The poor repeatability of cone penetration results (i.e. the deviation of multiple test results by the same operator, equipment, and sample in a short period of time exceeds the standard allowable range, usually requiring a relative deviation of ≤ 3%) is mainly due to non-standard operating procedures, inconsistent control of key parameters, or abnormal equipment status. A standardized operating system needs to be established throughout the entire process of "sample processing equipment calibration testing operations environmental control". The specific solution is as follows:

1、 Sample processing: Eliminating repeatability bias caused by "sample unevenness"

The uniformity and temperature consistency of the sample itself are the basis of repeatability. If there are local differences in composition/hardness of the sample, even if the operation is the same, the results will fluctuate. Three key points need to be controlled:

1. Reconfirm the uniformity of the sample to avoid "local differences"

If the sample is semi-solid (such as grease or ointment), a clean glass rod should be used to "cross stir" the sample from the center to the edge of the test cup three times before testing (stirring depth to the bottom of the cup, slow speed to avoid introducing bubbles), to ensure that the sample has no layering or local hardening;

If the sample is solid (such as paraffin), it is necessary to confirm that there is no local crystallization after constant temperature (which can be observed visually: the surface of the sample should be uniformly transparent/milky white, without obvious particles or crystal spots). If crystallization exists, it needs to be melted again (according to standard temperature) and gradually cooled down to ensure uniform crystallization.

2. Strictly unify the 'sample constant temperature time' to avoid temperature fluctuations

The samples for the same group of repetitive tests must be synchronously kept at the same constant temperature bath, and the constant temperature time must meet the following requirements: semi-solid samples ≥ 6 hours, solid samples ≥ 12 hours (it is forbidden to test some samples in advance if the constant temperature time is insufficient);

During the testing process, after measuring each sample, a thermometer (with an accuracy of 0.01 ℃) should be inserted into the center of the next sample to confirm that the temperature deviation from the set value (such as 25 ℃) is ≤ 0.05 ℃. If it exceeds the range, the temperature should be kept constant for 30 minutes before testing.

3. Avoid surface damage to the sample and ensure consistent initial contact state

Before each test, the surface of the sample should be kept flat and free of scratches. If there are pinholes on the surface of the sample after the previous test, a clean scraper (with a straight blade) should be used to gently scrape off the 1-2mm thick sample to expose a new flat surface (with consistent scraping direction to avoid texture);

After scraping, it cannot be tested immediately. It needs to be left in a constant temperature bath for 15 minutes to release the surface stress of the sample and avoid local hardness changes caused by the scraping process.

2、 Equipment calibration: eliminating repetitive deviations caused by "equipment abnormalities"

The "cone needle status", "lifting system", and "timing accuracy" of the fully automatic cone penetration tester are the core hardware factors that affect repeatability, and three key calibrations need to be completed before testing:

1. Cone needle calibration: Ensure compliance of "cone tip sharpness" and "cone needle weight"

Cone tip sharpness inspection: Observe the cone tip with a 10x magnifying glass, and there should be no obvious wear or deformation (the standard requires a cone tip angle of 30 °± 0.2 ° and a needle tip radius of ≤ 0.15mm). If worn, replace the cone tip with a new one;

Cone needle weight calibration: The total weight of the "cone needle+connecting rod" of the fully automatic instrument must meet the standard (such as GB/T 269 requirement of 100g ± 0.05g), and be weighed with a calibrated electronic balance (accuracy 0.1mg). If the deviation exceeds the range, the connecting rod weight needs to be adjusted or the components need to be replaced.

The cone needle is the core testing component. If the cone tip is worn (such as rounded corners) or there is a weight deviation, it will directly cause the depth to change:

2. Calibration of lifting system: Ensure the coaxiality of cone needle descent and displacement accuracy

Operation: Place the calibration "positioning ring" (inner diameter consistent with the test cup) on the sample stage, start the instrument to lower the cone needle, and observe whether the cone needle is completely centered on the positioning ring (deviation should be ≤ 0.5mm). If it is offset, adjust the horizontal screw or cone needle fixing bracket of the sample stage;

If the cone needle descends with a different axis (offset) from the test cup, it may cause the position of the cone needle in contact with the sample to be different (edge vs center, hardness may vary), and coaxiality needs to be calibrated:

Accuracy calibration of displacement sensor: Place a standard measuring block (such as 10mm, 20mm) on the sample table and let the instrument measure the height of the measuring block. Repeat 3 times, with an error of ≤ 0.01mm. If it exceeds the tolerance, contact the manufacturer for sensor calibration.

3. Timing accuracy calibration: Ensure that the "cone in time" is strictly 5 seconds (or the standard specified time)

Calibration method: Use a stopwatch (with an accuracy of 0.01 seconds) to synchronously record the timing of the instrument, repeat 5 times, and the timing deviation should be ≤ 0.05 seconds. If it exceeds the tolerance, enter the instrument parameter setting interface to adjust the timing module.

The core parameter of cone penetration testing is "the depth of penetration of the cone needle after 5 seconds of free descent". If there is a timing deviation (such as 4.8 seconds or 5.2 seconds in reality), it will lead to result deviation (especially when the sample viscosity is low, a time difference of 0.2 seconds may cause a depth difference of 1-2 units):

3、 Test operation: unify the "operation details" to avoid human differences

Even if the equipment is normal and the sample is uniform, small differences in operational details (such as sample stage positioning and cone needle zeroing) can still lead to poor repeatability, and strict adherence to the "four step standardized operation" is required:

1. Sample stage positioning: Ensure that the testing position is consistent after each test

Fully automatic instruments usually have "sample stage positioning holes" or "scale lines". Every time the test cup is placed, the notch/mark of the test cup should be aligned with the positioning hole, and the test cup should be gently pushed to the bottom by hand (to avoid displacement of the cone needle in contact with the sample due to different placement angles);

If the instrument does not have a positioning mark, a marking pen should be used to make "alignment marks" on the sample stage and test cup to ensure that the placement position is the same every time.

2. Cone needle zeroing: must be 'reset to zero' before each test

Zeroing operation: After placing the test cup, start the "Cone Needle Zeroing" program to slowly lower the cone needle to the surface of the sample (the instrument will automatically recognize the "contact moment" and reset it). After zeroing is completed, wait for 5 seconds before starting the test to avoid errors caused by testing immediately after zeroing.

It is prohibited to reset the zero value only before the first CI test, and use the previous reset value directly for subsequent tests (due to slight vibration of the sample stage and slight deformation of the cone needle, which may cause zero drift):

3. Test speed: Following the principle of "free descent", manual intervention is prohibited

The fully automatic instrument needs to confirm that the "cone needle descent mode" is "free fall" (without any damping), and it is forbidden to turn on the "slow descent" mode (which will cause the depth to be too small);

During the testing process, it is prohibited to touch the instrument, sample stage, or constant temperature bath (vibration can cause the cone needle to shake and become deeply unstable), and the laboratory fan and air conditioning vents should be turned off (airflow can affect the cone needle's descent trajectory).

4. Data recording: After a single test, it is necessary to "clean the cone needle" to avoid cross contamination

Cleaning method: Use a clean degreased cotton dipped in anhydrous ethanol to wipe the surface of the cone needle (from the cone tip to the connecting rod), then dry it with a hair dryer (cold air) before conducting the next test;

After cleaning, it is necessary to perform a "cone needle zeroing" again to avoid changes in the position of the cone needle caused by wiping.

After each test, there will be residual samples (especially viscous samples) on the surface of the cone needle. If the next sample is tested directly without cleaning, it will cause an increase in the weight of the cone needle (the residual sample weight may reach 0.1-0.5g), which will lead to a larger depth of immersion:

4、 Environmental control: stabilize the "testing environment" and eliminate external interference

The temperature, humidity, and vibration of the environment are easily overlooked as "hidden error sources", and the following conditions must be met:

1. Temperature control: laboratory temperature and constant temperature bath temperature deviation ≤ 1 ℃

If the laboratory temperature fluctuates greatly (such as frequent on-off of air conditioning in summer causing the temperature to rise from 23 ℃ to 27 ℃), it will increase the load on the constant temperature bath, thereby causing fluctuations in the sample temperature (even if the bath displays a normal temperature, there may be slight deviations in the actual sample temperature);

Solution: Install a "constant temperature air conditioner" in the laboratory to control the room temperature at 25 ± 1 ℃ (or consistent with the test temperature), and monitor it in real time with a temperature and humidity meter. Record every 30 minutes to ensure that the temperature fluctuation is ≤ 0.5 ℃/hour.

2. Humidity control: Maintain 45% -65% RH to avoid sample moisture absorption/loss

Excessive humidity (>70% RH): Samples (such as ointments and paraffin) are prone to moisture absorption, causing the surface to become soft and leading to a deeper immersion; Low humidity (<40% RH): The sample is prone to water loss, the surface becomes hard, and the depth of immersion is relatively small;

Solution: Turn on the dehumidifier when the humidity is too high and the humidifier when it is too low to ensure that the humidity remains stable within the specified range.

3. Vibration control: stay away from "vibration sources" to ensure instrument stability

Do not place the instrument near the centrifuge, shaker, or fume hood (where there is vibration during fan operation), and ensure that the instrument is placed on a "horizontal shock-absorbing table" (which can absorb ground vibrations and avoid shaking of the sample table);

Before testing, use a spirit level to check if the instrument is level (with a horizontal deviation of ≤ 0.1 °). If it is not level, adjust the horizontal screw at the bottom of the instrument.

4、 Validation and improvement: Verify the effectiveness of specifications through "repetitive testing"

After completing the above specifications, it is necessary to confirm whether the repeatability meets the standards through the "standard sample verification method":

1. Select standard samples: use "standard substances" with known cone penetration values

Purchase qualified standard samples (such as the "Cone Penetration Standard Lubricant" issued by the National Institute of Statistics, with a known value of 200 ± 3 units at 25 ℃), and conduct 6 repeated tests according to the above specifications.

2. Determine whether the repeatability meets the standard: calculate the "relative standard deviation (RSD)"

Calculation formula: RSD=(standard deviation/mean) × 100 (%)

Qualification criteria: RSD should be ≤ 3% (e.g. 6 test results of 198, 200, 199, 201, 200, 199, with an average value of 199.5, standard deviation of 1.0, RSD=0.5%, meeting the requirements);

If the RSD is still greater than 3%, it is necessary to investigate one by one: first check whether the conical needle is worn, then check the sample temperature holding time, and finally check the environmental vibration until the root cause of the problem is found.

Summary: The core solution logic for poor repeatability

The essence of poor repeatability in cone penetration results is that "variables are not controlled", and the core of standardized operation is to "fix all variables (samples, equipment, operations, environment) that may affect the results", ensuring that the "initial conditions are completely consistent" for each test. By establishing the "Standardized Operation SOP for Cone Penetration Testing" (including sample processing, equipment calibration, operating steps, environmental requirements, and exception handling) and providing training and assessment to operators, the repeatability deviation can be stably controlled within 2%, meeting the standard requirements.