Pond Boss Forums INTRO Questions & Observations True definition of Salinity

The term salinity has been used several times concerning the hatch in a pond. In my case I have high salinity(3500 ppm) which is helpful in some ways but detrimental concerning the spawn. I have an Extech meter which gives me salinity,TDS,and conductivity. I just get TDS(total dissolved salts),conductivity, and various elements on my water samples from A&M.So when you Pond Bosses say salinity you are referring to my Extech meter reading but on the A&M water sample which one is it or do I have to convert conductivity to salinity on this sample with some conversion ratio. As you can tell I am no scientist but I want to be able to compare the Extech meter with the lab samples. Also am I right in saying TDS(total dissolved salts) is of no use in evaluating my pond just salinity? Maybe you can refer me to a paper on the matter - Thanks

Is the TDS - total dissolved solids ? I don't see that on reports like the one you note. Not sure what your meter is testing.

KCH, yer meter is most assuredly giving you total dissolved solids (TDS) as there is a numerical relationship between that and electrical conductivity. i use a hanna water meter that gives temp, pH, conductivity, and TDS.

check out this thread, ewest had some amazing posts there:
http://www.pondboss.com/forums/ubbthreads.php?ubb=showflat&Number=38784&fpart=1

come back with more questions after reading this, it is a very interesting topic for me as well.

Thanks guys for the feedback but my water report from Texas A&M says Total Dissolved Salts(TDS) = 5000 ppm. My conductivity says 5450 umhos/cm. No where does this report say salinity and that is what confuses me. The term salinity in ppm is often used to describe fresh,brackish, and salt water. What is that number relative to the 2 I previously stated. Is there a correlation? On my Ectech meter it does calculate salinity which is constantly coming in at around 3000ppm. The TDS it gives is also around 5000 ppm and conductivity is 5400 umhos so there is correlation with the sample report. So really my question is can I take the A&M water report and calculate salinity so I can compare it with the Ectech meter? - Thanks

See if this helps.

Salinity is commonly reported in units of total dissolved solids (TDS) or electrical conductivity (EC). TDS are measured by evaporating a sample of water and weighing the residue. The results are reported in parts per million (ppm) or mg/L, depending on whether the calculation is on a weight or volume basis. Since 1 liter of water typically weighs about 1 million milligrams, for practical purposes the units of ppm and mg/L are equal.


The EC of a solution is a measure of the ability of the solution to conduct electricity. When ions (salts) are present, the EC of the solution increases. If no salts are present, then the EC is low, indicating that the solution does not conduct electricity well. As a result, EC indicates the presence or absence of salts, but does not indicate which salts are present. A high EC in soil water may result from irrigation with salty water or from dissolved fertilizer salts from a recent application. To determine the source of the salts in a sample, further chemical tests must be performed.

EC measurements are taken with conductivity meters and results are given in units of conductance. Hand-held conductivity sensors (Fig. 4) are convenient for measuring conductivity in the field. They come in a variety of designs and can range in cost from $40 to several hundred dollars.



Figure 4. Typical handheld conductivity meter.


The SI (metric) unit of salinity measurement is deci-Siemens per meter (dS/m), which is the same as millimho/cm (mmho/cm). Both of these terms are generally in the range of 0-5. If the numbers reported are higher, in the range of 100-5,000, the units are typically micro-Siemens per centimeter (µS/m) which is equivalent to micromho/cm (µmho/cm).


The conversion from electrical conductance to TDS depends on the particular salts present in the solution. The conversion factor of 700 x EC (in dS/m) is applicable for converting EC values to TDS for Florida irrigation waters. Commercially available meters will read directly in ppm. Care must be taken when using these meters so that results are reported consistently. Most of these meters use conversion factors of 630 or 640 x EC to get ppm. However, some meters may use a factor as low as 500 or as high as 800 to convert from dS/m to ppm. It is important to know what conversion factor your unit uses so you can properly interpret results for Florida conditions.


Conversions:


1 mg/L = 1 ppm

dS/m x 700 = ppm (for salts typically found in Florida surface and ground water)

µS/cm x 0.7 = ppm

µS/cm = µmho/cm

Example 2


Determine the salinity in ppm for a water sample with EC of 2.3 dS/m.

2.3 dS/m x 700 = 1,610 ppm

Example 3


A meter that has a built-in conversion factor of 1 dS/m = 630 ppm has a reading of 2,300 ppm. What would be the TDS if the factor of 700, suitable for most Florida water was used instead?

Convert to dS/m using the meter factor of 630

2300 ppm / 630 ppm/dS/m = 3.65 dS/m

Then convert back to ppm using a factor of 700

3.65 dS/m x 700 ppm/dS/m = 2,555 ppm




TDS meters are calibrated for Sodium Chloride salt

0.1 microSiemens corresponds to 0.04 ppm (parts per million) of Sodium Chloride salt.

1 microSiemens corresponds to 0.4 ppm of Sodium Chloride salt.

10 microSiemens corresponds to 4.3 ppm of Sodium Chloride salt.

99.9 microSiemens corresponds to 48 ppm of Sodium Chloride salt.


What is the difference between ppm and EC?

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Total Dissolved Solids (TDS) is the best measurement of the nutrient concentration of a hydroponic solution. To estimate TDS, one can use a meter that measures the Electric Conductivity (EC) of a solution, and convert the number to TDS in parts per million (ppm). Many meters will do this conversion.

Total dissolved solids (TDS) is typically expressed in parts per million (ppm). It is a measurement of mass and determined by weighing, called a gravimetric analysis. A solution of nutrients dissolved in water at a strength of 700 ppm means that there are 700 milligrams if dissolved solids present for every liter of water. To accurately calculate total dissolved solids (TDS), one would evaporate a measured filtered sample to dryness, and weigh the residue. This type of measurement requires accurate liquid measurement, glassware, a drying oven, and a milligram balance. Example: 50 mL of the 700ppm solution would leave 35 mg of salt at the bottom of a crucible after drying.

Electrical Conductivity (EC) is expressed in siemens per centimeter (s/cm) or milliseimens per centimeter(ms/cm). It can be determined with an inexpensive hand held meter. Nutrient ions have an electrical charge, a whole number, usually a positive or negative 1, 2, or 3. EC is a measurement of all those charges in the solution that conduct electricity. The greater the quantity of nutrient ions in a solution, the more electricity that will be conducted by that solution. A material has a conductance of one siemens if one ampere of electric current can pass through it per volt of electric potential. It is the reciprocal of the ohm, the standard unit of electrical resistance. A siemens is also called a mho (ohm backwards).

For convenience, EC measurements often are converted to TDS units (ppm) by the meter.

The meter cannot directly measure TDS as described above, and instead uses a linear conversion factor to calculate it. Everyone’s nutrient mix is different, so no factor will be exact. The meter uses an approximate conversion factor, because the exact composition of the mix is not known. Conversion factors range from .50 to .72, *depending on the meter manufacturer, which do a good job of approximating a TDS calculation from the meter’s measurement of EC.
* All ppm pens actually measure the value based on EC and then convert the EC value to display the ppm value, having different conversion factors between differing manufacturers is why we have this problem communicating nutrient measurments between one another.

EC is measured in millisiemens per centimeter (ms/cm) or microsiemens per centimeter (us/cm).

One millisiemen = 1000 microsiemens.

EC and CF (Conductivity Factor) are easily converted between each other.
1 ms/cm = 10 CF

"The communication problem"...
So again, the problem is that different ppm pen manufacturers use different conversion factors to calculate the ppm they display. All ppm (TDS, Total Dissolved Solids) pens actually measure in EC or CF and run a conversion program to display the reading in ppm's.

There are three conversion factors which various manufacturers use for displaying ppm's...

USA 1 ms/cm (EC 1.0 or CF 10) = 500 ppm
European 1 ms/cm (EC 1.0 or CF 10) = 640 ppm
Australian 1 ms/cm (EC 1.0 or CF 10) = 700 ppm

For example,
Hanna, Milwaukee 1 ms/cm (EC 1.0 or CF 10) = 500 ppm
Eutech 1 ms/cm (EC 1.0 or CF 10) = 640 ppm
Truncheon 1 ms/cm (EC 1.0 or CF 10) = 700 ppm

Calculating the conversion factor
If your meter allows you to switch between EC and TDS units, your conversion factor can be easily determined by dividing one by the other.

Place the probe in the solution and read TDS in ppm. Change to EC on the meter and read EC in ms/cm.

Conversion factor = ppm / ec.
[Note: ms must be converted to us: One millisiemen = 1000 microsiemens (1.0 ms/cm = 1000.0 us/cm)

According to the chart below:
1.0 ms/cm = 500 ppm (USA Hanna)
1000 us/cm = 500 ppm

Conversion factor = ppm / (ms/cm * 1000)
.50 = 500ppm / (1000us/cm) ]

The answer is your meter's convertion factor and should be a number between 0.50 and 0.72 To improve accuracy, take ec and ppm readings from your res daily for about ten days. Average the conversion factors. The more data points that you use, the closer you will be to finding your true conversion factor.

When reporting your PPM in a thread, please give the conversion factor your meter uses. For example: 550 PPM @0.7 or give the reading in EC, which should be the same meter to meter.

It may also be advisable to give the starting value of your water; there is a huge difference between RO and distilled water with a PPM of approximately 0 and hard tap water of PPM 300 @.5 (notice the conversion factor so others can work out the EC) or well water with a conductance of 2.1 ms/cm.

I don't know squat about water chemestry, but I did not see any one mention that the TDS (Solids or Salts) can include magnesium, aluminum, calcium, sodium, etc.

Am I correct that it does NOT mean just table salt (NaCl) with respect to "salinity"???

Rainman see blue text above. Good point!

yes. salt is a broad term like tissue. table salt (NaCl) is like kleenex.