Technical Insights > Benchtop

Five Good Reasons to Improve Your Measurement Accuracy with Your DAQ's ACAL Feature

2019-10-28  |  8 min read 

In my previous blog on How to Make Accurate Measurements with DMM’s Autocalibration Function, I have already described the workings of the autocalibration (ACAL) function in the Keysight’s 34465A Truevolt digital multimeter (DMM). I have also given two good reasons when you will actually need to use the autocalibration function. Please do click the highlighted link above to fully understand the workings and the reasons to use the autocalibration function.

The Keysight DAQ970A has built-in 6.5 digits DMM with the autocalibration (ACAL) function similar to the 34465A Truevolt DMM. So, my purpose in writing this blog to you is to give you five good reasons to use ACAL with your DAQ.

1) It is a feature available only in high-performance test instruments

ACAL function is not readily available in all test and measurement instruments. It requires extra precise electronic components and a lot of time invested in characterizing the instruments throughout its operating temperature range, not just room temperature but at extreme operating temperature ranges.

Hence, this ACAL feature is normally only available in high-end test and measurement instruments such as Keysight’s 8.5 digits digital multimeter. Now, some modern DAQs come with this ACAL feature, allowing you to enjoy higher accuracy and measurement stability. You can schedule your ACAL automatically based on the selected interval. See Figure 1.

DAQ-ACAL-scheduling
Figure 1. Schedule autocalibration menu on a DAQ

2) DAQ normally measures data over a long period of time

One of the key features of a DAQ is its data logging capability. The data logging period can be in seconds, minutes, hours, or even days. Test instruments such as DAQ are made up of active and passive electronic components that have electrical characteristics that can change over time, individually and collectively.

This is especially important for electronic components within the measurement path such as the ADC, reference source voltage, signal conditioning, and filters. With ACAL function, the DAQ compensates for temperature drifts and internal errors over time. This is sometimes known as self-calibration.

3) DAQ temperature measurement accuracy

The most common sensor to measure temperature is the thermocouple type sensor. For this reason, let’s use a DAQ with thermocouple as an example to illustrate how the temperature measurement accuracy can be affected along the temperature measurement path. See Figure 2.

temperature-accuracy-variables
Figure 2. Potential areas affecting accuracy along the temperature measurement path

To get accurate temperature measurements,

  • Make sure the thermocouple is properly constructed and that it has the right accuracy for the job.
  • A cold junction reference is needed in the DAQ to eliminate the thermocouple effect at the connection point to the DAQ terminals.
  • Once you have a good accurate voltage to the junction of the DAQ, there is a need to convert the analog voltage into digital binary output signals via an ADC. As for a DAQ with built-in ACAL function, the built-in temperature reference tracks the temperature at which the DAQ was calibrated. It knows what the current temperature is, the calibration, and the temperature change. Hence, it can correct and improve accuracy.

4) DAQs are regularly used as multiple distributed systems

Let’s take an example of a manufacturing environment with three separate work cells; see Figure 3. Each work cell has a dedicated DAQ system, albeit a small DAQ mainframe, but each cell connects via Ethernet or LAN to DAQ software running on a central control location. This is an example of a distributed DAQ system.

DAQ-distributed-system
Figure 3. A diagram of a distributed DAQ system


Here are some examples that show how ACAL can help in a distributed DAQ system with multiple work cells shown in Figure 3:

  • When each of the work cells has its own temperature environment or is partitioned by walls. The temperature changes in each of the work cells will affect the measurement accuracy of their respective DAQ system. When all the data are consolidated, measurements will not be accurate and consistent overall.
  • When each of the work cells has different applications, for example, one cell measures high temperatures and high power while others measure low temperatures and low power.
  • When each of the distributed DAQ systems has to monitor measurement over very long periods of time.

The ACAL function will be able to reduce measurement errors due to environmental changes in temperature and measurement drifts over time. Although measurement errors are likely more apparent with a distributed DAQ system, it also benefits a centralized DAQ system or a single DAQ measurement system.

5) DAQs are also used in remote locations with extreme temperatures

DAQs are often used in remote locations such as monitor temperature in solar harvesting farms, monitor multiple sensor signals from outdoor factory processes, monitor temperature and humidity in various locations of an “intelligent building,” and more.

At such remote locations, sometimes the DAQ’s environment is not temperature regulated. This will obviously contribute to additional measurement errors if the DAQ does not have ACAL function.

 

Finally, to recap, the two benefits of using ACAL for DMM in my previous blog on How to Make Accurate Measurements with DMM’s Autocalibration Function. They are also relevant to the DAQ,

  • Compensating temperature variations in a system test rack
  • Making highly sensitive measurements in a test lab, perhaps for characterization

Conclusion

I hope you have learned the five important reasons to use the autocalibration or ACAL function to your advantage in a DAQ to improve your measurement accuracy. Even though DAQs are used in a lab or an R&D benchtop, they can often be used in a distributed DAQ system, in an unstable temperature environment, in a remote location with extreme temperatures and so on.

Take advantage of the ACAL feature that is available in the Keysight’s DAQ970A data acquisition system. To learn more about the DAQM970A, please visit www.keysight.com/find/DAQ970A.

DAQ970A-with-ACAL
Figure 4. DAQ970A benchtop data acquisition system