PRT 140: Lesson 3 Temperature

Objectives

• Define ‘temperature’ and related terms
• Identify common types of Temperature instrumentation
• Perform temperature and pressure conversion calculations
• Review Temperature instrumentation symbols on PID’s

Temperature – Terms to Know

• Sensor – will respond to the process variable
• Temperature
• Heat Transfer, Conduction, Convection, Radiation
• Phase Change, Heat of Vaporization
• Latent Heat, Sensible Heat
• BTU
• Fahrenheit/Rankine; Celsius/Kelvin
• Absolute Temperature

Heat Transfer Terms

Convection

• Mixing two substances or direct contact with one substance (exhaust gases)
• hot coffee and cold cream
• Convection oven – blows hot air around food

Conduction

• Contact through a solid – no mixing; indirect contact
• Shell/tube heat exchangers
• Pan on electric stove top – pan conducts heat from burner to food

• Electromagnetic waves – no contact
• Sunshine, heat from woodstove, burner flames in furnace
• Pan under a broiler – radiant heat from burner/flame onto food

Absolute Temp/Pressure

Absolute temperature – temperature scale where 0 = ‘absolute zerothe temperature where no more heat can be removed from a system. This corresponds to 0 K or -273.15°C.   Theory – no more molecular movement.

Absolute pressure – pressure measured from 0 = full vacuum. 0 psia = full vacuum

Gauge pressure – pressure measured from 0 = current atmospheric pressure. 0 psig = atmospheric pressure

Temperature Scales vs. Actual Temperature

Absolute Temperature:

(Kelvin) K = C + 273

(Rankin) R = F + 460

F = (C x 9/5) + 32

C = (F-32) x 5/9

K = R x 5/9

R = K x 9/5 Temperature Scales vs. Actual Temperature [image 140-3-01-01]

Latent Heat vs Sensible Heat

• Sensible Heat – heat that can be ‘sensed’ by a thermometer – i.e. the temperature changes
• Latent Heat – heat that cannot be ‘sensed’ by a thermometer – i.e. temperature doesn’t change when phase is changing.
• Boiling water stays at 212 F until all of it is steam – even while we keep adding heat. This is called the latent heat of vaporization.
• Same idea when water freezes – releases latent heat.

Temperature Instruments

• Thermowell
• Thermometer
• BiMetallic Strip
• RTD – Resistance Temperature Device
• Thermocouple
• Thermistor
• Temperature Gauge

Thermowell

• Not an instrument – it holds the instrument and protects it from the process, while allowing heat transfer
• TW

Thermometer

• Glass Bulb – standard
• Also IR – Infrared – non-contact Infrared Train, By Jagokogo (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons[image 140-3-03]

BiMetallic Strip

• Two Dis-similar metals, bonded together
• Expansion/Contraction with Temperature different for 2 metals
• Movement of strip, moving temperature dial
• Dial thermometer very common

RTD – Resistance Temperature Device

• Electrical resistance (ohms) will change with temperature – varying output signal
• More accurate than Thermocouple
• Smaller operating range (-200F to 900F)
• Ohms resistance in the RTD correlates to temperature – see tables

100 (1000) Ohm Platinum RTD Resistance Chart
Generally RTDs are a 3- or 4- wire configuration – fine electrical wires

Thermocouple

• Most common, simplest
• Two dissimilar metals – generate voltage at their junction when they are heated.
• Measured junction – connects to process
• Cold junction/reference – connects to transmitter
• mV generated across Thermocouple correlates to temperature – see tables Thermocouple – excerpt from voltage chart [image 140-3-08]

Thermocouples

• Generally 2-wire configuration
• Wires are thicker
• made of the dissimilar metals ANSI MC96.1 Color Coding table [140-3-9]

Thermistor

• Ceramic resistor – same principle as RTD –
• Electrical resistance through thermistor changes with temperature
• Usually small bead/disk
• Registers very small temperature differences

Temperature Gauge

No transmitter – gauge face (like PI)

Temperature transmitters – How to identify them in the field

• Read the nameplate on the instrument
• Should include calibration range
• Will include model number – can look it up
• May include facility tag number
• Look at how it’s connected to the process
• Temperature sensors almost always have THERMOWELLS to house the sensor
• Temperature sensors usually extend into the process stream – so there will NOT be an isolation valve on the process connection.

Temperature Conversion Calculations

• Absolute: (Kelvin) K = C + 273
• Absolute: (Rankine) R = F + 460
• F = (C x 9/5) + 32
• C = (F-32) x 5/9

Temperature Conversion

32 C = ?? F

F = (C x 9/5) +32

F = (32 x 9/5) + 32

F= 89.6

32 C = 89.6 F

25 F = ?? C

C = (F-32) x 5/9

C= (25-32) x 5/9

C= -3.9

25 F = -3.9 C

25 C = ?? F

F = (C x 9/5) +32

F = (25 x 9/5) + 32

F = 77

25 C = 77 F

100 C = ?? K

K = C + 273

K = 100 + 273

K = 373

100 C = 373 K

Temperature ‘linear scaling’ – NEW

The conversions between F and C temperature scales are an example of how we use the linear scaling calculation.

We looked at this equation in week 1 – it seems very complicated:

VALUEB = {[(VALUEA – LRVA)/SPANA] x SPANB} + LRVB

But it’s not, and you’ve been doing it already:

LRV = lower range value = the lowest value in the operating range

URV = upper range value = the highest value in the operating range

Span = URV – LRV

We pick two equivalent operating ranges:

Temperature – range from freezing to boiling:

F:   LRV=32°F
URV=212°F
Span = 180°F
C:  LRV = 0°C
URV = 100°C
Span = 100°C

Convert any C reading to F
(Whenever we get data, that set of units becomes the “A” data)

F = {[(C reading – 0 C)/100 C] x 180 F} + 32 F

This equation is mathematically equivalent to:

F = (C X 9/5) + 32

Try it out! Convert 140°C to F

F = {[(C reading – 0 C)/100 C] x 180 F} + 32F

F = {[(140C – 0C)/100C] x 180F} + 32 F

F = {[140C/100C] x 180 F} + 32 F

F = {1.4 x 180 F} + 32 F

F = 252 F + 32 F

F = 284 F

140 C = 284 F

F = (C X 9/5) + 32

F = (140 x 9/5)+32

F = (252) + 32

F = 284

140 C = 284 F

Linear Scaling Calculation VALUEB = {[(VALUEA – LRVA)/SPANA] x SPANB} + LRVB

The linear scaling calculation is used to relate all kinds of linearly related scales. For example:

• Operating temperature range will be 200 F to 600 F
• We want the 4-20 mA signal from the transmitter to reflect that range.

We can use the scaling calc to calculate the mA reading at any temperature in the range – try 440 F.

F:  LRV=200 F  URV= 600 F  Span = 400 F

mA:   LRV = 4 mA  URV = 20 mA  Span = 16 mA

mA = {[(440F – 200F)/400F] x 16 mA} + 4 mA

mA = {[240F/400F] x 16 mA} + 4 mA

mA = {0.6 x 16mA} + 4 mA

mA = 9.6 mA + 4 mA = 13.6 mA at 440 F

Process Variable Relationships

Pressure, Volume, Temperature in a closed container

Pressure/Temperature are in Absolute Units

T = K,R     P = psia

V1, V2 both in the same units