MODBUS-RTU RS485 Soil NPK Measure Sensor (IP68, 5-30V) - Very Inaccurate Values

Hello,

I am a user of the RS485 soil NPK Measure sensor in practice for a student project in measuring soil contents in a greenhouse application.

I bought the N, P and K sensor to estimate a good value for these elements but found myself very far from accurately measured contents and solutions prepared in chemistry labs.

I have been able to test the probe in liquid solutions of individual Nitrogen, Phosphorus and Potassium jars with different ppm values, and more practically, in a soil solution with different values of these elements, alongside some tests with a mixture of these.

I am interfacing this sensor using an RS485 to TTL adapter connected to my board and the sample code (npk-sensor.ino) which is attached with the results in NPK.zip.

For an idea of how inaccurate my probe readings are to the measured contents of the chemicals, the document MODBUS-RTU RS485 Soil NPK Measure Sensor (IP68, 5-30V), is divided into 3 pages:

- Page 1: Individual quantities of N, P and K in liquid jars

Not in soil medium. 

-Page 2: Individual and mixture quantities of N, P and K solutions in soil medium

Since this is a soil probe, I repeated the accurate tests in the soil medium.

Tests of individual N, P and K values and some mixtures with known quantities (more common in fertile soil) 

- Page 3: Chemical quantities and values used to prepare to solutions for page 2

This page is only useful to verify the chemical aspect of how this experimentation was performed.

I would like to know whether my setup is correct or if the sensor probes are outputting inaccurate values since they are not actually NPK probes.

Have these probes been tested in similar cases like mine? If so, am I missing any calibration additions to my program to make them read better results or not?

Thanks for all the help in advance.