3 Really Important Things You Need To Know About System Accuracy Test

AIAG - Automotive Industry Action Group- released CQI-9 in March of 2006. The goal is to "include a reduction of campaigns, spills, recalls and warranty claims related to heat treated components. The management system will emphasize continual improvement, defect prevention and the reduction of variation and waste in the supply chain." This is achieved by performing a Heat Treat System Assessment- HTSA.

This article will focus on CQI-9 System Accuracy Test requirements.

CQI-9 Special Process: Heat Treat System Assessment in Section 3.3 System Accuracy Test (SAT) goes into great detail regarding the requirements. 

Probe Methods

Section 3.3.4.1 calls out the need for control equipment to be within ± 5°C of the test thermocouple that is used. Any thermocouples that exceed the 5°C must be replaced.

Section 3.3.4.1.5 requires a System Accuracy Test be performed quarterly when using the probe method.

Section 3.3.4.3 describes using a comparative method. This requires a comparison to a calibrated test instrument.

Section 3.3.4.3.7 requires a System Accuracy Test be performed monthly when using the comparative method.

The comparative method has a maximum difference of ± 1°C which is substantially different than the probe method.

The reporting requirements for a System Accuracy Test are detailed in Section 3.3.5 and require the following:

• Identification of the Thermocouple being tested
• Identification of the test thermocouple
• Identification of the test instrument
• Date and time of day of the test
• Observed control instrument reading
• Test thermocouple and test instrument correction factors
• Corrected test instrument reading
• Pass or Fail status
• External calibration company of applicable
• Sign-off by the responsible person in the heat treat organization

What does all this mean when scheduling a System Accuracy Test with a calibration company? Communication. Communication. Communication. 

Do not assume that the calibration company knows that you want a System Accuracy Test conforming to CQI-9.  They may be performing other calibrations for your company and following ASME, ASTM or other recognized standards for those calibrations.

You need to ask your calibration company some questions BEFORE they arrive to perform the SAT?

• Do you have a copy of CQI-9?
• Has anyone shared our operating ranges?
• What thermcouples will you be using?
• What is the calibration status of the equipment you will be using?
• What is the measurement uncertainty of the methods/equipment you will use?

The last question may seem very esoteric, but it is a rather practical question. If the measurement uncertainty of the methods/equipment used is greater than the maximum differences allowed you need to know this. It is common for measurement uncertainty to change across the range of a thermocouple.

Here is a note from our Technical Manager, Kevin Radzik about thermocouples and uncertainty:

" Some standards treat thermocouple measurements as a chain of voltage calibrations. This would be all well and good if thermocouples actually worked that way, but they don’t.

Voltage production due to the joining of dissimilar metals is only part of how a thermocouple works. The voltage generated is a function of the Seebeck effect first discovered in 1821. The voltage of the hot junction however cannot predict the temperature in and of itself. There needs to be a “cold junction” in the circuit.

In the past this was performed by transitioning the thermocouple wire to copper and placing the junction in an ice bath, hence the name “cold junction.”

Modern electronic devices measure the temperature of the transition from thermocouple wire to copper and compensate electronically.

This is a very useful trick!

However, it is also the single largest contributor of uncertainty to the measurement.

Consider the following example:

•  Published specification of a Fluke 5500 calibrator @ 50 mVDC is ±0.005mV
•  With a “K” type thermocouple 50mV is about 2250°F and 1°F is represented by a change of 0.02 mV
•  Based on the 2 statements above you would expect the accuracy of the Fluke 5500 to be ±0.25°F @ 2250°F (0.005mV being ¼ of 0.02mV)
•  The actual published specification of the Fluke 5500 for a measurement @ 2250°F is ±0.72°F. That is nearly three times the amount predicted by the difference in voltage.

Where does this additional uncertainty come from?

The cold junction compensation is responsible for the majority of the inaccuracy of the measurement. The problem is that it is not very easy to measure the temperature of a set of connections and their contribution to measurement inaccuracy can be several degrees in extreme cases."

There is more to this uncertainty issue than meets the eye.

You may want to visit our International Standards Used In Calibration page for more information.