Temperature Sensors (App Note)

Overview

If you are measuring in the range of -55 to +150 °C, consider using a silicon temperature sensor.  They are generally the cheapest solution, the easiest solution, and the most accurate solution.

Beyond silicon sensors, thermocouples are usually the best option. See the Thermocouples App Note.

Thermistors and RTDs can seem to be very accurate when you look at the raw specs, but that is just the accuracy of resistance versus temperature.  It can be difficult to measure that resistance with enough accuracy to achieve the stated accuracy of the sensor element itself.

Digital sensors have similar range limits as analog silicon type sensors, and are a great solution depending on which LabJack and software plans.  The T4 and T7 have high-level support for SBUS sensors (EI-1050, SHT1x, SHT7x) in hardware, so it is easy to read temperature and humidity in any software.  Older devices (U12, U3, U6, UE9) provide high-level support through software, so require a software application that makes specific calls to the UD or U12 library.  Other sensors speaking SPI, I2C, Asynch, or 1-Wire, are also an option but will require a software application that makes specific calls to the LJM, UD or U12 library.

Subsections

Temperature Range

The following table provides a broad overview of temperature ranges supported by different temperature sensor types. Note that the effective measurement range can be limited due to other factors in the circuit.

_{[1] Thermistor temperature range can vary by device. Additionally, thermistor accuracy is typically worse outside the range of 0 to 100 °C.}

_{[2] RTD temperature range could vary by device.}

_{[3] Thermocouple temperature range and sensitivity varies by thermocouple type. See the following thermocouple type information.}

Silicon Temperature Sensors (Analog)

In the range from -55 to +150 °C, analog silicon temperature sensors are generally cheaper, easier to use, and more accurate, than other types of temperature sensors. With no or minimal extra components, they provide a high-level linear voltage output that connects directly to a LabJack's analog inputs.

Measurement Range

The following table describes some silicon temperature sensors and their useful measurement ranges with LabJack hardware. Measurement range can be limited due to the input voltage range of some LabJack analog inputs.

_{[1] Specification when using the flexible I/O. Both the U3-HV and T4 have 4 ±10V analog inputs that could measure the full sensor span, however measurement resolution is lower when using the HV inputs.}

_{[2] Stated range requires use of the special 0-3.6 V input range setting.}

EI-1034

A silicon-based temperature probe made by Electronic Innovations and sold by LabJack.  It uses an LM34CAZ sensor element from National Semiconductor with a 10k load resistor from signal to ground.  The LM34 provides an easy-to-use 10 mV/°F.  Range with 5V/0V supply is -17 to +110 °C (0 to 230 °F).  Accuracy (max) is +/-0.56 °C (+/-1.0 °F) at room temp and +/-1.1 °C (+/-2 °F) across range.  Non-linearity is +/-0.3 °C (+/-0.6 °F) max across range, so a simple calibration can provide more accurate measurements.  Assembly has a 6ft cable, which can be extended to 25ft, or much longer if you add a 10k series resistor to prevent oscillation.  Uses a 6" x 0.25" waterproof stainless steel probe.

EI-1022

A silicon based temperature probe made by Electronic Innovations and sold by LabJack.  It uses an LM335A sensor element from National Semiconductor with a 2k current setting resistor.  The LM335A provides an easy-to-use 10 mV/°K.  Range with 5V/0V supply is -40 to +100 °C (-40 to 212 °F).  Accuracy (max) is +/-3 °C (+/-5.4 °F) at room temp and +/-5 °C (+/-11 °F) across range.  Non-linearity is +/-1.5 °C (+/-2.7 °F) max across range, so a simple calibration can provide more accurate measurements.  Assembly has a 6ft cable, which can be extended to much longer distances with no added components.  Uses a 4" x 0.25" plastic probe.

LM34CAZ

TO-92 package with 10 mV/°F output.  Buy from LabJack, Digikey, and others.  Range with 5V/0V supply is -17 to +110 °C (0 to 230 °F).  Accuracy (max) is +/-0.56 °C (+/-1.0 °F) at room temp and +/-1.1 °C (+/-2 °F) across range.  Non-linearity is +/-0.3 °C (+/-0.6 °F) max across range, so a simple calibration can provide more accurate measurements.  For lower temperatures, to -40 °C (-40 °F), you need to add a negative bias on the signal output (on the U6/T7 you can use a 220k pull-down to VM-).  Note that even if you want to measure in Celsius, the LM34 is better than the LM35 because you get more voltage per temperature (18 mV/°C versus 10 mV/°C) and you can measure lower with a single supply (-17 °C versus +1 °C).  Regardless of cable length we always recommend a 10k resistor from Vout to GND (preferably right at the sensor), and this is usually good for cables up to 25ft.  Beyond 25ft see the "Capacitive Loads" section in the LM34 datasheet and consider adding a series resistor.

LM34AH

TO-46 package with 10 mV/°F output.  Buy from Digikey and others.  Range with 5V/0V supply is -17 to +150 °C (0 to 300 °F).  Accuracy (max) is +/-0.56 °C (+/-1.0 °F) at room temp and +/-1.1 °C (+/-2 °F) across range.  Non-linearity is +/-0.4 °C (+/-0.7 °F) max across range, so a simple calibration can provide more accurate measurements.  For lower temperatures, to -40 °C (-40 °F), you need to add a negative bias on the signal output (on the U6/T7 you can use a 220k pull-down to VM-).  Note that even if you want to measure in Celsius, the LM34 is better than the LM35 because you get more voltage per temperature (18 mV/°C versus 10 mV/°C) and you can measure lower with a single supply (-17 °C versus +1 °C).  Regardless of cable length we always recommend a 10k resistor from Vout to GND (preferably right at the sensor), and this is usually good for cables up to 25ft.  Beyond 25ft see the "Capacitive Loads" section in the LM34 datasheet and consider adding a series resistor.

LM135A

TO-46 package with 10 mV/°K output.  Buy from Digikey and others.  Range with 5V/0V supply is -55 to +150 °C (-67 to 300 °F).  Accuracy (max) is +/-1.0 °C (+/-1.8 °F) at room temp and +/-2.7 °C (+/-4.9 °F) across range.  Non-linearity is +/-0.5 °C (+/-0.9 °F) max across range, so a simple calibration can provide more accurate measurements.  Very long cables can be used with no added components.  The other sensors are 3-terminal series sensors, but the LM135A is a 2-terminal shunt sensor that requires a resistor to control current (self-heating might need to be considered).

LM60BIZ

TO-92 package with 6.25 mV/°C output (plus 0.424V offset).  Buy from Digikey and others.  Range with 5V/0V supply is -40 to +125 °C (-40 to 230 °F).  Accuracy (max) is +/-3 °C (+/-5.4 °F) across -25 to +125 °C range.  Non-linearity is +/-0.6 °C (+/-1.1 °F) max across range, so a simple calibration can provide more accurate measurements.  Specified for use with long cables without added components.

TMP36

TO-92 package with 10 mV/°C output, and provides a 750mV output at 25°C. From Analog Devices, which also sells the TMP35 and TMP36. The TMP35 is similar to the TMP36, but outputs 250mV at 25°C. The TMP37 uses 20mV/°C scaling. The typical range of TMP35/36/37 devices with 2.7V-5.5V supply is -40 to +125°C (- 40 to +257 °F). With reduced accuracy, operation extends to +150°C (+302°F) when operating from a 5V supply. The max accuracy is +/-3°C (+/-5.4 °F) at room temp and +/-4°C (+/-7.2 °F) across range. Non-linearity is +/-0.5 °C (+/-0.9 °F) max across range, so a simple calibration can provide more accurate measurements. The TMP36 is more stable than the LM34, and will provide a stable output over long cables with no added components, though the TMP36 is less accurate.

LMT70

The LMT70 uses an unfriendly DSBGA package, but has great accuracy and range.  Running off a 2.0V to 5.5V supply, this sensor outputs 1400 mV to 300 mV corresponding to -55 °C to +150 °C and has an accuracy of +/-0.36 °C across that entire range.  Linear interpolation or a 3rd order equation is required to convert voltage to temperature.  Flex probe carriers make it much easier to attach wires.  The LMT70 datasheet says that it is stable with capacitive loads up to 1 nF.

Thermocouples

If you can't use a silicon sensor, a thermocouple is usually the next best option.  They are not particularly accurate, but are often fine when you are measuring hundreds of degrees.  See the Thermocouples App Note for more detail.

Thermistors

Thermistors often have great-looking accuracy specs for low cost, but that is the accuracy of resistance versus temperature.  Trying to measure the resistance with an accuracy that matches the accuracy of the sensor spec can be difficult.

The LJTick-Resistance is recommended for handling thermistors.

T-series devices can handle thermistor math in hardware using their Thermistor AIN-EF ability.

RTDs

RTDs (PT100, PT500, PT1000, etc.) often have great-looking accuracy specifications at reasonable prices, but the problem is that specification is the accuracy of a small resistance change versus temperature, as opposed to voltage versus temperature like you get with a silicon sensor or thermocouple.  Trying to measure resistance with an accuracy that matches the accuracy of the sensor spec can be difficult.

The LJTick-Resistance is recommended for handling RTDs.  Get 1 LJTR-1k for each 2 RTDs.

T-series devices can handle RTD math in hardware using their RTD AIN-EF ability.

See our RTDs App Note for more information.

Digital Sensors

Digital sensors have similar range limits as analog silicon sensors, and are a great solution depending on which LabJack and software plans.  In addition to temperature, digital sensors are available for many other parameters such as humidity, acceleration, and light.

SBUS is a serial protocol used with SHT1x and SHT7x sensors from Sensirion, which measure temperature and humidity.  SBUS is similar to I2C, but not exactly the same.  The EI-1050 (EOL) probe assembly used the SHT11 sensor and SBUS. The T4 and T7 have high-level support for SBUS sensors (EI-1050, SHT1x, SHT7x) in hardware, so it is easy to read temperature and humidity in any software.  Older devices (U12, U3, U6, UE9) provide high-level support through software, so require a software application that makes specific calls to the UD or U12 library.

Newer replacements for the SBUS based SHT1x sensors are the I2C based SHT3x and SHT4x sensors from Sensirion. We have high level SHT3x support through the SBUS interface on T-series devices.

All other sensors speaking synchronous protocols SPI (all devices), I2C (all except U12), or 1-Wire (all except U12), are also an option but will require a software application that makes specific calls to the LJM, UD or U12 library.  In our experience, SPI is pretty easy to use, I2C is not too bad, and 1-Wire is definitely the trickiest.  All devices also have varying support for asynchronous serial communication.  This asynch support is compatible with logic-level UARTs, and with added transceiver circuity is compatible with RS-232, RS-485, and RS-422, although with the RS- protocols you are usually better off using a specific USB dongle that supports that protocol unless there is a good reason to go through a LabJack.