Which ppm scale?

When people on here are referring to ppm's - is it usually the x 500 scale or x 700 scale?

PPM is actually the amounts of salts in the solution as in mg/liter. This is based upon the actual weight of the ingrediants added to the fertilizer sold to you. If it says (for an example) add 18 ml per gallon to obtain a mix at 200 ppm nitrogen, 100 ppm phosphorus and 200 ppm potash then it is based upon actual weights of the salt. A EC or TDS meter reads the conductivity of the solution then converts mathematically to an approximation of the total ppm of the solution. It will not be the same reading as the ppm stated by the manfacturer. TDS meters are also just reading conductivity like an EC meter and mathematically giving TDS estimate. TDS is a term based uopon the amounts of solids that would be left in a one liter flask after all the water evaporated from one liter of water and all organics were baked off so that only the mineral salts remained.

Saying ppm's is not referring to an estimated ppm but an actual weight of salts dissolved in a set amount of water. They actually should sell EC meters without TDS or ppm conversions as the conversions are not producing tri ue ppm amounts regardless of the meter conversion scale.

So should you use an EC meter instead of a TDS meter ?

My ppm meter is actually a EC meter - but it converts to ppm using either the 500 or 700 scale - my question is, which one is generally used around here?

Depends on if your from Europe or the US. There is the 442 conversion (40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride) which some say is the closest thing to a hydroponic solution. The 442 conversion is approximately 700 x EC in miliSiemens (mS). Then there is the NaCl conversion (sodium chloride) which others say is the closest thing to a hydroponic solution. The NaCl conversion is approximately 500 x EC in miliSiemens (mS). Most US manafctured EC meters use the Calcium Chloride conversion factor as a default. However many are adjustable to allow for any conversion rates aaaaaaaaaaaabetween 500 and 700. The European manafacturers more often use 700 as the conversion factor by default. It is really best to just use EC and ignore the TDS or ppm estimations. Sadly most people do not even calibrte their EC meters. Ideally they should be calibrated with a calibration solution of calcium chloride near to the EC/ppm you will commonly be testing.

So in a nut shell the answer to your question depends on if the persons is from the US/Canada, or from Europe. The 442 conversion factor was established for use with natural water such as untreated well water, rivers, lakes and streams.

Sounds like you have a Blue Lab or other higher end meter. Most nutrient manufacturers will put what conversion rate they use. Check their website and/or give them a call. I do know Advanced uses X700, says so right on their bottles. I once had a cheapo pen style and it only read x500 but it didnt say wether it did or not. I had to figure that out on my own. Peice of junk, had to calibrate it every other day.

Coals said:

Sounds like you have a Blue Lab or other higher end meter.

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Yeah i have a bluelab meter. Apparently these don't need calibrating? There is no mechanism to change it even if you have a solution that you knew the exact EC of...

fatman7574 said:

It is really best to just use EC and ignore the TDS or ppm estimations.

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I agree, because unless you state which conversion method you are using, its kinda meaningless.