TDS sensor

I think you have to try the 707ppm buffer solution along with the command 

‘cal:707’ as shown on the wiki page. If nothing fixes the problem, maybe you need a replacement. 

If you succeed, you can try this Water pollution monitoring project too. 

https://www.pcbway.com/blog/technology/GSM_SMS_Enabled_AI_driven_TinyML_Water_Pollution_Monitor_7684be2c.html

This is a machine learning project using TinyML. This project uses the DFRObot TDS sensor too. 

Try to troubleshoot the problem by:

Make sure the sensor probe is clean and free of any debris or buildup. Use a soft cloth or sponge to gently clean the probe if necessary.

Ensure that the calibration solution you are using is fresh and within its expiration date. Also, make sure that the solution is mixed correctly according to the instructions on the package.

The temperature of the calibration solution should be close to the temperature of the water you will be testing with the TDS sensor. Make sure the temperature is within the recommended range for the calibration solution.

If the current calibration solution is not working, try using a different one to see if it makes a difference.

In summary, if the TDS sensor calibration is failing, you can try cleaning the probe, checking the calibration solution and temperature, trying a different calibration solution, or contacting the manufacturer or customer support company for assistance.