Electrical Temperature Instruments
Introduction to Electrical Temperature Instruments
Temperature is one of the most commonly measured physical quantities in science and engineering. Electrical temperature instruments convert temperature into electrical signals that can be measured, displayed, and used in control systems.
Temperature measurement is essential in many fields, including:
- Manufacturing processes
- Environmental monitoring
- Medical diagnostics
- Food processing
- Chemical reactions
- Electronics cooling
Basic Principles of Temperature Measurement
What is Temperature?
Temperature is a measure of the average kinetic energy of particles in a substance. It determines the direction of heat flow between objects in thermal contact.
At the molecular level, higher temperature means faster-moving particles.
Temperature vs. Heat
Temperature: Intensity of thermal energy (°C, K, °F)
Heat: Total thermal energy content (Joules, calories)
A small cup of boiling water has higher temperature but less heat content than a swimming pool at room temperature.
Classification of Temperature Measuring Instruments
Contact Methods
- Thermocouples: Based on thermoelectric effect
- RTDs: Resistance Temperature Detectors
- Thermistors: Temperature-sensitive resistors
- Semiconductor Sensors: Silicon-based IC sensors
Non-Contact Methods
- Infrared Thermometers: Measure thermal radiation
- Thermal Imaging Cameras: Create heat maps
- Optical Pyrometers: Measure incandescent objects
Temperature Sensor Characteristics
| Characteristic | Description |
|---|---|
| Accuracy | Closeness to true temperature (±°C or %) |
| Range | Minimum to maximum measurable temperatures |
| Sensitivity | Change in output per degree change (mV/°C, Ω/°C) |
| Response Time | Time to reach 63.2% of final value after step change |
| Stability | Ability to maintain calibration over time |
| Self-heating | Temperature rise due to power dissipation in sensor |
When selecting a temperature sensor for an application, consider:
- Temperature range required
- Accuracy needed
- Response time requirements
- Environmental conditions (chemicals, moisture, vibration)
- Cost constraints
- Size limitations
Temperature Measurement System
A complete temperature measurement system consists of:
- Temperature Sensor: Converts temperature to electrical signal
- Signal Conditioning: Amplifies, filters, and linearizes the sensor output
- Processing Unit: Converts signal to temperature value, may apply calibration
- Display/Interface: Shows temperature reading in user-friendly format
Temperature Scales
Temperature scales provide standardized ways to express temperature measurements. The three most common temperature scales are Celsius (°C), Fahrenheit (°F), and Kelvin (K).
Celsius Scale (°C)
- 0°C = Freezing point of water
- 100°C = Boiling point of water (at standard pressure)
- Most commonly used scale in scientific and everyday applications worldwide
Fahrenheit Scale (°F)
- 32°F = Freezing point of water
- 212°F = Boiling point of water (at standard pressure)
- Commonly used in the United States for everyday applications
Kelvin Scale (K)
- 0K = Absolute zero (theoretical lowest temperature)
- 273.15K = Freezing point of water
- 373.15K = Boiling point of water (at standard pressure)
- SI unit of temperature used in scientific calculations
- No degree symbol is used with Kelvin
Temperature Scale Conversions
Celsius to Fahrenheit: °F = (°C × 9/5) + 32
Fahrenheit to Celsius: °C = (°F - 32) × 5/9
Celsius to Kelvin: K = °C + 273.15
Kelvin to Celsius: °C = K - 273.15
Fahrenheit to Kelvin: K = (°F - 32) × 5/9 + 273.15
Kelvin to Fahrenheit: °F = (K - 273.15) × 9/5 + 32
Temperature Reference Points
| Physical Phenomenon | Celsius (°C) | Fahrenheit (°F) | Kelvin (K) |
|---|---|---|---|
| Absolute Zero | -273.15 | -459.67 | 0 |
| Freezing Point of Water | 0 | 32 | 273.15 |
| Room Temperature (approx.) | 20-25 | 68-77 | 293-298 |
| Normal Human Body Temperature | 37 | 98.6 | 310.15 |
| Boiling Point of Water | 100 | 212 | 373.15 |
The Kelvin scale is used in scientific calculations because:
- It avoids negative numbers (0K is absolute zero)
- It's directly proportional to average molecular kinetic energy
- Many physical laws are simpler when expressed in Kelvin
Temperature Range Comparison
-273.15°C
-459.67°F
0°C
32°F
100°C
212°F
226.85°C
440.33°F
Temperature Conversion Interactive Demo
Temperature: 25°C = 77°F = 298.15K
Thermocouples
A thermocouple is a temperature sensor that consists of two different types of metals joined together at one end. It produces a small voltage (typically in microvolts) that is proportional to the temperature difference between the junction (measuring point) and the reference junction.
Working Principle: The Seebeck Effect
The Seebeck Effect
When two dissimilar metals are joined at two points and these junctions are at different temperatures, an electrical potential (voltage) is generated.
This voltage is proportional to the temperature difference between the junctions.
V = α × (T₁ - T₂)
Where:
- V = Voltage generated
- α = Seebeck coefficient
- T₁ = Temperature at measuring junction
- T₂ = Temperature at reference junction
The reference junction (cold junction) is traditionally kept at a known temperature (historically an ice bath at 0°C). Modern thermocouple instruments use electronic cold junction compensation to account for the ambient temperature at the reference junction.
Types of Thermocouples
| Type | Materials | Temperature Range | Sensitivity | Common Applications |
|---|---|---|---|---|
| Type K | Chromel-Alumel | -200°C to +1350°C | 41 µV/°C | General purpose, ovens, kilns |
| Type J | Iron-Constantan | -40°C to +750°C | 55 µV/°C | General purpose, reducing environments |
| Type T | Copper-Constantan | -250°C to +350°C | 43 µV/°C | Low temperatures, food processing, cryogenics |
| Type E | Chromel-Constantan | -200°C to +900°C | 68 µV/°C | High sensitivity applications |
| Type N | Nicrosil-Nisil | -270°C to +1300°C | 39 µV/°C | High temperature stability, oxidizing environments |
| Type S | Platinum-Rhodium | -50°C to +1768°C | 10 µV/°C | High temperature, standards, laboratories |
| Type R | Platinum-Rhodium | -50°C to +1768°C | 10 µV/°C | High temperature, similar to Type S |
| Type B | Platinum-Rhodium | 0°C to +1820°C | 10 µV/°C | Highest temperature range, furnaces |
Advantages of Thermocouples
- Wide temperature range (-270°C to +2300°C, depending on type)
- Rugged and durable
- No power supply required (self-powered)
- Fast response time
- Relatively low cost
- Can be used in harsh environments
Limitations of Thermocouples
- Low voltage output (microvolts)
- Non-linear response
- Reference junction compensation required
- Lower accuracy compared to RTDs
- Prone to electrical noise
- Aging and drift over time
Thermocouple Measurement Circuit
Thermocouple Installation and Usage
Thermocouples are available in different constructions:
- Exposed Junction: The welded junction extends beyond the protective sheath. Fastest response time but less durable.
- Grounded Junction: The junction is welded to the inside of the protective sheath. Good response and protection.
- Ungrounded Junction: The junction is electrically isolated from the sheath. Provides electrical isolation but slower response.
When extending thermocouple wires, you must use the correct type of extension wires:
- Thermocouple Extension Wire: Made of the same materials as the thermocouple but generally with lower grade materials.
- Compensating Cable: Made of different materials but designed to have similar thermoelectric properties over a limited temperature range.
Using standard copper wires to extend thermocouples will create additional unintended junctions, causing measurement errors!
- Cold Junction Error: Incorrect compensation for the reference junction temperature
- Extension Wire Error: Using incorrect extension wires
- Galvanic Voltages: Caused by dissimilar metals in contact with conductive solutions
- Thermal Shunting: Heat is conducted away from the measurement point through the thermocouple wires
- Electrical Noise: Thermocouple signals are low-level (microvolts) and susceptible to electromagnetic interference
- Calibration Drift: Changes in thermoelectric properties over time due to contamination or high-temperature exposure
Thermocouple Color Codes
Thermocouple wires and connectors are color-coded to identify the type. However, color codes vary by region/standard:
| Type | IEC Colors (International) | ANSI Colors (North America) |
|---|---|---|
| Type K | Green (+) / White (-) | Yellow (+) / Red (-) |
| Type J | Black (+) / White (-) | White (+) / Red (-) |
| Type T | Brown (+) / White (-) | Blue (+) / Red (-) |
| Type E | Violet (+) / White (-) | Purple (+) / Red (-) |
RTDs and Thermistors
Resistance Temperature Detectors (RTDs) and Thermistors are temperature sensors that operate based on the principle that the electrical resistance of certain materials changes with temperature.
Resistance Temperature Detectors (RTDs)
RTDs are precision temperature sensors made from pure metals (usually platinum, nickel, or copper). Their resistance increases fairly linearly with temperature.
- PT100: Platinum RTD with 100Ω resistance at 0°C
- PT1000: Platinum RTD with 1000Ω resistance at 0°C
For platinum RTDs (simplified equation):
Rt = R0(1 + αt)
Where:
- Rt = Resistance at temperature t
- R0 = Resistance at 0°C (100Ω for PT100)
- α = Temperature coefficient (≈0.00385 Ω/Ω/°C for platinum)
- t = Temperature in °C
For more accurate calculations, the Callendar-Van Dusen equation is used:
For t ≥ 0°C: Rt = R0[1 + At + Bt²]
For t < 0°C: Rt = R0[1 + At + Bt² + C(t-100)t³]
Where A, B, and C are calibration coefficients
Thermistors
Thermistors are semiconductor devices whose resistance changes significantly with temperature. They come in two types:
- NTC (Negative Temperature Coefficient): Resistance decreases as temperature increases
- PTC (Positive Temperature Coefficient): Resistance increases as temperature increases
For NTC thermistors:
Rt = R0eB(1/T - 1/T0)
Where:
- Rt = Resistance at temperature T (Kelvin)
- R0 = Resistance at reference temperature T0 (Kelvin)
- B = Beta constant (material property, in Kelvin)
- T = Temperature in Kelvin
- T0 = Reference temperature in Kelvin
A simpler approximation (Steinhart-Hart equation):
1/T = A + B(ln R) + C(ln R)³
Where A, B, and C are calibration coefficients
Comparison between RTDs and Thermistors
| Characteristic | RTDs | Thermistors |
|---|---|---|
| Temperature Range | -200°C to +850°C | -100°C to +300°C |
| Accuracy | High (±0.1°C possible) | Medium (±0.2°C typical) |
| Linearity | Good (nearly linear) | Poor (highly non-linear) |
| Sensitivity | Low (0.4 Ω/°C for PT100) | High (hundreds of Ω/°C) |
| Stability | Excellent (long-term stable) | Fair (more drift over time) |
| Response Time | Slow | Fast |
| Self-heating | Low | High |
| Cost | Higher | Lower |
| Size | Larger | Smaller |
RTD Measurement Circuits
The simplest connection but least accurate:
- Lead wire resistance adds to RTD resistance
- Temperature changes in leads affect measurement
- Suitable only for short distances or where accuracy is not critical
Most common industrial RTD connection:
- Compensates for lead wire resistance
- Assumes all leads have equal resistance
- Good balance of accuracy and cost
Highest accuracy RTD connection method:
- Completely eliminates lead wire resistance effects
- Uses separate current source and voltage measurement circuits
- Used in precision applications and calibration
Thermistor Measurement Circuits
Simple Voltage Divider
Output voltage:
Vout = Vcc × R / (R + Rt)
Temperature can be calculated from Vout using the thermistor equation.
Wheatstone Bridge
Provides more sensitivity and allows for temperature compensation.
Self-heating error: When current flows through an RTD or thermistor, it generates heat that raises the sensor's temperature above the environment. This leads to measurement errors.
To minimize self-heating:
- Use the minimum excitation current needed
- Choose sensors with larger physical size
- Use pulsed measurements rather than continuous power
Digital Temperature Sensors
Digital temperature sensors combine a temperature sensing element with signal conditioning, analog-to-digital conversion, and a digital interface, all in a single integrated circuit (IC).
Advantages of Digital Sensors
- Direct digital output (no need for ADC)
- Built-in calibration and linearization
- Immune to noise and lead resistance issues
- Multiple sensors can share the same bus
- Often include additional features (alarms, registers)
- Simple to interface with microcontrollers
Limitations of Digital Sensors
- Limited temperature range compared to thermocouples
- Higher cost for basic applications
- Requires programming knowledge
- Power consumption (not passive like thermocouples)
- Not suitable for harsh environments
- Fixed accuracy (typically ±0.5°C to ±2°C)
Types of Digital Temperature Sensors
I²C (Inter-Integrated Circuit) is a popular 2-wire communication protocol used by many digital sensors.
- Requires only 2 wires (SDA and SCL) plus power
- Multiple sensors can share same bus with different addresses
- Typical examples: TMP102, LM75, MCP9808
- Resolution typically 0.0625°C to 0.25°C
- Range typically -55°C to +125°C
1-Wire protocol allows data and power to be transmitted over a single wire.
- Requires only 1 data wire plus ground (sometimes "parasitic power" with no VCC wire)
- Each device has a unique 64-bit ID
- Can connect multiple sensors on one bus
- Slower than I²C but simpler wiring
- Most popular example: DS18B20
The DS18B20 is a popular 1-Wire digital temperature sensor with:
- Range: -55°C to +125°C
- Accuracy: ±0.5°C (from -10°C to +85°C)
- Resolution: Programmable from 9-bit (0.5°C) to 12-bit (0.0625°C)
- Unique 64-bit serial code (allows multiple sensors on one bus)
SPI (Serial Peripheral Interface) is a 4-wire synchronous serial communication protocol:
- Uses 4 wires: MOSI, MISO, SCK, and CS
- Faster than I²C and 1-Wire
- Requires a separate chip select (CS) line for each device
- Examples: MAX31855 (thermocouple interface), MAX31865 (RTD interface)
The MAX31855 is a digital thermocouple interface that combines a thermocouple input with cold junction compensation and digital conversion:
- Designed specifically for K-type thermocouples
- Cold junction compensation built-in
- 14-bit resolution (0.25°C)
- Range: -270°C to +1800°C
- SPI interface to microcontroller
Infrared (IR) Temperature Sensors
Non-Contact Temperature Measurement
IR temperature sensors measure the infrared radiation emitted by objects to determine their temperature without physical contact.
Based on the principle that all objects emit thermal radiation proportional to their temperature (Stefan-Boltzmann Law).
Stefan-Boltzmann Law:
P = εσT⁴
Where:
- P = Power radiated per unit area (W/m²)
- ε = Emissivity (0 to 1)
- σ = Stefan-Boltzmann constant (5.67 × 10⁻⁸ W/m²K⁴)
- T = Absolute temperature in Kelvin
Digital IR Temperature Sensors
Modern IR sensors combine an IR detector with processing electronics to output temperature directly.
Examples include:
- MLX90614: I²C output, -70°C to +380°C range, ±0.5°C accuracy
- TMP006/TMP007: I²C output, contactless temperature sensor
- AMG8833: 8×8 pixel thermal array ("thermal camera")
Key considerations:
- Emissivity setting (different materials emit different amounts of IR)
- Field of view (the area "seen" by the sensor)
- Distance-to-spot ratio (measurement area increases with distance)
Digital temperature sensors are commonly used in:
- Consumer electronics (smartphones, laptops, appliances)
- Environmental monitoring systems
- Home automation and HVAC controls
- Medical devices
- Automotive applications
- IoT (Internet of Things) devices
Arduino Example: Reading DS18B20 Temperature Sensor
Arduino code for reading a DS18B20 temperature sensor:
#include <OneWire.h>
#include <DallasTemperature.h>
// Data wire is connected to digital pin 2
#define ONE_WIRE_BUS 2
// Setup a oneWire instance to communicate with any OneWire devices
OneWire oneWire(ONE_WIRE_BUS);
// Pass our oneWire reference to Dallas Temperature
DallasTemperature sensors(&oneWire);
void setup() {
// Start serial communication
Serial.begin(9600);
// Start up the library
sensors.begin();
}
void loop() {
// Send the command to get temperatures
sensors.requestTemperatures();
// Print the temperature in Celsius
Serial.print("Temperature: ");
Serial.print(sensors.getTempCByIndex(0));
Serial.println(" °C");
// Print the temperature in Fahrenheit
Serial.print("Temperature: ");
Serial.print(sensors.getTempFByIndex(0));
Serial.println(" °F");
delay(1000); // Wait 1 second between readings
}
Temperature Measurement Calculators
Temperature Conversion Calculator
Conversion Results
Converted Value:
RTD Resistance Calculator
RTD Resistance Results
Resistance at °C: Ω
Thermocouple mV Calculator
Thermocouple Results
Output Voltage: mV
Sensitivity: µV/°C
Thermistor Resistance Calculator
Thermistor Results
Resistance at °C: Ω
