TDS microsiemens or PPM

[scott]

example: if my water tests at 85 ppm then what is the reading in microsiemens or is ppm and microsiemens a different reading altogether, like apples and oranges. i have always used chemical tests for hardness (2dkh and 2dgh)but i just got a tds meter which gives me the reading in parts per million.

 

[tjudy]

ppm and mS both measure conductivity of the water. There are actually different conversion factors that are used in different industries. Hanna instruments sets their meters to a conversion of 1 mS = 2ppm. So a ppm reading of 85 ppm would equal 42.5 mS on a Hanna meter. THe good meters can be adjusted. SpectraPure (makers of RO/DI systms) use a conversion factor closer to 1mS = 3ppm, which is apparently in line with the USDA.

 

[scott]

thanks ted, that's exactly what i needed. i got the hanna tds1 meter on aquabid. so going along with the conversion formula you gave for hanna meters the water in most of my breeding tanks ideal tds would be about 130 ppm which equals 65microsiemens. that works out to about 1tsp of r/o right to 30 gal of r/o water the way i figure it. that doesn't seem like too much does it? i've only been using half that up till now without any problems.

 

[fishgeek]

ted would the micrsiemens conductivty not be different for every solution

by this i mean some compounds dissassociate into more conductive molecules than other compounds
so you may still only have 1 particle of compound dissolved and more actual ability to conduct?

do you know what solution they calibrate with? saline?

or am i confused and is the ppm reading a number of collective ions dissolved

andrew
a very confused pseudochemist

 

[tjudy]

Conductivity is not a colligative property, so yes... different compounds will conduct electricity differenty. Some compounds that contribute to carbonate hardness barely conduct electricity at all, which is why carbonate hardness cannot be measured by a conductivity meter.

The concentration of ion particles is what determines the level of condictivity. Some ionic compounds will dissociate into a greater number of ions than others. For example... sodium sulfate (Na2SO4... I cannot subscript the numbers in the formula) will dissociate into three ions. SO if you put 1000 particles of that compound in water they would create 3000 charged ions. NaCl will dissociate into 2 ion particles, so 1000 NaCl particles fall apart into 2000 charged ions. Theory says that the sodium sulfate will create the greater conductivity.

Compounds that dissociate in aqueous solution and conduct electricity are electrolytes. There are 'strong' and 'weak' electrolytes. Strong electrolytes completely dissociate (every particle will fall apart), but weak electrolytes do not. Technically speaking it is possible to have a higher TDS than what a meter is reading if there are a lot of non-dissociated compounds in solution. Luckily most fo the compounds that were are likely to see in water are either carbonate compounds (and contribute to KH) or are strong electrolytes.

Remember though... I am not a chemist... I just play one for an hour a day.

 

[scott]

thanks again ted for your well thought out post. i have oversimplified the whole thing but i am begining to get the point.

 

[fishgeek]

thanks ted

 

[scott]

i just got an e-mail reply from a representative at hanna instruments and he said that on their meters 1ppm = 2uS example: 500ppm on the meter would be 1000uS. what was written in previous posts in this thread seems to be backwards. i saw a meter that had both ppm and uS and that seemed to confirm the 1ppm = 2uS. just thought i would let everyone know or else someone could be adding way to much r/o right or similar recipe to there tanks. i have the name and number of the hanna rep that gave me this info so if anyone wants to talk to him about it just send me a quick pm.

 

[tjudy]

Oops... typo. Sorry. I never use ppm, so I never hit that button on the meter.

 

[scott]

bo problem, i would rather use uS but unfortunately my meter was rather inexpensive(cheap- $28) and it only gives readings in ppm.