The NI USB-6008 has a 12-bit resolution with a minimum range of +/- 1 V, which translates to a minimum resolution of 480 uV. The NI USB-6009 has a 14-bit resolution and a minimum range of +/- 1 V, which translates to a 122 uV resolution. The poor voltage resolution of these input devices makes them non-ideal for low voltage measurements. Additionally, when acquiring in the +/- 1 V range, each device provides a typical accuracy of +/- 1.53 mV at 25 °C. The poor accuracy of these devices at low-level signals also makes them non-ideal for thermocouple measurements.
Here are some common thermocouple types and their associated voltage responses:
- K type: 41 uV/°C
- J type: 56 uV/°C
- T type: 52 uV/°C
Due to the large minimum voltage resolution, the USB-6008 and USB-6009 can only detect changes of approximately 8-9 °C and 2-3 °C respectively, and only accurate to within 30 °C. NI USB-6001/6002/6003:
The NI USB-6001/6002/6003 do have similar ADC resolutions (14 or 16-bit), however since their voltage input range is set to +/- 10 V the resolution is decreased. For example a 14-bit analog digital conversion of a signal ranging from +/- 10 V provides a resolution of around 1.22 mV.
A better alternative for measuring thermocouples is the NI 9211
as they are specifically designed for taking thermocouple measurements. The NI 9211 has built-in signal conditioning, 24-bits of resolution, and +/-80 mV input range which is better for thermocouple readings. The USB-TC01 is compatible with J, K, R, S, T, N, and B thermocouples and uses a standard miniplug for easy thermocouple connection.
These resolutions would mean that these devices can only detect changes of approximately 23-29 °C.