Measuring nitrogen using TDS meter ?

[Evendozen]

How do you measure the nitrogen content only with a TDS meter? I think you would look at the fertilizer number and divide by the proportion of nitrogen in the mix. Example --- a 1-1-1 would be a third of the meter reading ? Thank you in advance for replies.

[bil]

Me, I start off with RO water and add fertiliser in accordance with Ray's nitrogen calculator.
I think that doing it backwards like you are trying to do is comlicated. Plus, TDS meters read EVERYTHING in the water, so if the water is at all hard, you will be getting a false reading if you are relying on it to calculate nitrogen.

[Evendozen]

Originally Posted by bil

Me, I start off with RO water and add fertiliser in accordance with Ray's nitrogen calculator.
I think that doing it backwards like you are trying to do is comlicated. Plus, TDS meters read EVERYTHING in the water, so if the water is at all hard, you will be getting a false reading if you are relying on it to calculate nitrogen.

I use distilled water or rain water when available. My tap water is too hard--read 268 when I last checked it. I did find Ray's calculator after posting this question. A great handy tool !

[Ray]

First and foremost, TDS meters are notoriously inaccurate. They are just cheap electrical conductivity meters with a built-in conversion factor to TDS, and the true conversion would change from one fertilizer formula to any other.

If you want to use the meter as a guide, mix up a small quantity of fertilizer at a known concentration based upon calculations, and measure it's TDS. Then double that concentration (so you can cut it it half and still use it) and measure that.

Then make a graph of the TDS readings and the actual ppm N, draw a line through both data points, and you have a reasonable, if not entirely precise, calibration chart.

[Evendozen]

Originally Posted by Ray

First and foremost, TDS meters are notoriously inaccurate. They are just cheap electrical conductivity meters with a built-in conversion factor to TDS, and the true conversion would change from one fertilizer formula to any other.

If you want to use the meter as a guide, mix up a small quantity of fertilizer at a know concentration based upon calculations, and measure it's TDS. Then double that concentration (so you can cut it it half and still use it) and measure that.

Then make a graph of the TDS readings and the actual ppm N, draw a line through both data points, and you have a reasonable, if not entirely precise, calibration chart.

I believe this meter is fairly accurate. I was quite surprised at the small amount fertilizer needed. Without testing I would have used way too much !

[Ray]

Don't be so sure about your TDS meter. They are calibrated with a sodium chloride solution, which is nothing like a fertilizer solution. Read this.

[Evendozen]

This meter doesn't require a calibration solution. It might be better than nothing . I use it occasionally out of curiosity. I am just a hobby grower and like to try out new ways to do things. I do still measure the old way by counting drops for small batches of solution.

[Orchid Whisperer]

Originally Posted by Evendozen

This meter doesn't require a calibration solution. It might be better than nothing . I use it occasionally out of curiosity. I am just a hobby grower and like to try out new ways to do things. I do still measure the old way by counting drops for small batches of solution.

Meters that can't be calibrated will be inherently inaccurate due to instrument drift that can't be corrected for.

You can continue to use it for very rough estimates of TDS. Sort of like using your shoe length instead of an accurate ruler to measure length (I would be off by roughly an inch per foot, depending on the shoe).

[smokinjoe1952]

I use my cheap TDS meter for:

1. Tap water TDS versus RO water TDS.

2. Establishing a base TDS reading with new RO pre-filters and membrane.

3. Periodically testing my RO water to determine when to change pre-filters, then eventually the membrane.

So using it for relative readings seems to be a good use for these meters. My tap water is 205 PPM and my RO water is 11 PPM. I also had a reading of 110 PPM after mixing RO water with K-lite at a strength of 25 PPM N. (using Ray's calculator)

For what it's worth.

-Joe

[Ray]

I believe that's the best use for TDS meters, Joe.

If you deal with professional growers of any type of crop, you will NEVER hear the term "TDS" used. That is fairly unique the hobby orchid world. The pro's use electrical conductivity (EC) as the controlling factor, and fertilizer manufacturers commonly publish EC versus ppm N relationships.

For example, for K-Lite:
50 ppm N = 0.35 mS/cm (milliSiemens per centimeter)
100 ppm N = 0.71
150 = 1.06
200 = 1.46

MSU RO:
50 = 0.40
100= 0.80
150 = 1.20
200 = 1.60

MSU WW:
50 = 0.34
100 - 0.68
150 = 1.02
200 = 1.36