Ray said:
Lance's recommendation about calibrating a TDS meter is a good one, although I'm not sure I'd use table salt to do so. (Then again, I don't know how much it really matters, so he could be close enough.)
TDS meters just measure electrical conductivity of the solution and add an arbitrary conversion factor. EC is determined by the ionic mobility of the dissolved species. If you add table salt to pure water, the only things contributing to the EC are Cl- and Na+, which are both small in diameter so move easily in the solution. An ion of the same size but twice the charge moves twice as fast, while one with twice the size and the same charge moves half as quickly. Because of those ionic mobility differences, that conversion factor needs to be different depending on the exact chemical makeup of the solution.
Table salt is good to use to calibrate a meter because it is something everyone has. When mixed at exact ratios it provides an accurate known ppm to calibrate by.
It is not practical to have a seperate meter adjusted for each fertilizer element. It is also not reasonable to expect hobbyist growers to spend $300 on an accurate ec meter. So what we need to do is talk in terms of readings from ppm testers that everyone can afford ($20-$30). When calibrated to a table salt solution the accuracy is good enough to use for growing plants.
The cheap ppm tester I bought came calibrated very accurately and I felt no need to adjust it or compensate for the error.
Using the EC and concentration data from Blackmore, and a conductivity standard, I calibrated my two meters. At 150 ppm N of the MSU RO formulation, the calculated TDS contribution from the fertilizer should be about 425 ppm. My two meters told me about 600 and 750!
How did you make the fertilizer solution you tested? Measuring by weight or volume?
Above you say with 150 ppm N the total is 425 ppm.
On your web page you say 125 ppm N is 475 ppm total.
You written numbers don't support each other, can you define which is correct?
This is a quote from your website:
"As I typically shoot for 100-125 ppm N in my fertilizer solution, I simply use my TDS meters as a check, knowing that the Hanna TDS1 should show between 380 and 475 ppm TDS,"
That is not a problem, however, as I now know that if I measure the TDS at those levels, I am really getting the 150 ppm N. If, on the other hand, I use them and see 425 ppm and trust it at face value, I'm actually underfeeding.
I think the best thing for most folks to do is to trust the label on the fertilizer to start with. Then use the meter to make sure the concentrations in their pots are remaining at a safe level.
If you want to maintain 150 ppm of nitrogen and use a meter to keep track of the concentration here is a plan to follow....
(choose your desired nitrogen level, 150 ppm is used as an example only)
1. Make a table salt solution to calibrate your test meter. Save this solution to check your test meter in the future. Make a note as to what reading the meter gives for future reference
2.The label on the MSU (13-3-15) says to use 1 tsp per gallon to get 150 ppm. Use 1 tsp per gallon and trust it is correct.
3. Measure the total ppm with your test meter and remember this as the total ppm.
4. Periodically check the water in your pot reservoir or the small amout of water that drains from the pot with the
same test meter to see how it compares to the original total. If your ppm levels remain at or near the original level you are basically fertilizing correctly. If your levels are contstantly lower either your plants are consuming the nutrients or you are adding fresh water that dilutes the solution in the pot. If the concentration is lower you can safely increase the amount of fertilizer you are using. If the concentration is higher you should flush the media more often.
5. Occasionally test your meter by taking a reading of your saved table salt solution you made in step #1. If the meter no longer reads the same as when you first tested the solution you need to calibrate your meter's readout or get a new meter. If it is only of by a small amount don't worry about it.