Water chemistry

[BallAquatics]

It seems to me that conductivity readings are the more scientific method - normally when reading field reports the measurement is given in microsiemens.  Then again, maybe conductivity meters are more durable and better suited for field work??

Dennis

[Mugwump]

Neither is very expensive....but the conductivity testers are likely more durable than the run of the mill TDS pen,etc..

[b125killer]

It's a Hanna meter.  Let me go check the model number...It says HI 98311.

I like this meter it's waterproof floats and you don't need to calibrate it. I just need to understand water chemistry better. It's a little more involved then understanding the basic water parameters.

[BallAquatics]

I just need to understand water chemistry better.

Yea, back in the day, we used to say we kept water, not fish!  lol    It's like anything else, there's a bit of a learning curve, but once you've got it, you're good-to-go!

Dennis

[BillT]

There are several ways to get an idea of the amount of dissolved solids in water and different ways to put a number on it.
Total dissolved solids would be what you have left when you remove all the water and other liquids. This is mostly salts in most cases but can include uncharged solids also (such as uncharged sugars). Besides removing the liquids, which rarely done, you can measure the density of the water with various kinds of floating things (hydrometers) that will have a greater or lessor tendency to flow in water of different densities. Another method is to use a density refractometer. The amount of things in the water change the optical properties of the water which can be detected by looking at how much a light path bends when it goes through the water. Similarly, properties like freezing points and vapor pressure (ease of evaporation) are changed when other chemicals are dissolved in the water. These are used in labs with expensive machines (not realistic for normal people).

Details:
If a standard salt mix is present conductivity (which is the inverse of electrical resistance; 1/R) will provide a good indication of the solids present. Salts are molecules that break apart into two or more pieces in the water, each carrying an opposite charge. Because they are little bits of charge moving around in the water, they can transport current from one electrode to another resulting in a current which the instrument measures and conputes the resistance and therefore the conductivity (1/R). A small fraction of water molecules will also break apart into H+ and OH- ions, so that pure water can carry charge and has a residual level of conductivity.

Not all salt molecules of a particular kind will all break apart in this way and it will vary with the chemistry of the molecules and atoms involved. In addition, different ionized molecules or atoms will have different molecular weights and will therefore move more or less easily through the water, contributing more or less to the measured current.

Most electronic TDS meters are just conductivity meters with a conversion factor like what Dennnis was talking about built in in to their circuitry, probably based upon the most common kinds of salts they would be expected to encounter.

I use both conductivity meters and a refractometer for different purposes.