System Calibration

When developing a measurement system, you must make certain that the instrumentation you are using is calibrated. Calibration is the process of determining and adjusting an instrument’s accuracy to make sure it is within the manufacturer’s specifications. If your instrumentation hardware or hardware components are not calibrated, you run the risk of taking false data. Decisions to be made based on such data can potentially be incorrect. Furthermore, in a manufacturing test, improperly calibrated measurement systems can erroneously pass bad parts or fail good ones.

This article focuses on a particular type of calibration called system calibration. System calibration is accomplished through the use of software. It can remove errors from your measurements and therefore reduce the measurement uncertainty of your instrumentation system. Errors are introduced from the cabling between your instrumentation components, wiring from you sensor to your instrumentation front-end, and inaccuracies from the sensor itself. System calibration also can compensate for certain instrumentation component errors, such as offset errors.

Remember that since system calibration deals with software, it is still important to ensure your hardware components are calibrated. The simple way to know if the hardware in your measurement system is calibrated is to look for the calibration certificate that ships with the hardware components you purchase.

System calibration treats the user's entire measurement system, sensor, wiring, conditioning hardware, digitizer hardware, cabling and conversion software (software that converts from voltages to engineering units) as a single entity. Because system calibration encompasses all the pieces of a measurement setup, it is also called an end-to-end calibration.

Steps When Performing System Calibration

These are the steps for performing a system calibration when you are taking an analog input measurement such as temperature, voltage, pressure, and so on.

1. Apply a precision source
2. Span your input range of interest with the source
3. Create a compensation function
4. Apply the compensation function to all future measurements

   See Also:
   Calibration
   

   Example Using Traditional NI-DAQ
   

Here is an example system calibration using a Type K thermocouple with National Instruments (NI) data acquisition hardware. The products used are a National Instruments SCXI-1102 32-channel thermocouple module, a multifunction (E Series) data acquisition board, and LabVIEW 6.1 to control the hardware.

The precision source is a thermocouple calibrator (a traceable source) with an operating range of 100 degrees C to 500 degrees C.

1. Set the calibrator to 90 degrees C and acquire a data point with the DAQ hardware using LabVIEW. To get one representative data point, you may want to acquire several points at each setting and average the data.
2. Set the calibrator to 100 degrees C and acquire a second data point.
3. Continue setting the calibrator and acquiring data points from the range of 90 to 510 degrees C in units of 10-degree increments on your calibrator. You can use these setpoints and acquired values to create a polynomial function that can confirm future measurements with these components.

Performing a system calibration can make your measurements up to four times more accurate, but remember that once you perform a system calibration with a particular set of hardware components, you cannot change out one of the system components without making the system calibration compensation function invalid. If you do change one of the system components, perform the system calibration again.

See Also:
SCXI-1102
LabVIEW
LabVIEW - SCXI Thermocouple System Calibration Example

Example Using NI-DAQmx 7.4 or Higher

In NI-DAQmx 7.4, a new channel calibration feature was added to perform system calibrations. After you have configured a DAQmx Channel or DAQmx Task, you can select the Calibration tab from the Properties window (Figure 1) and select Calibrate to perform the operation.

The Channel Calibration Wizard (see Figure 2) takes you through the steps to setting up calibration pairs for the range in which you are calibrating. This is essentially identical to configuring calibration pairs programmatically, but here the driver handles configuring calibration pairs and accounting for the difference in the software. After you configure the calibration pairs, the pairs are transparent to the user.



Figure 1: Calibration Tab in Properties Window


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Figure 2: Setting up Calibration Pairs in the Channel Calibration Wizard

Conclusion

System calibration is a correction method that can improve measurement accuracy by quantifying all of the measurement errors in your system and compensating for them in software. System calibration compliments, but does not replace, component calibration.
Related Links:
System Calibration VIs

Reader Comments | Submit a comment »

Article OK for beginner
The "System Calibration" article is ok for a beginner/manager, but lacks breadth and depth for the experienced Instrumentation Engineer.
- Ronald Olton, ERC, Inc.. ronald.olton@dfrc.nasa.gov - Aug 23, 2001