DAQ Overview
NI 9211 DAQ:
Figure 1: NI 9211 DAQ
Overview:
The NI 9211 DAQ offers the most appropriate utility for the desired function of our design. The 9211 offers an acceptable sample rate with accurate measurements. Additionally, this model would allow for simultaneous sampling of four thermocouples. The NI 9211 DAQ has a balance of low cost and high accuracy for our particular purpose.
Specifications:
For a thorough list of all specifications, the manufacturer’s data sheet can be found on EDGE.
- 24 bit ADC resolution
- Scanned sampling mode
- 4 thermocouple channels (positive and negative terminals for each thermocouple)
- 14 S/s
These specifications allow for accurate and desirable measurement capabilities. 14 S/s is more than enough for a slow changing temperature measurement, especially when taking readings during steady state. The four thermocouple channels and scanned sampling mode allow for simultaneous measurements of all four channels to observe a linear change in temperature used to calculate the thermal conductivity. A drawback to this model is the thermocouple connectors. Both the positive and negative terminals of the thermocouple are plugged in and a screw is used to tighten them down. This connection method can wear down the leads if constantly plugged in and out, and if not properly attached can give faulty readings. Unfortunately, National Instruments do not offer other connector options.
For a standard range of -40° C to 70° C:
Typical Error For K type thermocouples: <0.07° C
When slightly outside of the standard temperature range, measurements are still relatively accurate. Accuracy quickly degrades as temperatures approach -100° C and lower. When temperatures exceed 70° C the typical error does not breach 2° C until about 600° C. The intended range for our equipment will be 0 to 100° C so the accuracy using the 9211 DAQ will not be a problem. Figure 2 below can be consulted for a full range of accuracies.
Figure 2: Thermocouple Type K Errors
Cost:
NI 9211 Module: $339 ($305 after 10% student discount)
The module itself is relatively cheap, but a chassis is needed to connect the DAQ to a computer for data recording and analysis.
NI 9211 Measurement System: $607 ($546 after 10% student discount)
The measurement system includes the 9211 DAQ, a 1-slot chassis and a 2.3 meter 120 VAC power cord. The bundle includes all components necessary for quick and easy set-up and data acquisition.
*The above prices do not include shipping and handling and have been approximated by NI.
NI USB-TC01:
Figure 3: NI USB-TC01
Overview:
The NI USB-TC01 offers a simple solution to a complex problem. The TC01 has a single slot for a standard thermocouple connector so students could simply plug in a thermocouple and obtain a measurement; similar to a multimeter except the TC01 works through LabView. The USB-TC01 has a lower cost but would not be able to obtain all four temperature readings simultaneously.
Specifications:
For a thorough list of all specifications, the manufacturer’s data sheet can be found on EDGE.
- 20 bit ADC resolution
- 1 channel with plug and go connector
- 4 S/s
- Standard range of 0 to 65° C
- Typical 0.6° C accuracy over the standard range
These specifications are slightly lower than the 9211 DAQ, although they would be acceptable for our experiment. The drawback for this model is that it is effectively just a multimeter for LabView. The students would be unable to see simultaneous readings of all four spaced temperature probes.
Figure 4: Temperature Error of Type K Thermocouple
Cost:
NI USB-TC01: $99 ($90 after 10% student discount)
*The above price does not include shipping and handling and have been approximated by NI.
Comparison Chart:
Model / 9211 / TC01Cost / ~$550 / ~$90
Channels / 4 / 1
Sampling / Simultaneous, 14 S/s / Single, 4 S/s
ADC Resolution / 24 bits / 20 bits
Standard Range / -40 to70° C / 0 to 65° C
Typical Error / <0.07° C / <0.06° C
Conclusion:
While the TC01 model offers lower cost and an easier connector, the benefits gained from the 9211 model can justify the higher cost. If the customer truly would like an automated readout, the 9211 model can obtain readings simultaneously from all four thermocouples hooked to the sample and LabView could be used to calculate the differential and eventually the thermal conductivity. If automation is not truly required, the TC01 would do the job. Students would be required to plug each thermocouple into the TC01 and obtain results one at a time. Both models will work, it is truly up to the consumer at this point.
Revision / User / Date / Changes0 / JRH / 10/31/12 / Created document