28 Feb 2000 ISAT 300 1

Resistance Temperature Detectors (RTDs)

28 Feb 2000 ISAT 300 2

Bridge CircuitsISAT 300

Spring 1999

28 Feb 2000 ISAT 300 3

Wheatstone Bridge

A circuit designed to measure changes in resistance

In Instrumentation it is used as signal conditioning for strain gages

28 Feb 2000 ISAT 300 4

Vs

R1 R2

R3R4

+

-

- +Vo

A

B

C

D

28 Feb 2000 ISAT 300 5

Build a Wheatstone Bridge

Vs

+

-

R2

R3

V2

V3

32

33

32

22

RR

RVV

RR

RVV

s

s

28 Feb 2000 ISAT 300 6

Build A Wheatstone Bridge

Vs

+

-

R2

R3

V2

V3

R1

R4

V1

V4

Vo

41

1

32

2

12

RR

RV

RR

RVV

VVV

sso

o

Or

41

4

32

3

RR

RV

RR

RVV sso

430 VVV

Apply Kirchoff’s Voltage Law:

28 Feb 2000 ISAT 300 7

Balancing the Bridge

41

4

32

3

RR

RV

RR

RVV sso

Governing Equation

Multiply by a common denominator

4132

2413

RRRR

RRRRVV so

3241

324

4132

413

RRRR

RRRV

RRRR

RRRVV sso

Simplify

28 Feb 2000 ISAT 300 8

Balance The Bridge

The bridge is balanced if the output is zero

0.00 V

0

if 0

2413

4132

2413

RRRR

RRRR

RRRRVV so

28 Feb 2000 ISAT 300 9

]2

2[

][

2

221

21

2

41

4

2

41

4

32

3

os

osRTD

RTD

RTD

s

o

RTD

RTDso

sRTD

RTDso

sso

VV

VVRR

RR

R

V

V

RR

RVV

RR

RV

RR

RVV

RR

RV

RR

RVV

If R3 is the RTD, then

With some algebra,

For RTD circuitswe can get RRTD as a function of Vo

Start with

Design with R1=R4, then

28 Feb 2000 ISAT 300 10

RTDs: Characteristics and Applications

Characteristics: Resistive device, active, linear Large range: -200 to +850oC for Platinum High accuracy: 0.001oC Low sensitivity: 0.39 % per oC Don’t need reference temperature

Applications: Industries and laboratories where high

accuracy of temperature measurements are required.

28 Feb 2000 ISAT 300 11

Thin-Film RTDs

Thin-film RTD design is a newer technology and is gaining favor due to lower cost. It is designed to minimize strain on the platinum due to thermal expansion since strain also cause changes in resistance, R =(L/A).

28 Feb 2000 ISAT 300 12

Calendar-Van Dusen Equation

For platinum, the resistance temperature relationship is given by the Calendar-Van Dusen equation:

R R T T T T TT o { [ ( . )( . ) ( . )( . ) ]}1 0 0 1 1 0 0 1 0 0 1 1 0 0 1 3

w h ere an d a re co n s tan ts , d ep en d en t o n

th e p u rity o f p la tin u m . , an d fo r

an d fo r .

, ,

1 4 9 0 0

0 11 0

. = T >

= . T <

For the U. S. calibration curve, = 0.003851/°C

(U.S. calibration curve, text p 248)

28 Feb 2000 ISAT 300 13

Platinum RTD: R versus T (U.S. Calibration)

0

50

100

150

200

250

300

-100 0 100 200 300 400

Temperature (C)

Re

sis

tan

ce (

Oh

m)

Equation (9.10)

Table 9.3

28 Feb 2000 ISAT 300 14

RTD’s small resistance change requires

Bridge circuit: Can detect small resistance changes If R1=R4, RRTD= R2(Vs-2Vo)/(Vs+2Vo)

(eq. 9.11)

R1 R2

RRTDR4

VoVs

“Supply”Voltage

28 Feb 2000 ISAT 300 15

Circuits Used to Determine the Resistance of an RTD

Two-wire: Non-linear relationship between the measured voltage and the RTD resistance.

Three-wire: Better results. Four-wire: Resistance is a linear function of the

measured voltage.

Four Wire Design

28 Feb 2000 ISAT 300 16

Example: An RTD probe has a resistance of 100 at 0oC. The Calendar-Van Dusen constants are = 0.00392, = 1.49, and = 0 for T > 0oC. What will be the resistance at 350oC.

R T

1 0 0 1 0 0 0 3 9 2 3 5 0 1 4 9 0 0 1 3 5 0 1 0 0 1 3 5 0

0 0 0 0 1 3 5 0 1 0 0 1 3 5 0

2 3 2 0 8

3

{ . [ . ( . )( . )

. ( . )( . ) ]}

. .

Alternatively, we could use table 9.3 (p248) and obtain RT = 231.89 .

(RT=RRTD)

28 Feb 2000 ISAT 300 17

Summary Thermocouples

Passive, non-linear, increase temperature increase voltages, big temperature range.

Types K and T are common devices. Need reference temperature

Thermistors Active, highly non-linear, increase temperature

decrease resistance. Medical use, not available above 300oC.

RTD’s Requires a Bridge, Linear by nature. High accuracy, use in industry & laboratory.

ALL: time constant of a first order system