Temperature input module for RTD and thermocouple sensors (4-channel Pt100 and thermocouple input card for PLC and EtherCAT remote I/O)

01

Pt100 inputs: 2-, 3- or 4-wire connection and the lead resistance each one leaves in the reading

Texas Instruments' guide to RTD measurement sets out the three wiring options. With two wires, the lead resistances cannot be separated from the RTD resistance, so they add an error; two-wire is the least accurate and is used when accuracy is not critical or leads are short. With three wires, lead resistance can be cancelled, but the compensation assumes the lead resistances match. With four wires, excitation flows through one pair and the RTD voltage is sensed on the other, so the lead resistance contributes no error. [1]

The module itself converts resistance to temperature. TI gives the Callendar-Van Dusen equation with the IEC 60751 coefficients for a Pt100 (R0 = 100 Ω at 0 °C; A = 3.9083×10⁻³, B = −5.775×10⁻⁷, C = −4.183×10⁻¹²), and notes that a straight-line fit from −200 to 850 °C would be off by more than 16 Ω, which is why the curve is computed or looked up rather than treated as linear. [1]

Editorial calculation: near 0 °C a Pt100 changes by about 100 Ω × 3.9083×10⁻³ ≈ 0.39 Ω per °C. Assuming two copper leads of 10 m each at 0.5 mm² (copper about 0.0172 Ω·mm²/m), the loop adds about 0.69 Ω, which a 2-wire input would read as roughly 1.8 °C too high. On a 3-wire input the error cancels to the extent the leads match, and on a 4-wire input the leads contribute none, per the wiring descriptions above.

EDATEC's EtherCAT temperature module accepts 2-, 3- and 4-wire RTD connections, defaulting to 3-wire, and lists Pt100, Pt200, Pt500, Pt1000 and Ni20 sensors, with ranges of −200 to +800 °C for the platinum types and −79 to +309 °C for the nickel type. [3]

State the RTD type (Pt100, Pt1000, Ni), the number of wires at the sensor, the lead length and cross-section, and whether the module must detect a broken or shorted sensor.

02

The sensor's IEC 60751 class adds to the module's error

A module accuracy figure covers the electronics; the RTD has its own tolerance. TI's table of tolerance classes shows the error each grade allows at 100 °C. [1]

TI notes that the temperature range over which each class applies narrows for the tighter classes and depends on the sensor construction, and that the North American ASTM E1137 grades A and B are defined differently. [1]

Editorial calculation: at 200 °C a Class B Pt100 is allowed ±(0.3 + 0.005 × 200) = ±1.3 °C. Paired with a module rated at ±1 °C, such as EDATEC's [3], the measurement chain could be off by up to about 2.3 °C in the worst case, or about 1.6 °C if the two errors are combined as independent (root-sum-square). A loop that has to hold a tighter band needs a better sensor class, a better module, or both.

03

Thermocouple inputs: microvolts per degree and cold-junction compensation at the terminals

TI's thermocouple guide tabulates the output of types J, K, T, E and S and their Seebeck coefficients at 0 °C. A thermocouple input module has to resolve signals of this size. [2]

The thermocouple voltage depends on the difference between the hot junction and the cold junction, where the thermocouple wires meet the module's copper terminals. TI describes cold-junction compensation as measuring that block's temperature with a diode, thermistor or RTD, converting it to the equivalent thermocouple voltage, adding it to the measured voltage, and only then converting the sum to temperature; because the curves are non-linear, the cold-junction temperature cannot simply be added in degrees. TI also notes that air currents can disturb the block and an enclosure may be needed. [2]

EDATEC's thermocouple module accepts types K, J, T, E, N, S, R, B and C, with type K the default over −100 to +1,370 °C. [3] Editorial calculation: at type K's 39.48 µV/°C, the module's 0.1 °C resolution corresponds to about 4 µV at the terminals; for type S at 10.19 µV/°C, the same 0.1 °C is about 1 µV, which is why the noble-metal types are harder to read precisely.

State the thermocouple type, whether extension or compensating cable runs to the module, where the cold junction sits (module terminals or an external block), and the cabinet temperature swing.

04

Reading a temperature module data sheet: channels, resolution versus accuracy, isolation

EDATEC sells its EtherCAT temperature modules as two separate 4-channel parts, ED-EIO4RTD for RTDs and ED-EIO4TC for thermocouples, each with a per-channel LED that flashes while a signal is detected and is off with no input. [3]

Resolution and accuracy are separate figures: the module reports in 0.1 °C steps, but the stated accuracy is ten times wider. Editorial note: where channels are not isolated from one another, as here, ask how the module handles sensors at different potentials, for example grounded-junction thermocouples on separate machine parts.

Ask for accuracy per sensor type and range (not one figure for all), the cold-junction error for thermocouple modules, conversion time per channel, sensor-break detection, and channel-to-channel isolation.

05

Data for a temperature input module quotation

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Sensors: [RTD type and class, or thermocouple type; number per module]
Wiring: [2-, 3- or 4-wire; lead length and cross-section; extension cable]
Range and accuracy: [temperature range per channel; accuracy needed in °C]
Cold junction: [at module terminals or external; cabinet temperature range]
System: [PLC or coupler; EtherCAT, PROFINET or other; update time]
Isolation and diagnostics: [field-to-bus; channel-to-channel; broken-sensor detection]

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