EC tests of various mixtures

I bought an EC wand on eBay which can be calibrated to 1.4 mS/cm with a test solution and a magnet

This version is calibrated by inserting in the 1.4 standard solution and then the wand is stroked with a magnet.
It then sets it's reading to 1.4.

The recommended EC is quoted for different crops on many web sites but seldom do those sites remind readers that the base solution without the added nutrients can conduct substantially. Masterblend make a tomato mixture that has to be augmented with magnesium sulfate and calcium nitrate and added to the local water. Masterblend do not suggest a correct EC for the final mixture.

With a freshly calibrated wand as above I find that 500 mg of standard table salt (sodium chloride) in 1 Litre of my stored rainwater has an EC of 1.45 and I also find that the Masterblend mixture in rainwater is also 1.45. (You can only measure to the nearest 0.05). And yet people say 2 or 2.5 or even higher is correct for tomatoes. I assume that they have conductive tap water. I prefer to trust the Masterblend recommendation.

Since the salt solution EC is so close to my Masterblend figure I now intend to have a large vessel of standard salt water at the side of my main feed tank. Every week or so I will check the 4 point probe in the salt. A second tank of raw rain water can be used to clean the probe as it is moved from salt standard to the feed tank.

(Do not assume that 500 mg of salt in one litre of YOUR water will have and EC of 1.45)
Even my rain water reads 0.26 when the salt solution is set to read 1.0 on the 4 point probe. My tap water then reads 0.49!

Note - the wand above was purchased in order to help calibrate the 4 element probe. The one I had before looked very similar and was more expensive but could not be calibrated with a magnet switch. That reads 0.85 in my 1.45 salt solution! eBay seem to offer just the two designs from several shops that import the wands from China. Some hydroponic stores catering for the "herbal" market sell them for £80!