How to Check the Accuracy and Repeatability of Colorimeter?

Photometric accuracy refers to the deviation between the transmittance (absorbance) read by the instrument on the absorption peak value and the known true transmittance (absorbance). Photometric repeatability refers to the operator repeatedly measuring certain repeatability of the transmittance (absorbance) of an absorption peak. The photometric accuracy of the portable colorimeter varies with the instrument and the measurement range, generally ±0.015A–±0.005A. The photometric repeatability is generally ±0.001- ±0.002A.

There are two ways to check the photometric accuracy: one is the standard solution method, and the other is the standard filter method. In the standard solution method, the photometric accuracy can be checked with potassium dichromate and sulfuric acid solution (0.005mol1′). Take about 60mg (precision weighing) of the benchmark potassium dichromate dried at 120°C to constant weight, dissolve and dilute to 1000ml with a sulfuric acid solution (0.005mol1′), the relative deviation should be within ±1%.

Use the standard filter method to check the photometric accuracy

In the standard filter method, the NBS standard filter or the International Physics Laboratory standard filter is generally used for inspection. The inspection method is: turn on the instrument, calibrate 0%T and 100%T or 0A after the instrument is stable, insert a standard filter with known transmittance or absorbance under the specified conditions, and the display value of the instrument can be determined. The metering accuracy of the instrument. If the neutral filter is not clearly marked when measuring the absorbance, after correcting the measurement conditions, first measure and adjust the transmittance of the filter, and then measure the absorbance (for example, the measured transmittance is about 10%T), change the wavelength to make it accurate to 10%T, and then measure the A value should be 1A). In use, the absorbance is often calibrated by adjusting the slope potentiometer of the logarithmic conversion circuit.

The following are the causes of some low-accuracy and repeatability phenomena in colorimeters and the corresponding countermeasures:

Whether the most accuracy spectral line is selected

There are often many emission spectral lines for a hollow spotlight of an element, and the most accurate spectral line should be selected for measurement. Some resonance spectral lines are relatively close to each other. For example, the manganese lamp has a latent line at 279.5 nm., 279.8 nm, and 280.1 nm, and the spectral line at 279.5 mm is the most accurate. If there is a certain error in the wavelength indication of the instrument, it is easy to be confused if it is not carefully distinguished.

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Is the lamp current setting appropriate?

The light intensity of the hollow shadow lamp is related to the current of the lamp. Increase the operating current of the lamp. The emission intensity can be increased. However, if the working current is too high, the discharge will be abnormal and the light intensity will be unstable. The lamp current is too low. In turn, the light intensity of the lamp will be weakened, resulting in a decrease in certainty and signal-to-noise ratio. Therefore, an appropriate lamp current must be selected. The optimum lamp current varies with cathode elements and lamp design. In actual operation, 1/3 of the rated maximum current is often selected.

Is the transparent glass on the light window on the right end of the burner clean?

The glass ball on the light window facing the right end of the burner is well sealed so that the dispersion system of the instrument can maintain good optical performance without being affected by the external environment. At the same time, it should have the best light transmittance. Maximize the passage of the optical signal into the dispersion system and detector. Exposure for a long time will cause dust on the surface of the light window, and the light transmittance will be greatly affected by the contamination or corrosion of high-temperature particles ejected by the combustion head. Therefore, it needs to be cleaned in time.

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