SENSORES DE TEMPERATURA - RTDS Y

TERMISTORES

Liquido (Mercurio) en la botella

de banda bi-metalica

l = l0 (T T0) 105/C

Aplicaciones:Termostatos de aire acondicionadoTermómetros de horno (sensor en forma de espiral con “dial” al final)

Basados en la transmisión de presión en un fluido

Uso directo de un sensor de presión

APPLICATION NOTE January 1993 / June 2000 TD002

PRECISION THERMISTOR THERMOMETRY

I. Introduction.

Thermistors and thermistor probe assemblies are used in diverse fields of temperature sensing, fromneo-natal infant monitoring to tracking the temperature of astronauts in space, from measurement oftemperature in the oceans to maintaining critical temperature parameters of satellites, from a one-timeuse in a disposable medical probe to decades in telecommunications cables. They are the sensors for theautomatic reference junction compensators in thermocouple devices, and they are ubiquitous inelectronic circuits for compensation of component changes due to temperature fluctuations.

What are Thermistors: The name is derived from the device’s major characteristic, a thermally sensitiveresistor. There are two major types: NTC, with a negative temperature coefficient of resistance, andPTC with positive temperature coefficient. The most common type in use for temperature measurementis the NTC thermistor which exhibits a steep decrease in resistance as temperature increases. Theresistance of the thermistor changes approximately three orders of magnitude in a 150°C range. Thisprovides a means to very accurately measure very small temperature variations.

In comparison with other electrical temperature sensors, thermistors are the most sensitive devicesavailable. This is shown below for a type K thermocouple, a 100 ohm platinum RTD, and a 10k ohmNTC thermistor at 25°C:

Table 1. Comparison of Temperature SensorsK Thermocouple 100Ω RTD 10kΩ Thermistor

Sensitivity: 40 uV/°C 0.392 Ω/°C 438 Ω/°CFor 1mk Resolution 4x10-8 Volts 4x10-4 ohms 4x10-1 ohmsTemperature Range -200°C to 1300°C -200°C to 650°C -80°C to 150°C

Other advantages of thermistors include very low excitation power, two-wire connection, ruggedness,interchangeability, flexibility in characteristics, and a large variety of sizes.

Their major disadvantages are a rather limited temperature range, and an inherent nonlinear response.

Temperature Product Group 2670 Indian Ripple Road Dayton, Ohio45440-3605 USA

www.meas-spec.com

• Thermocouple: consist of two dissimilar conductors (or

semiconductors) that contact each other at one point

• RTD: composed of certain metallic elements whose change in

resistance is a function of temperature

• Thermistors: temperature sensitive resistors constructed of

semiconductor material

Measurement and Instrumentation Principles 285

Nickel

Copper

Platinum

Tungsten

200 400 600 800 1000 °C

7

6

5

4

3

2

1

RR0

Fig. 14.8 Typical resistance–temperature characteristics of metals.

14.3.2 Thermistors

Thermistors are manufactured from beads of semiconductor material prepared fromoxides of the iron group of metals such as chromium, cobalt, iron, manganese andnickel. Normally, thermistors have a negative temperature coefficient, i.e. the resistancedecreases as the temperature increases, according to:

R D R0e[ˇ1/T!1/T0] 14.8

This relationship is illustrated in Figure 14.9. However, alternative forms of heavilydoped thermistors are now available (at greater cost) that have a positive temperaturecoefficient. The form of equation (14.8) is such that it is not possible to make alinear approximation to the curve over even a small temperature range, and hencethe thermistor is very definitely a non-linear sensor. However, the major advantagesof thermistors are their relatively low cost and their small size. This size advantagemeans that the time constant of thermistors operated in sheaths is small, although thesize reduction also decreases its heat dissipation capability and so makes the self-heating effect greater. In consequence, thermistors have to be operated at generally

Resistance Temperature Detectors (RTDs)

Para Platinoa1 = 0.00392/C

Metales: resistencia aumenta con aumentos en temperatura

R = R0

1 + 1T + 2T

2 + 3T3 + · · · + nTn

R0 (1 + 1T )

RTD’S

• Platino

• más común

• más inerte químicamente

• respuesta aprox. lineal

• estable (no cambia con el tiempo)

• más caro que los otros

R(T ) R0 (1 + T )

α = coef. de temp. de R

http://archives.sensorsmag.com/articles/0101/24/index.htm

Sensing Element Materials and Temperature Limits

Material Usable Temperature Range

Platinum –450°F to 1200°F

Nickel –150°F to 600°F

Copper –100°F to 300°F

Nickel/Iron 32°F to 400°F

• Relación T versus vO

• Auto-calentamiento

• FSH : factor de auto-calentamiento

• error en T = FSH P

• P = potencia disipada en RTD

v1

R1

R(1+δ)

VREF

¿Cuanto es v1 versus δ?

v1

R1

R(1+δ)

VREF

Z:6)*

=6(+ )

6 + 6 + 6

=6

6 + 6 (+ )(6 + 6)

6 + 6 + 6

=6

6 + 6 6 + 6 + 6 + 6

6 + 6 + 6

=6

6 + 6

+

66 + 6 + 6

=6

6 + 6

+

66 + (+ )6

v1

R1

R(1+δ)

VREF

Podemos restar el primer término electrónicamente

Z:6)*

=6

6 + 6

+

6

6 + ( + )6

7M = 66 <<

Z:6)*

=6

6 + 6

+

6

6 + 6

=6

6 + 6+

66

66 + 6 + 6

=6

6 + 6+

+ 66

+ 66

v1

R1

R(1+δ)

VREF v2R1

R

-

+

RGvOUT

Circuito puente

v1

R1

R(1+δ)

VREF v2R1

R

-

+

RGvOUT

)NIQTPS 'SRWMHIVIYR HIXIGXSVHIXIQTIVEXYVEHI4PEXMRS 4PEXMRYQVIWMWXERGIXIQTIVEXYVIHIXIGXSV S68( GSRYRGSI½GMIRXIHIXIQTIVEXYVE = ./' ] 6 = E ' HIXEPQSHSUYI

6(8) = ( + 8)

'YERXSIWPEVIWMWXIRGMEEXIQTIVEXYVEEQFMIRXI]E '#

7M :6)* = : ]WIHIWIEPMQMXEVPETSXIRGMEHMWMTEHEIRIPWIRWSVTSVHIFENSHI .Q; TEVEIZMXEVIVVSVIWHIFMHSEPEYXSGEPIRXEQMIRXS HMWIyIYRGMVGYMXSGSQSIPUYIWIQYIWXVETEVESFXIRIVYREWIRWMXMZMHEHHI .:/'

)WXMQIIPIVVSVIRUYIWI MRGYVVIGYERHS 8 = ' ]WIYXMPM^E PEIGYEGMzRETVS\MQEHETEVEIWXMQEVIPZSPXENIHIWEPMHE

Ejemplo: Se desea usar un RTD de Platino para medir temperaturas entre 0 y 50 con un error debido a auto-calentamiento no mayor a 1. Seleccione VCC si

R0 = 100Ω a 0α=0.00392/ FSH=0.5/mW vO+ -

R

R

RT

R0

VCC

TERMISTORES• Cerámica semiconductoras como Mg, Co, Cu, Fe,

etc)

• ΔR≈kΔT; PTC (k>0) or NTC (k<0)

• Alta sensitividad

• típicamente opera a T<100

Ecuación de Steinhart-Hart1T

= A + B lnR + C(lnR)3

T = temperatura absoluta R = resistencia a temperatura T A, B, C = Coeficientes que se deben determinar mediante calibración

R(T ) = R0 exp

1T 1

T0

3000K < < 5000K

R versus T (en °K) para un termistor:

Calibración: medir R1,R2 y R3 a temperaturas (conocidas)

T1, T2, y T3 resolver las tres ecuaciones por A, B y C

Ejemplo: R(273°K) = 16.33kΩ R(348°K)=740Ω R(423°K)=92.7Ω

Determine A, B, y C.

−

+ −

+

Vref

R(1+δ) R

R1 R1

R2

vOUT

AO1

AO2

Otros circuitos

−

+

VREFR(1+δ)

RR1

R1

− +−

+R(1+δ)

VREF

R1

R

R1 R2

vOUT