Temperature Sensors

ECE 371 JB Prof. Bernhard

A Simple Thermal System

Sensor Work Load Temperature Controlling Device Heat Source Sensor Input Output

Types

Thermocouples Resistance temperature devices (RTD) Thermistors Infrared sensors

Thermocouples

Mostly widely used in industry Range: sub-zero to 4000 o F(2000 o C) Formed by joining two different metal alloy wires (A,B) at point called junction Junction called the measuring or “hot” junction Lead ends attached to temp indicator or controller Connection point called reference or “cold” junction Measuring Junction A B Reference Junction + -

Display Device

How does it work?

Measuring junction is heated, small DC voltage (millivolts) generated in thermocouple wires Thermocouple

converts thermal energy into electrical energy

Note: thermocouple only generates a millivoltage signal when there is

temperature difference

between “hot” and “cold” junctions “cold” junction usually set to 32 o F(0 o C)

Thermocouple Types

Made up of two different metal alloy wires.

Different alloys result in different temperature ranges Ex:

Standard Type B Metal Content (Pos. Leg) 70.4% (Pt) 29.6% (Rh) Metal Content (Neg. Leg) 93.9% (Pt) 6.1% (Rh) Temp. Range 1600-3100 o F 870-1700 o C E 90% (Ni) 10% (Cr) 55% (Cu) 45% (Ni) 32-1650 o F 870-1700 o C

Pros/Cons

Each thermocouple type has advantages & disadvantages – Cost: Rare metals (i.e. noble metals)  – Types B, R, S $$$ Common metals (i.e. base metals)  – Types E, J, K, N, T $ Rarer metals = high temperature range & better accuracy – Temperature Range – Accuracy a.k.a. tolerance – Life Expectancy

Type Max.

Operating Temp.

Temp.

Tolerances

Wire size Thermocouple protection Environment Accuracy required

B E 3100 1700 1650 o o o 900 o C F C F (+/-) 0.5% (+/-) 1.7

o C (+/-) 3.06

or (+/-) 0.5% o F Whichever greater

Life Expectancy

Failed = inaccuracy - When wires are heated/cooled changes take place on molecular level Physically: molecular structure changes Chemically: wires react with oxygen or other substances, changing chemical composition Result: millivolt signal “drifts” EMF (mV) Tolerance Band Time - Recalibration: adjust controller to compensate for errors

Thermocouple Constructions

3 General constructions – Insulated Wire – Ceramic-beaded – Metal-sheathed

Insulated Wire Thermocouples

Bare wires wrapped with insulation – Insulations Fibrous, woven material made of fiber-glass, mica, or ceramic fiber Plastics (Teflon) Polyimides (Kapton) – Purpose Electrically isolate wires Protects wires from contamination Easier wire installation

Metal - Sheathed Thermocouples

Junction and wires are assembled in small diameter metal tubes – Insulation Fiberglass MgO – Purpose Protects against contamination Defends against chemical attack Provides mechanical stability

Metal - Sheathed Thermocouples

Orientation of thermocouple junction during assembly – Grounded Weld junction directly to inside tip of sheath Ensures rapid heat transfer from sheath to junction Protects junction while minimizing heat transfer delays. – Ungrounded Similar to grounded except junction isolated from metal sheath Electrically isolates junction from sheath Prevents stray voltages from inducing measuring error More shock resistant & better under rapid temperature changes DISADVANTAGE: Slows down heat transfer to junction (2x-3x slower) – Exposed Junction protrudes from end of sheath, but insulated from it Due to direct exposure with heated material, very quick response to temp. changes No sheath to slow down heat transfer DISADVANTAGE: Not protected from mechanical damage & chemical attack

Resistance Temperature Devices (RTD)

Precision Temperature Sensors – More accurate than thermocouple elements – Maintain accuracy over longer period of time – Range up to 1200 o F (650 o C) Styles – Wire-Wound – Thin film – Kapton Insulated

How do RTDs work?

RTD’s resistance as temp. – Controller measures resistance value and converts to temp. reading, fairly linear relationship.

– Unlike thermocouple, no electrical signal generated – Controller measures resistance by passing current through RTD – Use a base resistance value (ex: for Platinum, value of 100 ohms at 0 o C (32 o F) Resistance (Ohms) RTD Resistance Vs. Temp. (TCR) Curve TCR = Temperature coefficient of resistance Temperature ( o C)

RTD Vs. Thermocouples

Advantages of RTDs – Stability – Repeatability – Accuracy Disadvantages of RTDs – Cost: Platinum = $$$, 2x more expensive – Temp. Range limited – Response Time slower, 2x-4x times slower Heat must transfer through epoxy or glass coating Entire RTD element must reach uniform temp. before accurate measurement taken.

Lead Wire Effect

Alters reading due to lead wire resistance Two approaches – Determine lead wire resistance and have controller compensate – Attach additional lead wire to one end of RTD – Connect a transmitter, converts resistance to low amp signal and sent to temperature controller 1 2 3 1 2 3 4 RTD 3-wire RTD RTD 4-wire RTD

Effect of Lead Resistance: Platinum Wire RTD

Most Common: DIN 43760 – Standard temp. coefficient (alpha=0.00385) For 100 ohm wire  +0.385 ohms/ O C @ 0 alpha = average slope from 0 o C – 100 o C o C – A 10 ohm lead impedance implies 10/3.85 = 26 o C error in measurement  Lead 100  RTD Lead R=5 

How to correct this problem?

Wheatstone: 3-Wire Bridge – Wires A & B are perfectly matched in length, respective impedances effects will cancel out due to being on opposite legs – Wire C acts as sense lead & carries no current

DVM

A C RTD B – Non-linear relationship between resistance change and bridge output voltage change – Additional equation required to convert bridge output voltage to equivalent RTD impedance

3-Wire Bridge Calculations

If V s & V o known, R g can be found.

Unbalanced V o

V o



V s

 

R

3

R

 3

R g

If R g =R 3  of bridge with R 1 =R 2   

V s

( 1 / 2 ) V o =0 & bridge is balanced To determine R g zero

R g



R

3  

V V s s

assuming lead resistance is  2

V o

 2

V o

 

V 3 /2

3-Wire Bridge Calculations

If R g R L located some distance from 3-wire configuration appears in series with R g & R 3 

R g



R

3  

V V s s

 2

V o

 2

V o

  

R L

 

V s

4

V o

 2

V o

  + V o + R L R L R g

Current Source

Another Approach

4-Wire Ohms – DVM is directly proportional to RTD resistance  equation required – Insensitive to length of lead wires – Accuracy better than 3-wire – Disadvantage: One more extension wire required.

1 conversion + i = 0 100 W RTD DVM i = 0 RTD=R g V s + V o +

Resistance to Temperature Conversion

RTD more linear than thermocouple, curve fitting still required Callendar-Van Dusen Equation

R T



R o

   

T

  

T

100  1  

T

100 )    

T

100  1    

T

100 3    R T = Resistance at Temperature T R o = Resistance at T=0 o C  = Temperature coefficient at T=0 o C   = 1.49 (typical value for 0.00392 platinum) = 0 T>0, 0.11 (typical) T<0

1

Identification

2-wire RTD uses same color lead wire for both leads 3-wire has 2 red leads & 1 white lead 4-wire has 2 red leads & 2 white leads 3 4 2 Lead-to-lead Measurement 1 to 2; 3 to 4 4-wire RTD 1 to 3; 1 to 4 2 to 3; 2 to 4 Distance at Room Temperature Less than 1ohm to a few ohms max.

107 to 110 ohms RTD

RTD Assembly

Wire Wound – For 500 o F (260 o C), element welded to copper or nickel lead wires – Sub-assembly placed in closed-end tube – Powder, cement or thermal grease fills tube – Epoxy seal seals out moisture & locks RTD/leads to tube Thin Film – For 1200 o F (650 o C), element fitted into cavity of MgO metal-sheathed cable – Wires in cable welded to RTD element – Cap filled with MgO and placed on element end & mounted

What are Thermistors?

Semiconductor used as temperature sensor Made from mixture of metal oxides pressed to bead or wafer form Bead heated under pressure at high temp & encapsulated with glass/epoxy RESULT: Distinct non-linear resistance vs. temp. relationship Non-linear decrease in resistance as temperature increases.

Resistance (Ohms) Temperature ( o C)

So Sensitive…

Very large resistance change = small temp. change 3 – 5% per o C (vs. 0.4% per o C for RTDs) Temp. changes as small as 0.1

o C Significantly smaller in size Temp range: -100 o C – 300 o C (-120 o F – 570 o F)

Thermistor Standards

No Industrial Standards Base resistance range: 10 3 – 10 6 ohms – Typically measured at 25 o C vs. 0 o C for RTDs TCRs vary widely Thermistor’s accuracy limited to small temp. range

Thermistor Lead Wire Effects

Lead wire does add overall resistance NOTE: base resistance of thermistor very large (>10 3 ohms), added lead wire resistance insignificant.

RESULT: No resistance compensation required!

Infrared Sensors

Intercepts portion of infrared energy radiated by  Waves focused through lens on infrared detector, converting to an electric output signal Non-Contact Temp. Sensor Heat Source Temp. Indicator Optics Infrared Detector

Emissivity

Def: The ability of a material to radiate or absorb electromagnetic waves. Higher = Better!

– Ex: Given values below & emissivity varies by 0.05, what is measuring error?

Ans: IR Sensor A 5.5% (0.05/0.9) IR Sensor B 10% (0.05/0.5) IR Sensor A e = 0.9

IR Sensor B e = 0.5

Field of View

All infrared radiation in this filed of view will be detected by the sensor 4.5 in (114 mm) 1.0 in (25 mm) 1.4 in (36 mm) 2.5 in (64 mm) 0.60 in (15 mm) 0.75 in (19 mm) Infrared Sensor 25 mm 76 mm 152 mm

Good vs. Bad Radiation

Position 1, IR sensor sees both target object & background objects Position 2, IR sensor only sees target object. True target temperature can now be measured.

RULE: target size should be at least 1.5 to 2 times the “spot size.” Infrared Sensor 2 Correct Target Placement 1 Incorrect Target Placement Background “Noise”

Scenarios to Avoid

Figure 1: Thin film materials & background radiation enter sensor Figure 2: Polished metals will not function well with infrared sensing due to the reflecting radiation.

Infrared Sensor Figure 1 Infrared Sensor Figure 2

Sensor to Target Distance

To reduce reflected radiant energy, set IR sensor at right angle with respect to target If space limitation, mount IR up to a maximum of 45 O Sensor <45 o Product

Operating Environment

Smoke, dust vapors absorb or reflect infrared radiation before getting to sensor lens.

Causes controller to maintain target at wrong temperature Target Infrared Sensor Smoke or Vapors

So which one is better? Advantages

Thermocouple RTD Thermistor Infrared

Simple, rugged High temp. operation Low Cost No resistance lead wire problems Point temp. sensing Fastest response to temperature changes Most stable over time Most accurate Most repeatable temp. measurement Very resistant to contamination/corrosion of the RTD element High sensitivity to small temperature changes Temperature measurements become more stable with use Copper or nickel extension wires can be used No contact with the product required Response times as fast or faster than thermocouples No corrosion or oxidation to affect sensor accuracy High repeatability

So which one is better? Disadvantages Thermocouple RTD

Least stable, least repeatable Low sensitivity to small temperature changes Extension wire must be of the same thermocouple type Wire may pick up radiated electrical noise of not shielded Lowest accuracy High Cost Slowest response time Low sensitivity to small temperature changes Sensitive to vibration Decalibration if used beyond sensor’s temperature ratings Somewhat fragile

So which one is better? Disadvantages Thermistor Infrared

Limited temperature range Fragile Some initial accuracy “drift” Decalibration if used beyond the sensor’s temperature rating Lack of standards for replacement High initial cost More complex – support electronics required Emissivity variations affect temperature measurement accuracy Field of view and spot size may restrict sensor application Measuring accuracy affected by dust, smoke, background radiation etc.