Temperature, pressure, & flow are the most used process measurements in industry.
As such there are many options to choose from.
For temperature in the process industry, 99% or more of the temperature loops use thermocouples or resistance temperature detectors (RTD). The RTD provides sensitivity (minimum detectable change in temperature), repeatability, and drift that are an order of magnitude better than the thermocouple.
Thermocouples are made up of 2 different metals that produce a voltage that varies with temperature.
That millivolt signal is then amplified for reading.
An RTD is a platinum resistor ( usually 100 ohms ) that varies with temperature.
A circuit is required to measure that resistance and produce a voltage usually 0 to 5 VDC.
A 3 wire RTD is generally good, but a 4 wire will give the best accuracy with an extra cost.
Accuracy, Range, and Usability of Temperature Sensing Elements
Sensitivity and repeatability are two of the three most important components of accuracy.
The other most important component, resolution, is set by the transmitter.
Drift is important for extending the time between calibrations
Thermistors, which have seen limited use in the process industry despite their extreme sensitivity and fast (millisecond) response, primarily because of their lack of chemical and electrical stability.
Thermistors are also highly nonlinear, but this can be addressed by smart instrumentation.
Also of importance depending on the desired accuracy and repeatability is the effects of noise and calibration.
Determining which type of temperature sensor to use is based on:
1) Mounting - How will you physically mount the sensor to make your measurements ?
For measuring air temperature in a clean environment, you can get away with using a thermistor or TMP36.
For measuring a liquids temperature, you will need a protective probe type enclosure.( Usually RTD or thermocouple ).
2) Temperature range is another limiting variable. Too small a range will not work.
3) Accuracy needed.( See Temperature Noise & Calibration.txt )
|RTD (Pt 100 )||Thermistor|
|Drift||1-20||0.01 - 0.1||0.01 - 0.1|
|Sensitivity ( C )||0.05||0.001||0.0001|
|Temperature Range ( C )||-200 to 2000||-200 to 850||-100 to 300|
|Signal Output (volts)||0-0.06||1 - 6||1 - 3|
|Power (watts at 100 ohm )||0.4||2||0.4|
NOTE: The TMP36 is a low voltage, precision centigrade temperature sensor. It provides a voltage output that is linearly proportional to the Celsius temperature. It also doesn't require any external calibration to provide typical accuracies of ±1°C at +25°C and ±2°C over the −40°C to +125°C temperature range.
NOTE: Thermocouples come in type of different bi-metal for different ranges.
The most common being J & K type.
J - 0 to 750°C
K - 0 to 1250°C
Some quick & simple Arduino solutions:
TMP 36 / DS18B20 ( Solid State )
These sensors use a solid-state technique to determine the temperature. They use the fact as temperature increases, the voltage across a diode increases at a known rate. (Technically, this is actually the voltage drop between the base and emitter - the Vbe - of a transistor.) By precisely amplifying the voltage change, it is easy to generate an analog signal that is directly proportional to temperature.
Temperature range: -40°C to 150°C / -40°F to 302°F
Output range: 0.1V (-40°C) to 2.0V (150°C) but accuracy decreases after 125°C
A simple 3 wire connection.
#1) 2.7 to 5.5VDC VCC
#2) Analog voltage out (A0)
Uses the Dallas 1-Wire protocol, which is somewhat complex, and requires a bunch of code to parse out the communication, but avoids signal issues with long runs.
- Usable temperature range: -55 to 125°C (-67°F to +257°F)
- 9 to 12 bit selectable resolution
- Uses 1-Wire interface- requires only one digital pin for communication
- Unique 64 bit ID burned into chip
- Multiple sensors can share one pin
- ±0.5°C Accuracy from -10°C to +85°C
- Temperature-limit alarm system
- Query time is less than 750ms
- Usable with 3.0V to 5.5V power/data
DHT22 ( thermister )
Good for 0-100% humidity readings with 2-5% accuracy
Good for -40 to 80°C temperature readings ±0.5°C accuracy
No more than 0.5 Hz sampling rate (once every 2 seconds)
This is a thermistor ( temperature sensitive varying resistor ) with a capacitor for humidity.
-200°C to +1350°C output in 0.25 degree increments - note that K thermocouples have about ±2°C to ±6°C accuracy
SPI data output requires any 3 digital I/O pins.
The MAX6675 performs cold-junction compensation and digitizes the signal from a type-K thermocouple.The data is output in a 12-bit resolution, SPI-compatible, read-only format.
Usable temperature range: -200 to 550°C (-328°F to +1,022°F)
±0.5°C Accuracy from -10°C to +85°C
Platinum RTD Sensor - PT100 - 3 Wire
Resistor material is Platinum with a value of 100 ohm at temperature 0°C
Platinum has a positive resistance temperature factor; resistance increases with rising temperature
Resistance variation is a function of temperature: 0.385Ω/°C nominal
RTD to MAX31865 0-5VDC
Note the file below Temperature_Sensor_Results.xlsx has 3 minute data variation of each sensor in a stable room temperature measurement with no extra electronics to show the individual variation. A temperature change from ambient > freezing > boiling > ambient was also done with an RTD, K thermocouple, and DS18B20 shielded probes to show their relative accuracies and rates of change.
The range variation for each sensor in degrees C are:
Raspberry Pi Solutions
Provides four thermocouple inputs for adding temperature measurement capability to Raspberry Pi based systems. It features 24-bit resolution and provides professional-grade accuracy which is best in class. The MCC 134 also offers open thermocouple detection so users can monitor for broken or disconnected thermocouples and each channel type is selectable on a per-channel basis. Up to eight MCC DAQ HAT devices can be stacked onto one Raspberry Pi.