1、 Accuracy verification: the core process of ensuring "accuracy"

The core of accuracy verification is to compare the measured value of the online viscometer with the benchmark value of known accurate viscosity, and calculate whether the error meets the standard (such as ± 1%, ± 2%, etc.). It mainly consists of two steps: "laboratory calibration verification" and "on-site comparative verification".

1. Laboratory calibration verification (offline benchmark calibration)

This is the fundamental step of accuracy verification, which requires establishing measurement benchmarks using standard viscosity substances in a controlled environment (temperature, pressure stability).

Step 1: Select a standard viscosity substance

Using certified standard solutions (such as NIST, ASTM certified Newtonian silicone oil, glycerol solution), whose viscosity values are known at specific temperatures and traceable to international standards. According to the measurement range of the online viscometer, select 2-3 standard solutions covering the range (such as low viscosity 10cP, medium viscosity 100cP, high viscosity 1000cP).

Attention: Avoid using non Newtonian fluids (such as coatings and slurries) as standard fluids, as their viscosity varies with shear rate and cannot establish a stable benchmark.

Step 2: Simulate online measurement conditions

Inject the standard solution into a "simulated testing device" (such as a constant temperature circulation bath, pressure tank), and control the temperature and pressure to be consistent with the actual online working conditions (temperature fluctuation ≤± 0.1 ℃, pressure fluctuation ≤± 0.01MPa) - temperature is the key influencing factor of viscosity (such as a 1 ℃ change in liquid viscosity may lead to 1% to 5% error), and must be strictly matched.

Step 3: Compare measurement and error calculation

Measure the viscosity of the standard liquid using an online viscometer, while synchronously measuring it using a laboratory reference viscometer (such as Ubbelohde viscometer, Brookfield/LDV type), and calculate the error between the two:

Relative error=(online measurement value - reference value)/reference value x 100%

If the relative error of all standard solutions is ≤ the nominal accuracy of the instrument (such as ± 1.5%), then the accuracy meets the standard; If the error exceeds the standard, the instrument parameters (such as shear rate, sensor sensitivity) need to be adjusted or recalibrated.

2. On site comparison verification (online actual working condition verification)

After laboratory calibration is passed, it needs to be verified on the actual production line to eliminate the influence of working conditions (such as flow rate, shear, pollutants) on accuracy.

Step 1: On site sampling comparison

Take samples before and after the measurement point of the online viscometer (sample volume ≥ 500mL), immediately use a portable reference viscometer (such as a handheld rotary viscometer) to measure at the same temperature and pressure as the pipeline, and compare the error between the online measurement value and the sampled measurement value.

Example: The online measurement value of resin in a certain chemical pipeline is 250cP. After sampling, the portable viscosity meter measured 248cP, with a relative error of (250-248)/248 ≈ 0.8%, which meets the accuracy requirement of ± 1%.

Step 2: Dynamic condition verification (for non Newtonian fluids)

If the measured object is a non Newtonian fluid (such as polymer melt, food slurry), the "shear rate matching" needs to be verified: the shear rate of the online viscometer (determined by the sensor structure and fluid flow rate) needs to be consistent with the shear conditions required by the process, otherwise accuracy deviation may occur due to "shear thinning/thickening". By adjusting the pipeline flow rate or replacing sensor heads with different shear rates, it is possible to ensure that the online measurement values match the actual viscosity of the process.

2、 Stability verification: ensuring long-term testing of "stability testing"

Stability verification focuses on the consistency of the time dimension, that is, whether the measured values of the same stable fluid fluctuate within the allowable range (such as ± 0.5%/24h) during continuous operation of the online viscometer. Mainly achieved through "short-term repetitive testing" and "long-term drift testing".

1. Short term repeatability test (intraday stability)

Verify the measurement consistency of the instrument within a few hours and eliminate accidental interference (such as voltage fluctuations and instantaneous changes in flow rate).

Testing method: In the laboratory or on-site, let the online viscometer continuously measure the same stable fluid (such as standard silicone oil or stable materials in the process), record the data every 10 minutes, and record it continuously for 30 times.

Evaluation indicators:

Repetitive error=(maximum minimum)/average x 100%

Requirement: Repetitive error ≤ nominal stability of the instrument (e.g. ± 0.3%). If the fluctuation is too large, it is necessary to check whether the sensor is loose and whether the circuit is affected by electromagnetic interference (such as nearby motors and frequency converters).

2. Long term drift test (weekly/monthly stability)

The core of verifying the "zero drift" or "sensitivity drift" of instruments during long-term operation is to eliminate the effects of component aging, pollution, and other factors.

Test method:

Zero drift verification: Regularly (such as once a week) place the sensor in a "zero viscosity medium" (such as clean air, nitrogen) and record the zero output value. If the zero drift exceeds the instrument threshold (such as ± 0.1cP), zero calibration is required.

Sensitivity drift verification: Measure monthly with the same standard solution (such as 100cP silicone oil) under the same operating conditions, compare the deviation between the initial calibration value and the current measurement value. If the drift exceeds the nominal stability (such as ± 1%/month), laboratory calibration needs to be repeated.

On site simplification plan: If frequent sampling is not possible, viscosity data in the "stable stage" of the process can be monitored (such as the constant temperature curing stage of the reaction kettle). If the fluctuation range of the data exceeds the historical stable range (such as the usual fluctuation of ± 0.5%, now it is ± 2%), it indicates a decrease in stability.