. The correct fix is a sensor trim at the lower point. Confusing re-ranging with calibration is one of the most common commissioning mistakes on temperature loops.
Watch: How to Set Range in a Temperature Transmitter Using HART
Temperature Transmitter Configuration FAQs
What is the difference between re-ranging and calibrating a temperature transmitter?
Re-ranging changes LRV and URV in memory, adjusting what temperature maps to 4 mA and 20 mA. Calibration (trim) corrects actual sensor or output errors. Re-ranging does not fix a measurement error.
Why is a 250-ohm resistor needed for HART communication?
HART superimposes a 1200 Hz FSK signal onto the loop. The communicator needs at least 250 ohms of resistance to decode this signal. Most DCS input cards have a built-in 250-ohm resistor.
What does the burnout direction setting do?
It sets the output on sensor failure: 21.5 mA (burnout up) or 3.6 mA (burnout down). The DCS reads this as a fault and triggers a safe-state action. See the burnout function guide.
What is the minimum span for a Pt100 temperature transmitter?
Most Pt100 transmitters require a minimum span of 10°C to 25°C. A span smaller than the minimum causes the transmitter to reject the configuration. Check the manufacturer data sheet.
What happens if I select the wrong sensor type in the transmitter?
The wrong linearisation curve is applied. A Pt100 configured as Type K will give completely wrong readings. The loop may pass a 4 mA and 20 mA check but give incorrect values at intermediate temperatures.
External References
- HART Temperature Transmitter: Configuration and 4-20 mA vs HART Guide -- Microcybers (2026)
- How to Calibrate a Temperature Transmitter With HART Communicator -- Megatek (2026)
What We Learn Today
- Re-ranging sets the LRV (4 mA) and URV (20 mA) in transmitter memory. Calibration (sensor and output trim) corrects actual measurement errors. They are different tasks.
- HART requires at least 250 ohms of loop resistance between the communicator connection and the power supply. Check before connecting.
- Configuration sequence: sensor type, sensor wiring, LRV, URV, damping, burnout direction, tag number, units. Verify output with the formula I = 4 + 16 × (T minus LRV) / (URV minus LRV).
- Burnout direction must match the safe failure action required by the process. Set burnout down (3.6 mA) where a low reading drives the controller to a safe state.
- HART poll address 0 = point-to-point mode with active 4-20 mA. Addresses 1 to 63 = multidrop mode with fixed 4 mA analog output.
- Selecting the wrong sensor type causes incorrect linearisation across the full range. Always verify the sensor type entry against the physical sensor connected.
“A temperature transmitter with an incorrect sensor type in its configuration will pass a loop check perfectly and still give wrong readings in service. Verify the sensor selection first, every time.”