Expert Knowledge
Impact of stir bar wear on cold finger test results
Magnetic stirrers are an essential component of cold finger tests. They ensure that the oil sample is mixed homogeneously during cooling and wax deposition. Over time, however, the performance of the stir bars can deteriorate, for example due to loss of magnetic force, mechanical wear, or repeated exposure to higher temperatures.
This gradual decline in performance often goes unnoticed, but can have a significant impact on the accuracy and reproducibility of test results. A weakened stirring bar reduces the effective shear in the sample, promotes uneven temperature distribution, and can cause local wax deposition.
As a result, the measured wax mass may deviate from the actual deposition behavior. Recent observations have shown that even a slight reduction in stirring efficiency can alter the test result by more than 10 percent.
How to identify the issue in practice
Visually
The stir bar no longer runs smoothly and centrally in the beaker. It wobbles, moves irregularly or even jumps.
Acoustically
The sound of the stirring process changes. The background noise becomes unstable. At times you can hear the stir bar hitting the glass or insert. Overall, the operation sounds louder than usual.
Such signs clearly indicate that the stirring performance no longer corresponds to the original condition and that the test results should be reviewed carefully.
Recommendations for reliable cold finger operation
To maintain high quality and comparability of your cold finger measurements, we recommend:
- Regularly checking the stirring performance by visual and acoustic observation during the test.
- Replacing stir bars that show irregular movement, interruptions or audible vibration.
- Documenting replacement intervals and any abnormalities as part of your laboratory quality control.
Consistent monitoring of this seemingly small component makes an important contribution to ensuring the reproducibility of your cold finger test results and helps to identify deviations at an early stage.
- Practical Tip
The PSL Cold Finger family provides good access and direct visual control of the stirring process. This makes it easy to assess stirring performance during operation and to react quickly when intervention is required.
Cold Finger Product Family
THE PROVEN
Multi-Place Cold Finger
CF6
The ‘Proven’ with 6 measuring stations with water bath and innovative lift system for easier operation. The affordable entry-level model for low to medium sample throughput.
THE PROFESSIONAL
Multi-Rack Cold Finger
CF15
The ‘Professional’ in the laboratory with maximum flexibility, accuracy and safety. From 5 to 15 places with a wide temperature range, many options and used by leading laboratories.
THE SPECIALIST
Coaxial Shear Cold Finger
cCF6
The ‘Specialist’ with 6 rotating cooling fingers with water bath and lift system. It simulates the coaxial shear in the pipeline and measures the effects on the wax deposits.
THE INDEPENDENT
Mobile Cold Finger
mCF2
The ‘Independent’ with 2 measuring stations for mobile use or for low sample throughput. The PSL technology from its big brother has been implemented here in the smallest of spaces.
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