Contents

## 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

## Reading

### 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

**Radiation**

- 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 zero****’**** – **the 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

### 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

### 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

### Thermocouples

- Generally 2-wire configuration
- Wires are thicker
- made of the dissimilar metals

### 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:

**VALUE _{B} = {[(VALUE_{A} — LRV_{A})/SPAN_{A}] x SPAN_{B}} + LRV_{B}**

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:

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

**VALUE**_{B}** = {[(VALUE**_{A}** — LRV**_{A}**)/SPAN**_{A}**] x SPAN**_{B}**} + LRV**_{B}

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

V_{1}, V_{2} both in the same units