there are 4 different types of meters that measure nutrient concentration, two are EC meters and two are TDS meters
of the 2 types of EC meters, (electrical conductivity), one displays with decimal places, the other without
for example
EC 2.0 and EC 2000 are the same
TDS meters come in 2 types, .5 and .7 conversion.
You seem to have a .5 conversion TDS meter
TDS meters take the electrical conductivity value that is measured internally by the meter (EC), but then multiplies the EC by a factor, either .5 or .7
so for example,
2000EC * .7= 1400ppm but also
2000EC * .5= 1000ppm
TDS is a strange beast and does not react logically, as you have found
for example 0-8-16 should read 1000ppm on .5 conversion meter, in RO water, but strangely, if you start with 250ppm water, and add the nutes, it wont necessarily come out to 1250, as you have noticed.
the reason I think this happens is that some of the TDS in the water gets neutralized by the nutes.
for example, much of the TDS in water is calcium, which is of very high pH. When nutes, of low pH are mixed in, some of the low and high values cancel each other out... this is sometimes called nute lockout and is the cause of some of the sediment in a res.
as far as the term TDS (total dissolved solids), in contrast to the term PPM (parts per million), they are the same thing stated in different words
here is an example of how the terms interact
An EC of 2000 corresponds to a TDS reading of 1400 PPM on a .7 conversion meter.
imho, .7 conversion is the primary hydroponic conversion factor, but many people own meters with .5 conversion, therefore, you MUST know your conversion, to understand my TDS numbers, which are all .7 values unless stated otherwise
