Ok guys I've decided to write this tutorial on the correct use of EC meters in hydroponics.
I know from reading diaries/posts on here that it is a subject that comes up regularly so hopefully this will address most of the issues.
Let's start at the beginning, EC refers to the electrical conductivity of an object, in our case the EC of our hydro reservoir.
The EC is a measure of the nutrient solutions ability to conduct electricity which is useful because it depends on the amount of nutes so more nutes means higher EC and vice versa.
Much of the confusion surrounding EC is caused because there are three different units of measurement that are used
1 )siemens per meter s/m or sm-1
2 )parts per million ppm
3 )total dissolved solids tds
The last two units are defunct and soon to be assigned to the dustbin of history while the first is the internationally recognized unit adopted by the scientific community so I will only be dealing with that.
OK as we have seen the basic unit of EC is the siemen per meter, i.e. it's ability to conduct electricity over a known distance.
Now that is all fine and dandy but for our purposes it can be a bit unwieldy because we are measuring very, very small readings.
For example if I go and take a reading from my res just now my digital EC meter might say
"123" with a small "x10" symbol flashing in the corner. So what does that mean?
Well first off you have to be aware that most hydro EC meters are calibrated to give readings in micro siemens per meter (us/m or usm-1) not siemens per meter.
This is shown by a "us" symbol above the display and most digital hydro EC meters will have this symbol.
So in this example my meter is telling me I have a EC of 123x10 micro siemens per meter or 1230 us/m.
OK great I have a reading from my meter in units I understand so how does this relate to my grow?
If you are a hydro grower then you will be familiar with feeding schedules like the one I use for my canna aqua nutes.
Pay attrention to the recommended EC levels on the right hand column of these table.
You'll see numbers like 1.2-1.4 so you probably wonder how that relates to my previous reading of 1230 us/m?
If you look closely at the bottom of the table you'll see that recommended maximum EC is given as 2.8ms/cm or 2.8 milli siemens per centimeter so what does this mean?
This tells me that all EC values in the table are given in milli siemens and not micro siemens so we have to take that into account.
2.8ms/cm =2.8 x10-3 siemens per centimeter = 0.0028 siemens per centimeter, done by moving the decimal point 3 places to the left as indicated by the "milli" prefix.
Another thing to notice is the move away from siemens per meter to siemens per centimeter but this doesn't matter because the measurement is still taken over a known distance and this is already dialled into you EC meter so don't worry about it just concentrate on the siemens value.
Basically the move away from siemens to milli siemens is done merely for convenience, it's easier to write 2.8 ms than 0.0028 s.
So basically what we want to do is convert our meter reading in micro siemens into the more commonly used milli siemens. How do we do that.
OK back to the example.
I have a reading of 1230 us that I want to express in ms.
1230 micro siemens = 1230 millionths of a siemen = 1230x10-6 = 0.001230 siemens done by moving the decimal point six places to the left as indicated by the micro prefix.
0.001230 siemens = 1.230 milli siemens done by moving the decimal point 3 places to the right.
So now I have my answer in terms I can work with, my EC is 1.23 milli siemens.
So that is what all the maths boils down to in the end, you will be converting from micro siemens to milli siemens to tie in your readings with the feeding guides and all you have to do is move the decimal point 3 places to the left.
1)meter reading = 500 = 0.5 milli siemens
2)meter reading = 100 x10 = 1000 micro siemens = 1.0 milli siemen
3)meter reading = 190 x10 = 1900 micro siemens = 1.9 milli siemen
Here is a useful online converter you migh find useful
Remember always pay attention to the flashing "x10" indicator if it appears!
Comments and questions are welcome.