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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › Robotics / Mechatronics › Industrial Sensors and Signal Conditioning for Mechatronics › Lesson 6
MFG-MECH15-04 · Lesson 6 · 14.5 clock hours · lab

Temperature Sensors: Thermocouples and RTDs

Objective. Students identify thermocouple and RTD sensor types, connect each to an appropriate transmitter, and calibrate the transmitter output against a reference thermometer over a specified temperature range.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed engineer or certified journey-level tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Temperature Sensors: Thermocouples and RTDs

Why Temperature Measurement Matters in Mechatronics

Temperature sensors appear throughout manufacturing and marine systems: motor windings, hydraulic fluid reservoirs, heat-treat ovens, food-processing lines, and engine cooling loops all depend on accurate temperature feedback. As a mechatronics technician you must be able to identify the right sensor, connect it correctly, condition its signal into a usable 4–20 mA loop, and verify accuracy before the system goes live.

Thermocouples

A thermocouple is made by joining two dissimilar metal wires at one end. The junction between the metals produces a small voltage — typically in the millivolt range — that changes predictably with temperature. This is called the Seebeck effect. Because the voltage depends on the difference in temperature between the measuring junction (the hot end you insert into the process) and the reference junction (where the wires connect to your instrument), all thermocouple instruments include cold-junction compensation (CJC) to account for ambient temperature at the terminals.

Type designations indicate the specific metal pair:

NIST (the National Institute of Standards and Technology) publishes reference tables that list the exact millivolt output for each thermocouple type at every temperature. These tables are the calibration standard you will use in today's lab.

Polarity matters. Always connect the positive lead of a thermocouple to the positive terminal of the transmitter. Reversed polarity causes the instrument to read backwards from the true temperature. Extension wire must match the thermocouple type — never substitute ordinary copper wire for extension leads.

RTDs (Resistance Temperature Detectors)

An RTD works on a different principle: the electrical resistance of a pure metal increases in a known, repeatable way as temperature rises. The most common type is the PT100 — a platinum element whose resistance is exactly 100 Ω at 0 °C. The resistance-temperature relationship for platinum follows the Callendar–Van Dusen equation, but for practical work technicians read the values directly from the standard PT100 table (IEC 60751).

A few key PT100 resistance values to know:

| Temperature | Resistance |

|-------------|------------|

| 0 °C | 100.00 Ω |

| 100 °C | 138.51 Ω |

| 200 °C | 175.86 Ω |

RTDs are more accurate and more stable over time than thermocouples, but they are also more fragile and respond more slowly. They require an external excitation current from the transmitter. A 3-wire or 4-wire connection is used to cancel lead-wire resistance error — always check the transmitter datasheet and wire accordingly. Never use a 2-wire connection on precision RTD work unless the lead run is very short and error is acceptable.

Signal Conditioning and the 4–20 mA Transmitter

Both thermocouples and RTDs produce signals too small or too variable to send directly over long cable runs. A head-mount transmitter mounts directly on the sensor connection head, converts the sensor signal to a standard 4–20 mA current loop, and provides isolation from electrical noise. Current signals are preferred over voltage signals in industrial settings because current does not degrade with cable resistance over long distances.

The transmitter is configured (or ordered) for a specific sensor type and temperature range — for example, Type J, 0–400 °C. At the low end of the range the transmitter outputs 4 mA; at the high end it outputs 20 mA. The PLC analog input module converts this current to a digital value and the scaling formula maps it to engineering units.

4–20 mA scaling formula:

> Temperature = (mA − 4) / 16 × Span + T_low

Where Span = T_high − T_low.

Safety

Before any wiring work, review the hazard:

Calibration Procedure Overview

1. Set the dry-block (or reference bath) to the first calibration temperature.

2. Allow temperature to stabilize — the block indicator should be steady for at least two minutes.

3. Record the reference temperature from the certified reference thermometer.

4. Record the transmitter output in mA using your calibrated meter.

5. Convert mA to temperature using the scaling formula.

6. Look up the expected millivolt output in the NIST table (for the thermocouple check) or the expected resistance in the PT100 table (for the RTD check).

7. Calculate error: Error = Indicated temperature − Reference temperature.

8. Acceptance criterion: all calibration points within ±2 °C (or ±2% of full-scale span, whichever the worksheet specifies).

Record every reading on the calibration worksheet. If a point is out of tolerance, check wiring, CJC setting, and transmitter configuration before adjusting the trim — never adjust without first diagnosing the cause.

Key terms8

Seebeck effectThe phenomenon by which a voltage is generated at the junction of two dissimilar metals when that junction is at a different temperature than the reference junction; the physical basis of thermocouple operation.
Cold-junction compensation (CJC)A correction applied by the thermocouple transmitter or instrument to account for the temperature at the point where the thermocouple leads connect to the instrument terminals, so the reading reflects only the process temperature.
PT100 RTDA platinum resistance temperature detector whose resistance is 100 Ω at 0 °C; resistance increases predictably with temperature according to the IEC 60751 standard table.
4–20 mA current loopAn industrial signal standard in which 4 mA represents the low end of the measurement range and 20 mA represents the high end; current signals are preferred because they are not affected by cable resistance over long distances.
Head-mount transmitterA compact signal conditioner that mounts directly in the sensor connection head and converts a thermocouple millivolt or RTD resistance signal into a standard 4–20 mA output.
NIST thermocouple reference tablePublished tables from the National Institute of Standards and Technology listing the exact millivolt output of each thermocouple type at every temperature; used as the calibration standard for thermocouple verification.
Dry-block calibratorA portable temperature calibration instrument that uses a heated or cooled metal block with precision-drilled sensor wells to apply a known, stable temperature to a sensor under test.
3-wire / 4-wire RTD connectionWiring configurations that use extra conductors to allow the transmitter to subtract lead-wire resistance from the measurement, improving accuracy compared to a 2-wire connection.

Worked examples4

1. A PT100 RTD reads 113.61 Ω on your precision multimeter at room temperature. What temperature does the standard PT100 table predict for this resistance, and what is the approximate reading you should expect if the room is actually 35 °C?
  1. Look up 100.00 Ω at 0 °C and 138.51 Ω at 100 °C in the standard PT100 (IEC 60751) table.
  2. The span from 0 °C to 100 °C corresponds to 138.51 − 100.00 = 38.51 Ω.
  3. Sensitivity ≈ 38.51 Ω / 100 °C = 0.3851 Ω per °C.
  4. Resistance above 0 °C point: 113.61 − 100.00 = 13.61 Ω.
  5. Temperature estimate: 13.61 Ω ÷ 0.3851 Ω/°C ≈ 35.3 °C.
  6. Compare to the actual room temperature of 35 °C: the difference is about 0.3 °C, which is within normal RTD accuracy.
Answer: The 113.61 Ω reading corresponds to approximately 35.3 °C, agreeing closely with a 35 °C reference temperature. The small difference is within typical measurement uncertainty.
2. A Type-J thermocouple transmitter is configured for 0–400 °C (4–20 mA). The dry-block is set to 200 °C. The NIST Type-J table gives 10.779 mV at 200 °C. Your precision multimeter reads 10.81 mV at the thermocouple terminals. What mA output should the transmitter produce at 200 °C, and what is the millivolt error as a percentage of the 0–400 °C span?
  1. Determine expected mA at 200 °C using the scaling relationship: 200 °C is the midpoint of a 0–400 °C span, so the expected output is midpoint of 4–20 mA = 12 mA.
  2. Confirm: (200 − 0) / (400 − 0) × (20 − 4) + 4 = 0.5 × 16 + 4 = 12 mA. ✓
  3. Find millivolt error: measured 10.81 mV − expected 10.779 mV = 0.031 mV.
  4. Find full-scale millivolt span for Type-J 0–400 °C: NIST table gives 21.848 mV at 400 °C and 0.000 mV at 0 °C, so span = 21.848 mV.
  5. Percent error: 0.031 mV / 21.848 mV × 100 = 0.14%.
  6. 0.14% is well within the ±2% acceptance criterion.
Answer: The transmitter should output 12 mA at 200 °C. The millivolt error is 0.031 mV, or approximately 0.14% of full-scale span — well within the ±2% acceptance limit.
3. The PLC analog input module reads the transmitter current as a raw count. The module converts 4 mA to count 0 and 20 mA to count 27,648 (a common 15-bit unsigned range). The transmitter is calibrated for 0–400 °C. The current count is 13,824. What temperature does this represent?
  1. Find the fraction of span: 13,824 / 27,648 = 0.5000 (exactly 50%).
  2. Apply the scaling formula: Temperature = (count / max count) × Span + T_low.
  3. Temperature = 0.5000 × 400 + 0 = 200 °C.
  4. Alternatively, convert count to mA first: mA = (13,824 / 27,648) × 16 + 4 = 0.5 × 16 + 4 = 12 mA, then Temperature = (12 − 4) / 16 × 400 = 200 °C. Both methods agree.
Answer: A raw count of 13,824 out of 27,648 corresponds to 200 °C.
4. During the wiring-to-PLC exercise the HMI displays 198 °C while the certified reference thermometer reads 200 °C. The configured range is 0–400 °C. Is this within the ±2% of full-scale acceptance criterion?
  1. Calculate the full-scale span: 400 − 0 = 400 °C.
  2. Calculate 2% of full-scale: 0.02 × 400 = 8 °C.
  3. Calculate the actual error: |200 − 198| = 2 °C.
  4. Compare: 2 °C ≤ 8 °C. The reading is within ±2% of full-scale.
  5. Also check the ±2 °C absolute criterion stated in the lab objective: 2 °C is exactly at the limit — borderline acceptable. Investigate wiring and scaling before signing off.
Answer: The 2 °C error is within the ±2% of full-scale criterion (which allows up to ±8 °C on a 400 °C span) but sits right at the ±2 °C absolute limit. The reading is marginally acceptable; verify scaling constants and cold-junction compensation before approving.

The math in this lesson2

Learners check whether a 2 °C HMI error falls within ±2 percent of a 0–400 °C span.
7.RP.3F-LE.1
Brush up in the math halls at newSchoolWorks.com:
In class: Bench comparison: students measure resistance of a PT100 RTD at room temperature, calculate expected resistance using the standard PT100 table, and compare; repeat with a Type-J thermocouple millivolt output measured with a precision multimeter. · Transmitter calibration lab: students connect a Type-J thermocouple to a head-mount transmitter, use a dry-block calibrator or ice/boiling-water reference points to apply known temperatures, record transmitter mA output, and calculate error against the NIST thermocouple table values. · Wiring-to-PLC exercise: students wire the 4–20 mA transmitter output to a PLC analog input, apply the scaling formula, and verify the HMI temperature display within ±2°C of the reference.
Materials: Type-J and Type-K thermocouples, PT100 RTD, head-mount transmitters, dry-block temperature calibrator or ice/boiling-water baths, precision multimeter, NIST thermocouple reference tables, PLC analog module, HMI, lab worksheet.
Check: Calibration worksheet with error column; all calibration points within ±2% of full-scale range; instructor verifies PLC HMI reading.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Siemens SMSCP Level 1 — Electrical Components and Drives
1. A Type-K thermocouple is connected to a transmitter with the positive and negative leads reversed. What is the most likely effect on the temperature display?
PMMI Mechatronics — Industrial Electricity
2. According to the standard PT100 table (IEC 60751), what is the resistance of a PT100 RTD at 0 °C?
PMMI Mechatronics — Programmable Logic Controllers / Industrial Electricity
3. A 4–20 mA transmitter is configured for a thermocouple range of 50–250 °C. The transmitter output is measured at 12 mA. What is the indicated temperature?
Siemens SMSCP Level 1 — Electrical Components and Drives
4. Which wiring configuration eliminates lead-wire resistance error and provides the highest accuracy for a PT100 RTD installation?
FANUC CERT / Siemens SMSCP — Safety
5. Before connecting a head-mount transmitter to an energized terminal strip, which standard requires you to verify the absence of voltage using a calibrated meter and to wear appropriate PPE?
PMMI Mechatronics — Industrial Electricity
6. A technician forgets to use thermocouple extension wire and instead runs ordinary copper wire from the thermocouple junction back to the transmitter. What is the primary error introduced?
PMMI Mechatronics — Industrial Electricity
7. During a transmitter calibration, the dry-block reference thermometer reads 150 °C and the transmitter display reads 154 °C. The sensor range is 0–400 °C. Is this point within the ±2% of full-scale acceptance criterion?
Siemens SMSCP Level 1 — Electrical Components
8. Which thermocouple type uses an iron / constantan metal pair and is commonly found in older industrial and plastics processing equipment?
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