RTD or Thermocouple

We know Rosemount is a major brand from Emerson for field instrumentation. Here we will discuss about temperature measurement thus continuing with accuracy improvement during sensor and transmitter assembled.

About RTD or thermocouple, simply the differences about range measurement. RTD for limited range typically until 600 degC and thermocouple until 1000 degC. Rosemount offers two sensors that widely used in many process plant. The series 65 Platinum RTD and Series 185 Thermocouple. Both specification cannot be compared one by one since the measurement method is different. But we can read the major specification with either Rosemount product as an example.

 

Series 65 Platinum RTD Specification

100 Ω RTD at 0 °C,

α = 0.00385 Ω x °C/Ω.

Temperature Range

–50 to 450 °C or –196 to 600 °C depending on type

Self Heating

0.15 K/mW when measured per method defined in DIN EN 60751:1996

Thermal Response Time

9 seconds maximum required to reach 50% sensor response when tested in flowing water according to IEC 751

Immersion Error

60 mm minimum usable depth of immersion when tested according to IEC 751

Insulation Resistance

1,000 MΩ minimum insulation resistance when measured at 500 Vdc and at room temperature

Sheath Material

316 SST / 321 SST with mineral-insulated cable construction

Lead Wire

PTFE insulated, silver-coated copper wire.

Identification Data

The model and serial numbers are marked on each sensor Ingress Protection (IP) Ratings Rosemount Series 65 sensor assemblies are IP65 / IP68 and NEMA 4X. This rating is applicable only for complete assemblies including either:

• a connection head, extension, and barstock thermowell

• a connection head and tubular thermowell

• a connection head, extension, and sensor

 

 

Series 185 Thermocouple

Construction

A thermocouple consists of a junction between two dissimilar metals that produces a change in thermoelectric emf in relationship to a change in temperature. Rosemount Series 185 thermocouple sensors are manufactured from selected materials to meet IEC 584 Tolerance Class 1. The junction of these wires is welded to form a pure joint, maintaining the integrity of the circuit and ensuring the highest accuracy. Ungrounded junctions are protected from the environment by the sensor sheath. The ungrounded and isolated junctions provide electrical isolation from the sensor sheath.

Sheath Material

Rosemount thermocouples are made of a mineral insulated cable design with a variety of sheath materials available to suit both the temperature and the environment. For temperature up to 800 °C in air, AISI 321 is standard. For Temperatures from 800 to 1100 °C in air, Inconel 600 is standard. For temperatures above 1100 °C, precious metal or ceramic protective sheaths are available upon request. For strongly oxidising or reducing atmospheres, to be consult to manufacturer.

Lead Wires

Thermocouple, internal – 18 SWG (16 AWG) solid wire (max), 19 SWG (18 AWG) solid wire (min.). External extension leads, Type J and K – 0.8 mm minimum stranded wire, PTFE insulated. Color coded per IEC 584. See Figure 3 for wire configuration.

Identification Data

The model and serial numbers are marked on each sensor.

Insulation Resistance

1 000 MΩ minimum insulation resistance when measured at 500

Vdc and at room temperature.

Ingress Protection (IP) Ratings

Rosemount Series 65 sensor assemblies are IP65 / IP68 and NEMA 4X. This rating is applicable only for complete assemblies including either:

• a connection head, extension, and barstock thermowell

• a connection head and tubular thermowell

• a connection head, extension, and sensor

Characteristic of Series 185 thermocouples

Type	Alloys (polarity)	Sheath material	Temp. Range degC	Limits of error (DIN EN 60584-2)	Tolerance class
J	Fe(+), CuNi (-)	1.4541 (AISI 321)	-40 to 375, 375 to 750	1.5oC, 0.004 t	1
K	NiCr(+), NiAl(-)	Inconel 600	-40 to 375, 375 to 1000	1.5oC, 0.004 t	1
N	NiCrSi(+), NiSi(-)	Nicrobell B	-40 to 375, 375 to 1000	1.5oC, 0.004 t	1

 

 

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Source: Rosemount