Indeed
Active member
In discussions about temperature sensors - in the context of 3d printing - the most commonly mentioned sensor types seems to be thermistor, pt100, pt1000 and thermocouple.
This is an attempt to shine some light on a few of the differences between them. Enough to be able to tell them apart, but without a lot of intricate details.
When the temp goes up the resistance goes down - high ohms when cold, low ohms when hot.
A common thermistor type in 3d printer kits is the generic "100k 3950" NTC resistor.
It has a nominal resistance of 100kΩ at 25°C, and a response characteristic labelled "3950".
The response is not linear. When heating from cold the (absolute) resistance drops quickly at first, then slower and slower the hotter it gets.
Accuracy can be high within a narrow temp range, but usually goes down as temperatures go up.
Not necessarily because of the thermistor only, but also because of the simple input circuit on the mcu (pullup resistor + AD conversion), and also to some minor extent because of cable resistance.
Even with the low quality thermistors included in some 3d printer kits, accuracy is often within a couple of degrees at moderate print temperaures below 250°C.
However, if you are unlucky and get a really bad "lemon" NTC thermistor you may see deviations as high as ±10°C or more, even at lower print temperatures 200-230°C.
Which isn't necessarily the end of the world since the deviation can be compensated for in the klipper (and reprap) config, if you have the time and determination.
Many NTC resistors are specified to work reliably only below 250°C or so, though some types are advertised as being usable up to 450°C or more.
The Nitpicker (that's me) notes that:
"pt" is for the platinum used for sensor material, and "100" is the resistance at 0°C.
When the temp goes up the resistance goes up - low ohms when cold, high ohms when hot.
The response is almost linear, the resistance change is about 0.385 Ω/°C.
pt100 (and pt1000) resistors are often used when the max temperature will exceed 300°C.
They are more accurate at high temps than the cheaper 100k NTC resistors, but they require a special amplifier.
Max allowed temperature can be 800°C or more, but the more affordable ones suitable for 3d printing are usually specified for max 400-500°C or thereabouts.
A "class A" pt100 sensor has better than ±1°C accuracy at 300°C while a cheaper "class C" sensor has around ±3.5°C accuracy at 300°C.
The accuracy of the amplifier circuit must also be accounted for of course, but I don't know how much deviation it can be expected to add.
"pt" is for the platinum used for sensor material, and "1000" is the resistance at 0°C, ten times higher than the pt100.
The resistance change is about 3.85 Ω/°C, also ten times higher than the pt100.
Due to the higher resistance it's possible for some mcu boards to obtain fairly accurate values from a pt1000 connected to a standard thermistor input.
If a special amplifier is used the total system accuracy will be about the same as with a pt100 sensor.
A thermocouple is a thermoelectrical device that generates a temperature dependent voltage.
Much like a peltier element, such as those found in self powered fireplace blowers.
A thermocouple is not used as a resistor, instead the voltage produced by the thermocouple is measured.
There are many types of thermocouple probes, some can measure up to 2300°C.
Type "K" seems to be the most common one for general purpose use (as far as I can tell).
If you have a multimeter with a thermocouple input and a general purpose thermocouple probe to go with it, you probably have a type K probe.
The range for a type K probe can be as wide as -200°C to +1350°C, but is usually less depending on the material used for tip protection and connecting cable insulation.
A special low noise amplifier is required to read the tiny voltage produced. For a type K probe the voltage change is just 41 µV/°C.
A type K thermocouple reading can deviate from actual temp some ±4°C.
±2°C is from the probe itself and another ±2°C can be caused by the amplifier circuit (where a "reference junction" temperature must be determined, often by the use of a thermistor).
So, a type K thermocouple may not be as accurate as a good quality pt100 or pt1000 but it has a wide temperature range.
Final notes by the Nitpicker:
If you spot any errors, let me know and I'll make changes.
This is an attempt to shine some light on a few of the differences between them. Enough to be able to tell them apart, but without a lot of intricate details.
Thermistor
When "thermistor" is mentioned in the context of a 3d printer discussion, it almost always refers to a kind of cheap and common NTC resistor with a "Negative Temperature Coefficient".When the temp goes up the resistance goes down - high ohms when cold, low ohms when hot.
A common thermistor type in 3d printer kits is the generic "100k 3950" NTC resistor.
It has a nominal resistance of 100kΩ at 25°C, and a response characteristic labelled "3950".
The response is not linear. When heating from cold the (absolute) resistance drops quickly at first, then slower and slower the hotter it gets.
Accuracy can be high within a narrow temp range, but usually goes down as temperatures go up.
Not necessarily because of the thermistor only, but also because of the simple input circuit on the mcu (pullup resistor + AD conversion), and also to some minor extent because of cable resistance.
Even with the low quality thermistors included in some 3d printer kits, accuracy is often within a couple of degrees at moderate print temperaures below 250°C.
However, if you are unlucky and get a really bad "lemon" NTC thermistor you may see deviations as high as ±10°C or more, even at lower print temperatures 200-230°C.
Which isn't necessarily the end of the world since the deviation can be compensated for in the klipper (and reprap) config, if you have the time and determination.
Many NTC resistors are specified to work reliably only below 250°C or so, though some types are advertised as being usable up to 450°C or more.
The Nitpicker (that's me) notes that:
- A thermistor is actually any kind of resistor intended for use as a thermometer, where the resistance change either up or down with temperature.
- Even a pt100 or pt1000 sensor is a thermistor.
- Sometimes a pt100 or pt1000 is referred to as a thermocouple. It's not, it's a thermistor.
- Many different thermistor types exist, with huge differences in resistance and temperature response.
- Some types have a close to linear response, some types have extremely nonlinear response. The nonlinear ones tend to be cheaper.
pt100
A pt100 sensor is a PTC thermistor with a "Positive Temperature Coefficient"."pt" is for the platinum used for sensor material, and "100" is the resistance at 0°C.
When the temp goes up the resistance goes up - low ohms when cold, high ohms when hot.
The response is almost linear, the resistance change is about 0.385 Ω/°C.
pt100 (and pt1000) resistors are often used when the max temperature will exceed 300°C.
They are more accurate at high temps than the cheaper 100k NTC resistors, but they require a special amplifier.
Max allowed temperature can be 800°C or more, but the more affordable ones suitable for 3d printing are usually specified for max 400-500°C or thereabouts.
A "class A" pt100 sensor has better than ±1°C accuracy at 300°C while a cheaper "class C" sensor has around ±3.5°C accuracy at 300°C.
The accuracy of the amplifier circuit must also be accounted for of course, but I don't know how much deviation it can be expected to add.
pt1000
A pt1000 sensor is more or less a pt100, but with a higher resistance."pt" is for the platinum used for sensor material, and "1000" is the resistance at 0°C, ten times higher than the pt100.
The resistance change is about 3.85 Ω/°C, also ten times higher than the pt100.
Due to the higher resistance it's possible for some mcu boards to obtain fairly accurate values from a pt1000 connected to a standard thermistor input.
If a special amplifier is used the total system accuracy will be about the same as with a pt100 sensor.
Thermocouple
The other sensor types mentioned above are thermistors, but a thermocouple is not.A thermocouple is a thermoelectrical device that generates a temperature dependent voltage.
Much like a peltier element, such as those found in self powered fireplace blowers.
A thermocouple is not used as a resistor, instead the voltage produced by the thermocouple is measured.
There are many types of thermocouple probes, some can measure up to 2300°C.
Type "K" seems to be the most common one for general purpose use (as far as I can tell).
If you have a multimeter with a thermocouple input and a general purpose thermocouple probe to go with it, you probably have a type K probe.
The range for a type K probe can be as wide as -200°C to +1350°C, but is usually less depending on the material used for tip protection and connecting cable insulation.
A special low noise amplifier is required to read the tiny voltage produced. For a type K probe the voltage change is just 41 µV/°C.
A type K thermocouple reading can deviate from actual temp some ±4°C.
±2°C is from the probe itself and another ±2°C can be caused by the amplifier circuit (where a "reference junction" temperature must be determined, often by the use of a thermistor).
So, a type K thermocouple may not be as accurate as a good quality pt100 or pt1000 but it has a wide temperature range.
Final notes by the Nitpicker:
- NTC resistors, PTC resistors, pt100 and pt1000 - they are all thermistors, they are not thermocouples
- thermocouples are thermocouples, thermocouples are not thermistors
If you spot any errors, let me know and I'll make changes.
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