In order to optimize the efficiency of plant growth, hydroponic nutrient solutions need to be at very specific levels. While following label directions will get dosages close to where nutrient and pH levels should be, this isn't enough if there are issues with water quality or evaporation and so a meter becomes essential.
The Straightforward Measurements, Temperature and pH
Most meters will measure temperature, the solution should be 15 – 24° C (60 – 75° F). Too cold and the growth of the plants will slow down and too hot will increase the risks of root born pathogens. Temperature is something most of us use everyday, and so there's no point going too far into it.
Another measurement that we monitor using a meter is pH. Most people do have a basic understanding of the scale. 7.0 is neutral, under 7.0 is acidic and over is basic (alkaline). There is a lot technical information on how these measurements are derived which I don't want to get into. The only piece of information that I would like to share from the technical side is that the scale is logarithmic meaning that each point it 10 times greater than the previous. For example a pH 5.0 solution is 10 times more acidic that a pH 6.0 solution. With this knowledge it's easy to see how getting to the extreme ranges of the scale will make the solutions very reactive.
With the exception of Fahrenheit still being used in 3 countries of the world, temperature and pH is pretty much universal.
Parts Per Million in the Chemical Industry
I have a confession to make. When I started work at Future Harvest and started hearing the term PPM I just assumed that it was the same unit I had been using in the chemistry labs where I previously worked. In chemistry Parts Per Million is a weight volume measurement corresponding to milligrams per liter (mg/l). Within a few months, and a bit of bewilderment I had figured out that the unit used in the hydroponic industry had no relation to the one that I was familiar with, but more on that later.
Electroconductivity How and Why
It turns out that there is no quick and easy way of measuring how much of a solid is dissolved in a solution. We could dry out all of the water and weigh up the remaining solids while it would be very accurate would be really impractical. It turns out that since fertilizers are salts they conduct electricity very well and the amount of conductivity is measurable, and the level of conductivity increases with the amount of salts. In fact this conductivity has its own unit, the Siemens (S). Most people are familiar with Ohms, the unit of resistance for electricity. Siemens essentially the opposite to that. Since conductivity and resistance will changed based on how far the electrodes are from one another in a solution, the measurements are kept at a standard distance which is 1 cm.
A big limitation of using conductivity is the fact that different salts conduct electricity in different degrees. A salt which ionizes completely conducts very well so for example a 0.7 gram per liter solution of potassium chloride will give a reading of around 1400 µS. On the other hand organic based chemicals conduct electricity very poorly, the same concentration of urea probably wouldn't show up on all but best lab grade meters.
Also keep in mind that the level of conductivity doesn't go up at a one to one rate like one would expect. So for example increasing the concentration by 50% doesn't mean there will be a 50% increase in conductivity. It could be significantly more or significantly less again depending on the salts being measured.
It turns out that Siemens are way too large of a unit for measuring how much fertilizer is dissolved in a solution, however because it's an SI (International System of Units) Unit we can simply add a prefix and the numbers become way more manageable. In Europe milliSiemens (mS, 1/1000 of a Siemens) are used for hydroponics, and here in North America microSiemens (µS, 1/1,000,000 of a Siemens) are more common. For example using mS we could take a measurement of 1.2, the same solution measured in µS would read at 1200. Both Europe and North America call their unit "EC", and while not technically wrong can be confusing.
TDS and CF, Simpler or More Complicated?
Back in the early days of hydroponics "Company A" decided that they wanted to market a meter to the public. Apparently they thought that using SI measurements were too confusing for growers and came up with the idea of TDS (Total Dissolved Solids) and used the unit PPM. Like I said previously this was not the unit that had been used in chemistry expressing mg/l but instead they took the EC measurement measured in microSiemens and multiplied it by 0.5. So we can take the 1200 µS from our previous example and multiply it by 0.5 and wind up with 600 PPM.
When "Company B" began marketing meters they didn't agree with the units that "Company A" were using so decided to fix the problem by using a 0.67 multiplier on their scale to get PPM. In this case we can take the 1200 µS and multiply by 0.67 giving a value of 804 (which would read at 800 PPM).
Are you confused yet? We're not done. "Company C" decided that both were wrong and came up with yet another scale. Instead of the previous TDS / PPM units they came up with another unit, CF or Conductivity Factor. This time instead of using microSiemens as the base unit milliSiemens were used. To get CF, we multiply the measurement in mS by 10. The 1.2 mS measurement we've previously been using would read at 12 CF using this scale.
Can or Should We Fix This?
"My car gets forty rods to the hogshead and that's the way I likes it."
So if you have been counting there are 5 scales (that I am aware of) being used in the hydroponic industry, so keep this in mind when talking to other growers that you may not be using the same units. There does seem to be a regional variation to this as well but it's not completely clear who uses what and so can't elaborate.
I doubt that any standard measurement that everyone agrees on will happen any time soon as there's no kind of government regulation to get everyone on the same page. Maybe if there was a large industry that was using these measurements and they came to a decision to standardize it would influence meter manufactures and the public to follow suit, but it's hard to foresee this happening. As far as meter manufacturers in the present day, it's relatively easy with design one that can switch between the different measurements.
It seems as though it's human nature that once someone learns one unit that it's very difficult for them to change to another one. Canada changed to the Metric system in the mid 1970s, and today the baby boomer generation prefers the old Imperial system, Generation X can use both, and Millennial's lean toward metric. My personal preference is to use microSiemens since it's both an SI unit and is fine enough for very accurate measurements. I started my career using 0.5 scale TDS but switched after the realization that it's an arbitrary unit. I do realize that most people would prefer sticking with what they know and am okay with that and whenever possible will try to communicate with others using the unit they feel comfortable with.
Ultimately use the measurement that you feel comfortable with but again, be aware that there are others out there and things like feed schedules and calibration standards could be in a different scale than what you're using.