Temperature Transmitters

What Is a Temperature Transmitter?

Temperature sensors express what they measure as a resistance change of a few ohms or as a thermoelectric voltage in the millivolt range. Over long cable runs these signals pick up electrical noise easily, and control-system input cards need sensor-specific linearization to interpret them. A temperature transmitter is the intermediate device, installed close to the sensor, that measures this weak signal, converts it into temperature and generates a standard output signal.

A two-wire transmitter draws its supply power from the 4-20 mA current loop itself. HART digital communication superimposed on the same loop carries configuration, diagnostic information and secondary variables to the control system or asset management software in addition to the measured value. The sensor input is user-selectable, so the same device can work with different RTD and thermocouple types or with a millivolt or ohm input.

A modern temperature transmitter is far more than a signal converter. It detects faults such as an open or shorted sensor, thermocouple degradation, a broken wire or electrical problems in the current loop before they affect the process, and with a dual-sensor input it provides redundant measurement and calculates average and differential temperature. Choosing the transmitter is therefore one of the key decisions that determine both the accuracy and the reliability of a measuring point.

How a Temperature Transmitter Works

The transmitter passes the sensor signal to the control system in four consecutive steps:

1.      Measuring the sensor signal: With an RTD input, the transmitter applies a small measuring current to the element and determines its resistance, compensating for lead resistance according to the 2-, 3- or 4-wire connection. With a thermocouple input it measures the millivolt signal and separately determines the cold junction temperature at the terminal block to compensate for it.

2.      Digital conversion and automatic self-calibration: At every measurement update, the analog-to-digital converter recalibrates itself by comparing against stable internal reference elements. The update time is about 0.25 s with one sensor and 0.5 s with two. Input-to-output isolation of 500 Vdc limits ground loops and field-induced noise.

3.      Linearization and the sensor-specific curve: The measured resistance or voltage is converted into temperature using the standard curve of the selected sensor type. When sensor-specific Callendar-Van Dusen constants are entered for an RTD, the transmitter uses the sensor's real curve instead of the ideal one, and the error contributed by the sensor drops markedly.

4.      4-20 mA and HART output: A digital-to-analog converter turns the calculated value into a 4-20 mA signal that is linear with temperature, while the HART protocol carries digital data over the same two wires. If the microprocessor or software fails, an independent circuit drives the alarm current in the direction (high or low) selected with a hardware switch.

How Is Total Probable Error Calculated?

The real uncertainty of a measuring point is not made up of transmitter accuracy alone. Total probable error (TPE) is calculated as the square root of the sum of the squares of digital accuracy, D/A accuracy, ambient temperature effects and sensor accuracy. Consider a high-performance-class transmitter with a Pt100 input, a 20 °C reference, 30 °C ambient and a 100 °C process span, used with a Class A RTD at 120 °C: the sensor contributes 0.39 °C and the total probable error comes to about 0.394 °C. The example shows that sensor tolerance dominates total uncertainty — and why sensor-transmitter matching matters.

Non-Intrusive Temperature Measurement

In the optional non-intrusive measurement assembly, the sensor is attached to the outside surface of the pipe with a dedicated clamp. The transmitter measures the pipe surface temperature and a secondary (terminal) temperature, then calculates the process temperature with a built-in algorithm based on the thermal conductivity properties of the assembly and the pipe material. This removes the process penetration, the thermowell and a potential leak point. A standard range of –50 to 300 °C (direct mount) and an extended range of –60 to 650 °C (remote mount) are available, and the solution is recommended for monitoring applications.

Mounting the transmitter close to the sensor keeps the low-level sensor signal on a short path, while a noise-resistant current signal and digital data carry the measurement to the control room.

Key Features

·         Two performance classes: The high-performance class offers ±0.05 °C digital accuracy with a Pt100 input and 20-year stability; the standard class offers ±0.08 °C accuracy and 10-year stability.

·         Broad range of inputs: 2-, 3- or 4-wire Pt, Ni and Cu RTDs; Type B, E, J, K, N, R, S, T, DIN L/U, GOST L and W5Re/W26Re thermocouples; –10 to 100 mV and 0–2,000 Ω inputs are supported.

·         Dual-sensor input: Two independent measurements, average temperature, differential temperature and sensor backup with automatic switchover when the primary sensor fails; dual 4-wire input in the high-performance class.

·         Sensor drift warning: In average temperature mode the difference between the two sensors is monitored; if it exceeds a set limit, the transmitter reports it in warning or alarm mode.

·         Automatic 4-wire to 3-wire fallback: If one wire of a 4-wire RTD breaks, corrodes or works loose, the transmitter keeps measuring without interruption in a 3-wire configuration and raises a maintenance alert.

·         Current loop diagnostics: The current loop is monitored continuously, and problems that affect the communication signal — such as corrosion, water ingress into the housing or an unstable supply — are flagged early.

·         Thermocouple degradation diagnostics and event log: The degradation trend of the thermocouple element is tracked; up to 100 diagnostic events on device condition are stored, and min/max value tracking generates process alerts.

·         One-button sensor recognition: With compatible sensors fitted with an ID tag, the sensor type, wire count and CVD constants are detected automatically at the push of a button, preventing manual configuration errors.

·         Bluetooth and local buttons: Bluetooth allows configuration and maintenance from a line-of-sight distance of typically at least 15 m; local buttons on the graphical display handle loop testing and sensor configuration.

·         Graphical LCD display: 128×128-pixel backlit display with a 0–100% bar graph, a screen that rotates in 90° steps, and a second line that can show measurement 2, differential, average, terminal temperature or loop current.

·         Non-intrusive measurement capability: Optional pipe-surface mounting measures temperature without penetrating the process; clamp options cover pipe sizes from DN15 to DN1500.

Technical Specifications

The values below are typical data for the field-mount temperature transmitter covered on this page. Accuracy and supply values depend on the performance class, sensor type and selected options and should be confirmed at project stage.

Total digital accuracy for thermocouples is obtained by adding the 0.25 °C cold junction accuracy to the value in the table. Digital accuracy is lower for Type B thermocouples between 100 and 300 °C and for Type K between –190 and –90 °C. Performance values for non-intrusive measurement should be assessed separately for each project.

Key Advantages

·         Tighter control around the set point: High digital accuracy and use of the sensor-specific curve improve control performance, especially in reaction and heating loops with narrow tolerances.

·         Longer calibration intervals: Stability of up to 20 years limits transmitter-related drift, allowing calibration periods to be extended on a risk-based basis.

·         Uninterrupted measurement: Sensor backup and automatic 4-wire to 3-wire fallback keep a single sensor or wire failure from turning into a process shutdown.

·         Predictive maintenance: Thermocouple degradation, sensor drift and current loop diagnostics notify maintenance teams before a fault corrupts the measurement.

·         Suitable for safety loops: IEC 61508 certification, FMEDA data and the ability to record SIS proof tests make the design and verification of safety instrumented functions easier.

·         Faster commissioning: Bluetooth access, local buttons and one-button sensor recognition speed up field configuration and reduce manual entry errors.

·         Measurement without a leak point: The non-intrusive option allows temperature monitoring without drilling into the pipe at points where thermowell design is difficult.

Application Areas

Field-mount temperature transmitters are used at process points where temperature data is passed to the control system for control, safety or monitoring. The groups below summarize the applications in which the device's key functions add the most value.

Precision Control Loops

In loops that must operate close to the set point, the high-performance class is combined with a CVD-matched RTD to reduce total uncertainty.

Safety Instrumented Systems

The IEC 61508 certified configuration, with its safety accuracy and 5 s safety response time data, is used in SIS temperature functions; proof tests can be recorded in the device.

Redundant and Differential Temperature Measurement

Backup and drift monitoring with two sensors at critical points; direct calculation of the differential temperature between two sensors in applications such as heat exchanger inlet and outlet.

Points Where a Thermowell Is Difficult to Use

Non-intrusive measurement from the pipe surface on small-bore lines, on lines where a leak point is unacceptable, or at monitoring points where thermowell design is difficult.

Marine and Offshore Installations

ABS, Bureau Veritas, DNV and Lloyd's Register type approval options are intended for shipboard and offshore use.

Hazardous Areas

Flameproof housings and intrinsically safe configurations with ATEX, IECEx and North American approvals are used in gas and dust atmospheres.

How to Select a Temperature Transmitter

Transmitter selection gives the right result when it is considered together with sensor selection. For a technical assessment we recommend clarifying the following:

Input and Performance

·         Sensor type and wire count (3-/4-wire Pt100, thermocouple type, mV or ohm input)

·         Single or dual input; need for backup, average or differential temperature

·         Span to be calibrated and recommended minimum span

·         Required accuracy and stability (standard or high-performance class)

·         Need for factory-entered CVD constants and a 5-point calibration certificate

Diagnostics and Safety

·         Basic sensor diagnostics, or extended coverage including thermocouple degradation?

·         Requirement for IEC 61508 certification and FMEDA data

·         Alarm direction and levels (NAMUR NE 43 or custom levels)

·         Requirement for a tamper-sealed calibration certificate

Electrical and Communication

·         Supply voltage and total loop resistance (250–1,100 Ω for HART)

·         Need for Bluetooth access and a local display/buttons

·         Transient protection (against surges from lightning, welding and heavy electrical equipment)

·         Selection of the 50/60 Hz line filter

Mechanical and Environmental Conditions

·         Housing material (aluminum or stainless steel) and cable entry thread

·         Direct mounting on the sensor, or remote mounting with a bracket on a 2-inch pipe or panel

·         Minimum ambient temperature (low-temperature option below –40 °C)

·         Area classification and required protection type; need for marine type approval

·         If non-intrusive measurement is considered: pipe size, pipe material and insulation

If the process temperature approaches the transmitter's operating limit, the transmitter should be moved away from the heat source by an extension or remote mounting. Sensor and thermowell lengths should then be defined together in line with this decision.

Certifications, Approvals and Applicable Standards

The safety, hazardous-area and communication characteristics of the transmitter are defined by the certifications and standards below. The applicable scope depends on the ordered configuration.

Hazardous-area and functional-safety suitability should be assessed for each project together with the sensor, connection head and cable glands, in cooperation with the TLY Enerji engineering team.

Temperature Transmitter Solutions from TLY Enerji

At TLY Enerji we never consider the temperature transmitter in isolation from the sensor and thermowell; we evaluate the total uncertainty, diagnostic needs and safety requirements of the measuring point together. Working from the process data, our engineers define the input type, performance class, dual-sensor function and diagnostic scope — and, where needed, back up the selection with a total probable error calculation.

We provide support with transferring safety data into SIS documentation, HART configuration, aligning alarm levels with the control system, loading CVD constants into the transmitter and PLC/DCS integration. At points where thermowell design is difficult, we evaluate the non-intrusive option technically, and we work alongside your teams during commissioning and field support. In existing plants, rationalizing transmitter types, linking diagnostic alerts to maintenance workflows and defining a spare-device strategy are also part of this approach.

Frequently Asked Questions

What does a temperature transmitter do?

A temperature transmitter measures the low-level resistance or millivolt signal produced by an RTD or thermocouple, converts it into temperature and sends it to the control system as a 4-20 mA current signal with HART digital data. This keeps the signal immune to noise over long cable runs, and diagnostics make it possible to monitor sensor failures, drift and loop problems.

Is transmitter accuracy the same as total measurement accuracy?

No. Total probable error combines the transmitter's digital accuracy, D/A accuracy, ambient temperature effects and sensor accuracy. For example, while a transmitter with a Pt100 input has a digital accuracy of ±0.05 °C, a Class A sensor contributes 0.39 °C at 120 °C, giving a total of about 0.394 °C. This is why sensor-transmitter matching improves overall accuracy so noticeably.

What functions does a dual-sensor input provide?

A dual input provides two independent measurements, average temperature, differential temperature and sensor backup. In backup mode, the transmitter switches to the second sensor automatically if the primary sensor fails. Average mode can be combined with the sensor drift warning. For differential temperature, accuracy is 1.5 times the worst sensor accuracy when both sensors are of similar type, and the sum of both accuracies when they differ.

Is the measurement lost if a sensor wire breaks?

With a 4-wire RTD, if one of the wires breaks, corrodes or loosens anywhere between the sensing element and the terminal, the transmitter automatically falls back to a 3-wire measurement. The reading continues without a process interruption and a maintenance alert is generated. For complete sensor failure, a dual-sensor backup configuration should be chosen.

Can the transmitter be used in SIL applications?

Yes. With the safety certification option, the transmitter is certified to IEC 61508, with a SIL 2 and SIL 3 claim limit and software data suitable for SIL 3 applications. Safety accuracy is ±2% of span for spans of 100 °C and above, or ±2 °C for narrower spans; the safety response time is 5 s. The FMEDA report is used in safety calculations.

Can process temperature be measured without a thermowell?

For monitoring applications, yes. In the non-intrusive assembly the sensor is clamped to the pipe surface; the transmitter measures the pipe surface temperature and a secondary temperature and calculates the process temperature with a thermal conductivity algorithm. The standard range is –50 to 300 °C and the extended range –60 to 650 °C. For control duties and points needing a fast response, a thermowell installation should be evaluated separately.

What should be considered for HART communication?

For dependable HART signaling, the loop needs a total resistance of 250 to 1,100 Ω. The supply voltage depends on loop resistance and performance class: 11.5–42.4 Vdc in the standard class and 16.7–42.4 Vdc in the high-performance class. Do not communicate with the transmitter while the voltage at its terminals is below 12 Vdc.

What ambient temperatures can the transmitter operate in?

The standard operating range, display included, is –40 to 85 °C, and every unit is characterized over this range at the factory. For colder sites, –50 °C and –60 °C low-temperature options are available; below –40 °C the ambient temperature effect increases. The display slows down below –20 °C, and the local buttons are locked out below –25 °C.