COD automatic monitoring instrument

COD automatic monitoring device, reducing inorganic substances in water samples, such as nitrite, sulfide, and ferrous ions, will react with potassium dichromate, affecting the measurement results. The amount of potassium dichromate consumed by them will be recorded in the measurement results, making the measurement results higher.

Principle of COD automatic monitoring instrument:

The mixture of water sample, potassium dichromate digestion solution, silver sulfate solution (silver sulfate as a catalyst can effectively oxidize linear fatty compounds), and mercury sulfate solution is heated to 165 ℃. After the oxidation of organic matter in the solution by dichromate ions, the color will change. The pollution source water quality detector detects this color change and converts it into COD value output. The amount of dichromate ions consumed is equivalent to the amount of oxidizable organic matter.

The reducing inorganic substances in water samples, such as nitrite, sulfide, and ferrous ions, will react with potassium dichromate, affecting the measurement results. The amount of potassium dichromate consumed by them will be recorded in the measurement results, making the measurement results higher.

The characteristics of COD automatic monitoring instrument:

1. It has the function of automatically collecting, storing, processing, querying, displaying, and outputting all data, instrument parameters, and operation logs.

2. It has a large storage capacity and can store historical data for no less than 5 years (with a measurement cycle of 1 hour per time).

3. Equipped with automatic standard sample verification function.

4. It has multiple ranges and can automatically switch ranges, and the instrument displays test results.

5. Capable of adding identification to different test data.

6. It has a digital communication interface that outputs commands, related data, and operation logs through the digital communication interface, and can receive remote control commands from the management platform, including remote startup, remote timing, and other functions.

7. Can achieve serial port output and network port output of monitoring data.

8. It has the function of collecting and storing waste liquid and cleaning wastewater separately.

9. Abnormal alarms and power outages will not result in data loss.

10. After abnormal reset and power failure, the instrument will automatically discharge residual reactants and restore working status.

11. Selecting valve components, selecting reagent sampling timing, flexible and diverse channels, with advantages such as minimum dead volume, easy maintenance, and high lifespan.

What are the main components to be introduced below? Let's take a look at the following content together.

1. Sample injection/metering unit: including sample, standard solution, reagent and other import parts (including sample channel and standard solution channel) as well as metering part.

2. Physical/chemical pretreatment unit: removes interference from the water sample matrix through physical and chemical means, and completes the enrichment, dilution, etc. of the analyte.

3. Reagent storage unit: a functional unit that stores various standard solutions and reagents, ensuring the safety and quality of their storage.

4. Analysis and detection unit: composed of a reaction module and a detection module. The online automatic analysis of the measured substance is completed through the control unit, and the measured value is converted into an electrical signal output part.

5. Control unit: including hardware and software for system control, implementing operations such as injection, digestion, and drainage. It has functions such as data collection, processing, display and storage, security management, data and operation log query and output, output sample retention, trigger sampling, etc.

Technical parameters of COD automatic monitoring instrument:

1. Method basis: Dichromate spectrophotometric method.

2. Measurement range: (0-5000) mg/L.

3. Quantitative lower limit: ≤ 15mg/L (indication error ± 30%).

4. Repeatability: ≤± 5%.

5. Low concentration drift: ± 5mg/L.

6. High concentration drift: ≤ 5%.

7. Indication error: Basic range 0-200mg/L.

8. Extended range: indication error: ± 3%, repeatability: ≤ 5%, high concentration drift: ≤ 3%.

9. Chloride ion influence test: ± 10%.

10. Measurement cycle: The measurement cycle is 20 minutes, and according to the actual water sample, the digestion time can be modified arbitrarily within 5-120 minutes.

11. Sampling period: time interval (adjustable from 10 to 9999 minutes) and whole point measurement mode.

12. Calibration cycle: 1 to 99 days, adjustable at any interval and time.

13. Maintenance cycle: usually once a month, about 60 minutes each time.

14. Output: RS-232, RS-485, 4-20mA (optional).

15. Environmental requirements: Indoor with adjustable temperature, recommended temperature range of+5-28 ℃; Humidity ≤ 90% (non condensing).

16. Power supply: AC220 ± 10% V, 50 ± 10% Hz, 5A.

So what should we pay attention to when testing chemical oxygen demand? The following interfering factors should be avoided:

1. Organic compounds in glass or the atmosphere may cause serious interference.

Therefore, when preparing reagents and diluting samples, it is necessary to avoid organic matter from entering distilled water as much as possible. The glassware used in the test should be blank processed to eliminate interference from organic matter.

2. A certain amount of chloride can interfere with the oxidation of potassium dichromate. Adding mercuric sulfate to the digestion bottle effectively eliminates the chloride interference from saltwater and estuarine samples.

3. If sulfuric acid is added during the heating of the water sample, the detection data may deviate due to the loss of some volatile substances. Therefore, in order to reduce this error, everyone should add sulfuric acid during the cooling process of the flask.

Stability is the ability of an instrument to measure a sample over a long period of time. According to the definition of stability in the "General Metrology Terms and Definitions", the change in metrological characteristics within a specified time interval is used to characterize the metrological characteristic.

In practical work, Type I instruments measure every 4 hours, while Type II instruments measure every 1 hour; Due to the regular automatic calibration function of the heavy metal water quality automatic analyzer and the relevant regulations in the environmental protection industry standards for its small maintenance cycle (the instrument maintains its metrological performance cycle without manual maintenance), the long-term stability of the instrument is well guaranteed. The preparation team found during the experiment that the 24-hour stability measurement results were basically consistent with the 4-hour stability measurement results. To increase on-site operability, a 50% range standard solution is used as the test sample, and after stabilization, continuous measurements are taken for 4 hours. The stability is determined by the percentage of the initial measurement value with a larger deviation relative to the initial measurement value. Referring to relevant standards, the recommended stability index is set at 5%.

Curve calibration method for COD automatic monitoring instrument:

1. Place the "calibration/measurement toggle switch" at the measuring position, and the instrument will display 00.00.

2. Insert the dedicated colorimetric tube containing blank samples into the colorimetric hole, press the "confirm" button to display the blank signal value. After the reading stabilizes, press the "confirm" button to confirm and the instrument will automatically zero.

3. Insert the dedicated colorimetric tube containing sample 1 into the colorimetric hole, enter the concentration value of the sample using the key header, and then press the "confirm" button to display the absorbance value of the sample. After the reading stabilizes, press the "confirm" button to confirm.

4. Repeat step 3 and calibrate the remaining standard samples separately until all standard samples are calibrated. Press the "End Calibration" button to end the calibration. The instrument will automatically calculate the standard curve equation and display the r value (9.9. x, x represents r value of 0.99 x). Enter the curve number (I=1-5) and press the "confirm" button to save the curve in the instrument.

Note: In general, the correlation coefficient r value of the standard curve should be above 0.990. If the r value of the calibrated standard curve is lower than 0.990, it indicates that there is a problem with the standard preparation or the colorimetric operation is not standardized. It should be carefully analyzed and eliminated one by one.