We're planning on using an NPK Sensor to measure the NPK amount in water. Will it work? If not, is there a better alternative sensor that we can use? Thanks!
When you contacted the potential supplier of your NPK sensor and explained your intended use, what did they advise?
Hi, @eyeyrown
Welcome to the forum.
Can you please post a link to the specs/data of the sensor?
Thanks.. Tom.... 
An NPK sensor should certainly detect the presence of NPK but to make quantitative measurements (how much NPK is in the water) you need a quantitative sensor that is calibrated.
Maybe you can follow the example of this project:
I wonder what it is that the gizmo linked above actually does?
Judging from the slickness and lack of information on the product page, it is almost certainly bogus.
With three stainless steel prongs, it certainly does not quantitatively measure nitrogen, phosphorous and potassium concentration in soil.
Ask any professional farmer whether there is a cheap and reliable method to measure NPK, and you will get an earful.
I've no need of one myself, but I often wondered how those soil probes worked (or not....).
EDIT: Seems that there may be some science behind those 3 prong NPK sensors. I found a paper published in the International Journal of Engineering Research & Technology (IJERT) that had this paragraph:
A] Conductivity Measurement Technique:
In conductivity measurement technique two or three
electrodes of same material are immersed in soil samples.
Materials used can be steel, silver, platinum, graphite or
copper. An A.C. voltage is applied to electrodes in sample.
Another electrode is connected to multimeter to measure the
current changes. The A.C. voltage results in movements of
ion which in turn results in variability of current of soil
sample. Use of A.C. voltage avoids neutralization of ions.
Varying current gives varying conductivity. Variability
between electrical conductivity and concentration N, P, and K
are observed. As concentration increases, variability in
electrical conductivity increases [3].
As per concentration of NPK in soil, conductivity of electrode
change. The change in conductivity is converted into
electrical signal for further electronic control system [9].
I have a vague understanding of what that means ...
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Again, ask a professional if those work. I have, and they don't.
It is currently a hot topic in agribusiness research to come of up a cheap method of estimating NPK concentrations using simple measurements like conductivity, pH, etc. The really big companies are trying to use machine learning soil models to solve the problem, which requires to collect millions of data points from active farms all over the world.
This company claims to make an NPK sensor that you RENT for $1200+/year, and you don't even have control of the data. "AS SEEN IN FORTUNE MAGAZINE" is a strong hint.
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