2 Measuring With Thermocouple R1

8/12/2019 2 Measuring With Thermocouple R1

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Temperature Sensors


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T

w

S T I R R E R

Tc

Ps

Tm Ti T T

I/P

TC

S T E A M

p

pt

V

C O N D E N S A T E

T c - T H E R M O C O U P L E

T C - T E M P -

C O N T R O L L E R

T T - T E M P - T R A N S M I T T E R

I / P - I / P C O N V E R T E R

w

// P N E U M A T I C

S I G N A L

T E M P - C O N T R O L S Y T E M F O R A S T I R R E D-

T A N K H E A T E R


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Applications Of Temp: Sensors

Control Process Control

Battery Charging

Monitoring

Portable Equipment

CPU Temperature

Battery Temperature

Ambient Temperature

h l


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Common Thermocouples


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TC Voltage O/p Extremes (mV)


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TC Voltage Vs Temp

V S T


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Thermocouple O/p Voltages (J, K,S)


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Seebeck Coefficient Vs Temp


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J-Type Thermocouple


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CJC:TC Law of Intermediate Metals


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CJC:Inserting an Extra Lead in theIsothermal Region


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Classical Cold-Junction Compensation (CJC)


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Classical Cold-Junction Compensation (CJC).


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CJC..

VMEAS= VJ1-VREF

T= a0+ a1v + a2v

2

..anv

n

V= co+ c1T + c2T2+ .. cnT

n


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NIST Polynomial Coefficients for V-to-T


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NIST Polynomial Coefficients for T -to- V


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CJC: Implementation Techniques

Software compensation

Hardware compensation

Reference junction temperature to besensed with a direct-reading sensor


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CJC Software Implementation

Direct Voltage Addition Method

Measure TREF

Convert to VREF

VJ1 = VMEAS + VREF

Convert VJ1 to T


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CJC Software Implementation

Direct Temperature Addition Method

Measure VMEAS and TREF

Convert VMEAS to TMEAS

TTC = TMEAS + TREF


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CJC Signal Connections (Software)


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CJC Hardware Compensation


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CJCHardware Compensation(Using Temperature Sensor)


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CJCSoftware VsHardware compensation

Hardware SoftwareLess accurate Voltage Addition:

more accurate

Separate CJC CKT foreach type

Temperature Addition:

Only fewer

computational steps

Fairly expensive


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Temperature Measurement Error


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Temperature Measurement Error

Total Error = Hot Junction Error +

Cold Junction Error +

Isothermal Bar Error


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Thermocouple Tolerance Table to BS4937 Part20, 1991 ( IEC 584-2 :1982 + A1 : 1989 ).


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Thermocouple Tolerance Table to BS4937 Part20, 1991 ( IEC 584-2 :1982 + A1 : 1989 )..

Compensation cables have tolerances in

between +- 0.5C and +- 5C.


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C


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Cold Junction Measurement

Error Linearization error

Conversion error

Range

ADC Resolution

Error in the voltage/current source

Error due to components tolerance/stability .

Thermistor error

Cold Junction Error = Linearization Error +Conversion Error + Thermister Error

Th l S l ti C it i


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Thermocouple Selection Criteria

Operating Environment:

Operating temperature range

Process fluid (affects life / performance of TC)

Sheath materialThermo well

Cost/Performance Ratio:

Accuracy classLongevity:


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Thermocouple Selection Criteria.

Response Time:

Thermocouple Selection Criteria


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Thermocouple Selection Criteria.

Response Time:


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Cold junction Temperature Measurement


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Specifications

These specifications are typical at 25 C unless

otherwise stated.Analog Input

Number of input channels ................... 1 DIFF

Input signals ....Thermocouples of types B, E, J,K, N, R, S, and T

Input signal range.......................... 100 mV

Input impedance................ 10 M. powered on,

10 k. powered off or overload

Bandwidth ............................................ 2 Hz


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Transfer Characteristics

Gain........................................................ 100

Gain error ...................... ........ 0.08% max

Gain stability ................... 0.0005%/C max

Offset error ........ 5 V max (post calibration)

Offset stability...................... 0.6 V/C max

Nonlinearity ............................ 0.004% max

Recommended warm-up time ................ 5 min


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Cold-Junction Sensor

Sensor accuracy.........0.4 C max from 15 to 35 C,

0.75 C max from 0 to 15 C

0.75 C max from 35 to 55 C

Output ..........1.91 V (0 C) to 0.58 V (55 C)


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Open Thermocouple Detection

Pull-up resistor .................................... 10 M.

Bias resistor......................................... 10 M.

Maximum field wire gauge ................ 16 AWG


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Maximum Working Voltage

Maximum working voltage = signal voltage +common-mode voltage

Each input must remain within 12 V of chassisground.


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Electromagnetic Compatibility

Emissions...................EN 55011 Class A at 10 mFCC Part 15A above 1 GHz

Immunity ..............EN 61326:1997 + A2:2001,


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DAQ Measurement Accuracy


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TC Measurement Issues

TC Wire Decalibration

Insulation Resistance Failure TC Isolation

Auto-Zero Correction/Calibrating System

Offsets Line-Cycle Noise Rejection

Open TC Detection

Errors Due to Open-TC Detection Ckt

Galvanic Action

Thermal Shunting


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Open Thermocouple Detection


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Insulation Resistance Failure


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Auto-Zero Correction

2 Measuring With Thermocouple R1

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