Tds Theory

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Test

12

Water Quality with Computers 12 - 1

Total Dissolved Solids

INTRODUCTION

Solids are found in streams in two forms, suspended and dissolved. Suspended solids include silt,

stirred-up bottom sediment, decaying plant matter, or sewage-treatment effluent. Suspended

solids will not pass through a filter, whereas dissolved solids will. Dissolved solids in freshwater

samples include soluble salts that yield ions such as sodium (Na

+

), calcium (Ca

2+

), magnesium

(Mg

2+

), bicarbonate (HCO3

–

), sulfate (SO4

2 –

), or chloride (Cl

–

). Total dissolved solids, or TDS,

can be determined by evaporating a pre-filtered sample to dryness, and then finding the mass of

the dry residue per liter of sample. A second method uses a Vernier Conductivity Probe to



determine the ability of the dissolved salts and their

resulting ions in an unfiltered sample to conduct an

electrical current. The conductivity is then converted to

TDS. Either of these methods yields a TDS value in

units of mg/L.

The TDS concentration in a body of water is affected by

many different factors. A high concentration of

dissolved ions is not, by itself, an indication that a

stream is polluted or unhealthy. It is normal for streams

to dissolve and accumulate fairly high concentrations of

ions from the minerals in the rocks and soils over which

they flow. If these deposits contain salts (sodium

chloride or potassium chloride) or limestone (calcium

carbonate), then significant concentrations of Na

+

, K

+

,

Cl

–

will result, as well as hard-water ions, such as Ca

2+

and HCO3

–

from limestone.

TDS is sometimes used as a “watchdog” environmental

test. Any change in the ionic composition between



testing sites in a stream can quickly be detected using a

Conductivity Probe. TDS values will change when ions

are introduced to water from salts, acids, bases, hardwater minerals, or soluble gases that ionize in

solution.

However, the tests described here will not tell you the specific ion responsible for the increase or

decrease in TDS. They simply give a general indication of the level of dissolved solids in the

stream or lake. Further tests described in this book can then help to determine the specific ion or

ions that contributed to changes in the initial TDS reading.

There are many possible manmade sources of ions that may contribute to elevated TDS readings.

Fertilizers from fields and lawns can add a variety of ions to a stream. Increases in TDS can also

result from runoff from roads that have been salted in the winter. Organic matter from

wastewater treatment plants may contribute higher levels of nitrate or phosphate ions. Treated

wastewater may also have higher TDS readings than surrounding streams if urban drinking water

has been highly chlorinated. Irrigation water that is returned to a stream will often have higher

concentrations of sodium or chloride ions. Acidic rainwater, with dissolved gases like CO2, NO2,

or SO2, often yields elevated H

+

ion concentrations.

Sources of Total Dissolved Solids

• Hard-Water Ions

- Ca

2+

- Mg

2+

- HCO3

–

• Fertilizer in agricultural runoff

- NH4



+

- NO3

–

- PO4

3 –

- SO4

2 –

• Urban runoff

- Na

+

- Cl

–

• Salinity from tidal mixing, minerals,

or returned irrigation water

- Na

+

- K

+

- Cl

–

• Acidic rainfall

- H

+

- NO3

–

- SO3

2 –



, SO4

2 –Test 12

12 - 2 Water Quality with Computers

TDS

If TDS levels are high, especially due to dissolved salts, many forms of aquatic life are affected.

The salts act to dehydrate the skin of animals. High concentrations of dissolved solids can add a

laxative effect to water or cause the water to have an unpleasant mineral taste. It is also possible

for dissolved ions to affect the pH of a body of water, which in turn may influence the health of

aquatic species. If high TDS readings are due to hard-water ions, then soaps may be less

effective, or significant boiler plating may occur in heating pipes.

Expected Levels

TDS values in lakes and streams are typically found to be in the range of 50 to 250 mg/L. In

areas of especially hard water or high salinity, TDS values may be as high as 500 mg/L.

Drinking water will tend to be 25 to 500 mg/L TDS. United States Drinking Water Standards1

include a recommendation that TDS in drinking water should not exceed 500 mg/L TDS. Fresh

distilled water, by comparison, will usually have a conductivity of 0.5 to 1.5 mg/L TDS.

Table 1: TDS in Selected Rivers

Site Season TDS

(mg/L)

Season TDS

(mg/L)

Rio Grande River, El Paso, TX Spring 510 Fall 610

Mississippi River, Memphis, TN Spring 133 Fall 220

Sacramento River, Keswick, CA Spring 71 Fall 60

Ohio River, Benwood, WV Spring 300 Fall 143

Hudson River, Poughkeepsie, NY Spring 90 Fall 119

Summary of Methods



Method 1: TDS Using a Conductivity Probe

A Vernier Conductivity Probe is used on site, or placed into samples

collected at sites, to measure TDS concentration of the solution. It offers

the advantage that it can be performed without filtration, providing

instantaneous feedback about total dissolved solids concentration in a

stream.

Method 2: TDS By Evaporation

Using this method, samples are first filtered to remove suspended solids. A precise amount of

sample is added to a carefully cleaned, dried, and weighed beaker. The water is then evaporated

in a drying oven. The difference in mass between the two weighings is the mass of the total

dissolved solids. Calculations are then performed to convert the change in mass to mg/L of TDS.

This procedure does not require a sensor, but does require an analytical balance (0.001 or

0.0001-g resolution).

1

Established by 1986 Amendments to the Safe Drinking Water ActTotal Dissolved Solids


TDS

Method 1: TDS USING A CONDUCTIVITY PROBE

Materials Checklist

___ laptop computer (Power Mac or Windows) ___ wash bottle with distilled water

___ Vernier computer interface, battery-powered ___ tissues or paper towels

___ Logger Pro ___ small paper or plastic cup (optional)

___ Vernier Conductivity Probe ___ 50 mg/L TDS standard solution (optional)

___ 500 mg/L TDS standard solution

Collection and Storage of Samples

1. This test can be conducted on site or in the lab. A 100-mL water sample is required.



2. It is important to obtain the water sample from below the surface of the water as far away

from shore as is safe. If suitable areas of the stream appear to be unreachable, samplers

consisting of a rod and container can be constructed for collection. Refer to page Intro-4 of

the Introduction of this book for more details.

3. If the testing cannot be conducted within a few hours, place the samples in an ice chest or a

refrigerator.

Testing Procedure

1. Position the computer safely away from the water. Keep water away from the computer at all

times.

2. Prepare the computer for data collection by opening

“Test 12 Total Dissolved Solids” from the Water

Quality with Computers experiment files of Logger

Pro. On the Graph window, the vertical axis has

concentration scaled from 0 to 1000 mg/L TDS. The

horizontal axis has time scaled from

0 to 10 seconds. There is also a Meter window

which displays live TDS concentration readings.

3. Prepare the Conductivity Probe for data collection.

a. Plug the Conductivity Probe into Port 1 or

Channel 1 of the Vernier computer interface.

b. Set the switch on the probe box to the 0-2000 µS

range (2000 µS = 1000 mg/L TDS).

4. You are now ready to prepare the computer and the Conductivity Probe for calibration.

• If your instructor directs you to use the calibration stored in the experiment file, then

proceed to Step 5.

• If your instructor directs you to perform a new calibration for the Conductivity Probe,

follow this procedure:Test 12




TDS

First Calibration Point

a. Choose Calibrate from the Experiment menu and then click .

b. Perform the first calibration point with the probe in the air (e.g., out of any solution).

c. Type “0” in the edit box.

d. When the displayed voltage reading for Input 1 stabilizes, click Keep .

Second Calibration Point

e. Place the Conductivity Probe into the 500 mg/L TDS standard solution. The hole near the

tip of the probe should be covered completely.

f. Type “500” (the concentration in mg/L TDS) in the edit box.

g. When the displayed voltage reading for Input 1 stabilizes, click Keep , then click OK .

5. You are now ready to collect TDS concentration data.

a. Rinse the probe tip with distilled water.

b. Place the tip of the probe into the stream, or into a cup with sample water from the stream.

The hole near the tip of the probe should be completely covered.

c. If the TDS value appears stable, simply record it on the Data & Calculations sheet and

proceed to Step 7.

6. If the TDS value displayed in the Meter

window is fluctuating, determine the

mean (or average) value. To do this:

a. Click Collect

to begin a 10-second

sampling run. Important: Leave the

probe tip submerged for the 10

seconds that data is being collected.

b. Click on the Statistics button, , to



display the statistics box on the

graph.

c. Record the mean TDS value on the

Data & Calculations sheet.

7. Return to Step 5 to obtain a second

reading.Total Dissolved Solids


TDS

DATA & CALCULATIONS


Stream or lake: ___________________________ Time of day: __________________________

Site name: ______________________________ Student name: ________________________

Site number: ____________________________ Student name: ________________________

Date: __________________________________ Student name: ________________________

Column A

Reading TDS

(mg/L)

1

2

Average

Column Procedure:

A. Record the TDS value (in mg/L) from the computer.

Field Observations (e.g., weather, geography, vegetation along stream)

_________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

Test Completed: ________________ Date: ______Test 12




TDS

Method 2: TDS BY EVAPORATION

Materials Checklist

___ sampling bottles ___ drying oven

___ one 600-mL beaker for filtration container ___ 100-mL graduated cylinder

___ large funnel (>10 cm diameter) ___ two 250-mL beakers

___ filter paper to fit large funnel ___ milligram balance (0.001 g)

___ tongs or gloves to hold beaker

Collection and Storage of Samples

1. This test can be conducted on site or in the lab. Collect a 500-mL water sample per site so

that you can run two 200-mL trials. Note: If your stream or lake could have low levels of

TDS, then collect a larger sample volume (see Step 6 of the Testing Procedure).

2. It is important to obtain the water sample from below the surface of the water as far away

from shore as is safe. If suitable areas of the stream appear to be unreachable, samplers

consisting of a rod and container can be constructed for collection. Refer to page Intro-4 of

the Introduction of this book for more details.

3. If samples cannot be tested immediately upon returning to the lab, they should be refrigerated

until the time of analysis to avoid microbiological decomposition of solids. Samples should

not be tested after seven days.

Testing Procedure

Day 1

1. Filter any solid particles or suspended solids from your sample.

a. Set up a funnel and funnel support on a ring stand. Place a 600-mL beaker or other large

container below the funnel.

b. Place a folded piece of filter paper in the funnel and moisten it with distilled water so that

it adheres to the funnel sides.



c. Slowly add your 500-mL sample to the funnel, being sure not to let the level of liquid in

the funnel go above the top of the filter paper. Continue adding your sample to the funnel

until you have more than 400 mL of filtrate in the beaker below the funnel.

2. Prepare two 250-mL beakers for drying and sample evaporation.

a. Carefully clean two 250-mL beakers and place them in a 100-

105°C drying oven for one hour to dry.

b. Remove the beakers from the oven. Allow them to cool.

c. Using a pencil, number your beakers “1” and “2”. Do not use

label tape.

d. From this point on, always handle the beakers with tongs or

gloves to prevent the oils on your hands from affecting their

mass. Weigh each beaker on a milligram balance to the nearest

0.001 g. Record the data on the Data & Calculations sheet.

e. If you complete Step 2 before collecting samples, leave the

beakers in a clean, dry, dust-free space until you return to the lab.Total Dissolved Solids


TDS

3. Transfer the samples to the beakers.

a. Using a 100-mL graduated cylinder, carefully measure 200.0 mL of filtered sample water

into each beaker.

b. Place the remaining sample water into a refrigerator for possible future use.

4. Using tongs or gloves, place the beakers into the oven and allow the water to evaporate

overnight at 104°C.

Day 2

5. Measure the mass of the beakers and solids.

a. Using tongs or gloves, remove the

beakers from the oven and place them in



a dessicator, if available, to cool. A

dessicator will keep the samples from

absorbing any water from the air that

would increase their mass. If no

dessicator is available, the beakers can

be cooled on a table top. Proceed to the

next step as soon as possible to

minimize any absorption of water.

b. Use an analytical balance to measure the

mass of each beaker with the solids now

left behind. Record the values on the

Data & Calculations sheet (round to the

nearest 0.001 g).

c. Obtain the mass of the solids by

subtracting the mass of the empty

beaker from the mass of the beaker with

the solids. If the mass of the solids is at least 0.025 g, proceed to Step 7. If the mass of the

solids is less than 0.025 g, proceed to Step 6.

6. If the mass of the solids is less than 0.025 g, add another 200.0 mL of sample to each beaker

and repeat Steps 4 and 5. Make a note on the Data & Calculations sheet that your total

volume is now 400.0 mL instead of 200.0 mL.

7. Record the mass of each beaker plus the solids on the Data & Calculations sheet.

8. Soak the beakers in hot soapy water.

Cool sample in a

dessicator, if available.Test 12


TDS



DATA & CALCULATIONS

Method 2: TDS by Evaporation

Stream or lake: __________________________ Time of day: _________________________

Site name: _____________________________ Student name: _______________________

Site number: ____________________________ Student name: _______________________

Date: _________________________________ Student name: _______________________

Column A B C D E F

Beaker

Number

Mass of

empty beaker

(g)

Mass of beaker

plus solids

(g)

Mass of solids

(g)

Mass of

solids

(mg)

Total volume

(L)

TDS

(mg/L)

Example 95.245 g 95.277 g 0.032 g 32 mg 0.200 L 160 mg/L

1

2



Average TDS

(mg/L)

Column Procedure:

A. Mass of empty beaker

B. Mass of beaker with dried solids

C. Mass of solids (g) = B – A

D. Mass of solids (mg) = C5 1000

E. Total volume (L) = mL water / 1000

F. TDS = D / E

Field Observations (e.g., weather, geography, vegetation along stream) ________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

Test Completed: ________________ Date: ______Total Dissolved Solids


TDS

ADDITIONAL INFORMATION

Tips for Instructors


1. There is a nearly linear relationship between conductivity and

total dissolved solid concentration (of dissolved ionic

substances). A curve similar to the one shown here can be

obtained using standard TDS solutions. In the figure shown

here, the ratio of TDS concentration in mg/L to conductivity

in µS/cm is 0.5 to 1, and represents the approximate

relationship between TDS concentration as sodium chloride

and conductivity.



2. Step 4 of the student procedure provides several alternatives for loading or performing a TDS

calibration:

• The easiest option is to use the TDS calibration that is stored with the Logger Pro

experiment file, “Test 12 Total Dissolved Solids.” The stored calibration corresponds to

your range setting on the switch box (0 –2000 µS/cm, corresponding to 0 –1000 mg/L

TDS). This calibration measures mg/L TDS as NaCl; therefore, it is based on a sodium

chloride standard, and does not take into account the variation in ion composition of

different streams. If your purpose in measuring TDS is to monitor a stream for changes in

total ion concentration, this calibration will work very well. If you want to determine a

precise TDS concentration value at one location, either of the next two options will provide

better results.

• The stored TDS calibrations the Logger Pro experiment file, “Test 12 Total Dissolved

Solids,” make use of the equation, TDS = 0.50 5 Conductivity (in µS/cm)—a relationship

based on sodium chloride calibrations. Most freshwater streams, however, have higher

concentrations of hard-water ions (Ca

2+

and HCO3

–

) than salt ions (Na

+

and Cl

–

). Large

variations in ionic composition of streams result in the 0.50 “constant” actually ranging

from 0.50 to 0.90. An average value of 0.70 is often used in freshwater studies

TDS = 0.70 5 Conductivity (in µS/cm)

If you are making measurements in a freshwater (non-brackish) samples, improved TDS



data can usually be obtained if you use a calibration that reflects the 0.70 value. Simply

open “Test 12 Total Dissolved Solids,” then choose Calibrate from the Experiment menu.

Click on the Details tab. In the Details dialog box, you will see the values for the intercept

and slope values displayed for the TDS calibration. Click on the Unlock button. You can

now manually enter the intercept and slope values for a calibration based on the constant,

0.70. The values to be manually entered are

2

Intercept = 0 Slope = 593

• This calibration will give TDS readings (in mg/L TDS) over a range of 0 -1400 mg/L when

using the 0-2000 µS/cm switch setting of the Conductivity Probe.

3

To save the calibration

values, choose Save As from the File menu and resave the file as “Test 12 Total Dissolved

Solids”. When the file is reopened, the new calibration values stored with the file will be

used during data collection.

2

These calibration values were obtained by multiplying the calibration for the 0-2000 µS/cm

conductivity

calibration by the constant, 0.70: Intercept = 0 5 0.70 = 0, Slope = 847.2 5 0.70 = 593.

3

If you are using the low-range switch setting (0-200 µS/cm) for samples with low TDS values, you can

manually

enter a calibration corresponding to the 0.70 constant: Intercept value = 0, Slope = 46.0

conductivity

1000 mg/L

2000 µS

TDS concentrationTest 12




TDS

• A third option has students perform a two-point calibration. To obtain a standard that

reflects the unique ionic composition of your particular stream, you can collect a 1-liter

water sample, and determine the TDS concentration of 500 mL of the sample using

Method 2 of Test 12: TDS by Evaporation. After finding its concentration, you can now

use the remaining 500 mL of the water sample as a standard solution. To do this, choose

Calibrate from the Experiment menu. For the first calibration point, simply hold the probe

in the air (out of solution), and enter a value of “0” (mg/L TDS). For the second calibration

point, place the Conductivity Probe into the water sample, and enter the value you obtained

using Method 2 (e.g., 142 mg/L TDS).

3. Having accurate standard solutions is essential for performing good calibrations. The Sodium

Chloride Calibration Solution that was included with your Conductivity Probe (500 mg/L

TDS) can last you a long time if you take care not to contaminate it with a wet or unrinsed

probe. You should perform and save a calibration with your probe while the solution is new

and uncontaminated. To prepare your own standard solutions using solid NaCl, use a

container with accurate volume markings (e.g., volumetric flask) and add the amount of solid

shown in the first column of Table 2. This standard can be used to calibrate Conductivity

Probe for Test 12, using the amount shown in mg/L as TDS (second column).

Table 2

Add this amount of NaCl to

make 1 liter of solution

TDS and Conductivity values equivalent to

the NaCl concentration in the first column

Total dissolved solids (TDS) Conductivity

0.0474 g (47.4 mg/L) 50 mg/L as TDS 100 µS/cm

0.491 g (491 mg/L) 500 mg/L as TDS 1000 µS/cm



1.005 g (1005 mg/L) 1000 mg/L as TDS 2000 µS/cm

5.566 g (5566 mg/L) 5000 mg/L as TDS 10,000 µS/cm

4. Flinn Scientific (P.O. Box 21, Batavia, IL 60510, Tel: 800-452-1261, www.flinnsci.com)

sells a set of four standard solutions in 500-mL bottles. The concentrations correspond to the

four solutions shown in Table 2. Here is the ordering information:

− Conductivity Calibration Kit with four 500-mL bottles (50 mg/L, 500 mg/L, 1000 mg/L,

and 5000 mg/L TDS, order code AP 9111)

5. Your Vernier Conductivity Probe is automatically temperature compensated between

temperatures of 5 and 35°C. Readings are automatically referenced to a conductivity value at

25°C. The Conductivity Probe will, therefore, give the same conductivity reading in a

solution that is at 15°C as it would if the same solution were warmed to 25°C. This means

you can calibrate your probe in the lab, and then use these stored calibrations to take readings

in colder (or warmer) water in a lake or stream. If the probe was not temperature

compensated, you would notice a change in the conductivity reading as temperature changed,

even though the actual ion concentration did not change.

Method 2: TDS by Evaporation

6. A larger sample size should reduce the percent error in this test. The problem with a very

large sample is that most analytical balances do not have the capacity to weigh beakers larger

than about 250 mL. You could use a larger beaker if your balance has the capacity to do so.

Alternatively, as stated in Step 5 of the procedure, you can add a second sample of water to

the beaker. If the water has very low TDS, you could continue adding daily samples of water

until the mass change is in the 0.025-g range. You would need to know this ahead of time,Total

Dissolved Solids


TDS

however, to ensure that you initially collect a large enough water sample. If you use this

approach, make sure you keep the sample in the refrigerator to cut down on any microbial

action that may affect the results. Change your calculations on the Data & Calculations sheet



accordingly.

7. If you do not have a milligram balance, satisfactory results can usually be obtained by using a

centigram balance and a larger sample size. Use the largest beaker that can be weighed on

your balance and increase the sample size accordingly. You could add another sample of

water on Day 2, as described in Step 5 of the Testing Procedure.

8. If you are concerned about the water boiling and splattering out of the beaker in the oven,

you can dry the sample for a few hours at around 80°C so that it won’t boil. Then, when the

water level has gone down sufficiently, turn the oven up to 104°C and finish the drying

process.

9. If you have trouble cleaning the dried residue out of the beaker, try swirling a little 1 M

hydrochloric acid, HCl, in the bottom. Make sure you dispose of the acid properly.

10. Some substances, such as calcium, magnesium, chloride, and sulfate, may attract water.

These types of substances are hygroscopic. If you have a sample containing high levels of

hygroscopic substances, you should dry the samples for a longer period of time and weigh

them as soon as possible after removing them from the oven. Dessicate the samples while

they cool, if at all possible.

11. Do not turn up the heat in the oven to evaporate the water at a faster rate. A higher oven

temperature will volatilize some of the organics and cause some heat-induced chemical

decomposition. This will give a false reading for total solids.

12. If you will be testing water with unusually high levels of dissolved ions (brackish water or

very hard water), students may encounter readings above the upper limit of the 1000-mg/L

TDS range used in this test. Have them use the highest switch setting on the Conductivity

Probe (0 –20,000 µS/cm), and load the calibration for 0 –10,000 mg/L TDS. If readings

exceed 10,000 mg/L, follow the directions in the next section.

Sampling in Ocean Salt Water or Tidal Estuaries

Salt-water samples may exceed the high range of the Conductivity Probe, 0 to 20,000 µS/cm

(0 to 10,000 mg/L TDS). Seawater from the mid-Atlantic ocean has a conductivity value of



53,000 µS/cm (or a TDS concentration of about 26,500 mg/L). Samples in this range will need to

be diluted in order for them to be measured using the high range. For example, you can take a

sample of ocean water, and dilute it to _ of its original concentration by adding 100 mL of the

salt-water sample to 300 mL of distilled water. This diluted sample can then be measured using

the Conductivity Probe at the high-range setting. If the TDS value for the diluted sample is

measured to be 6,600 mg/L, then this answer is multiplied by a factor of 4 to obtain the TDS

value of the original sample: 456,600 = 26,400 mg/L TDS.

How the Conductivity Probe Works

The Vernier Conductivity Probe measures the ability of a solution to conduct an electric current

between two electrodes. In solution, the current flows by ion transport; therefore, an increasing

concentration of ions in the solution will result in higher conductivity values.Test 12


TDS

The Conductivity Probe is actually measuring conductance, defined as the reciprocal of

resistance. When resistance is measured in ohms, conductance is measured using the SI unit,

siemens (formerly known as a mho). Aqueous samples are commonly measured in microsiemens,

or µS.

Even though the Conductivity Probe is measuring conductance, we are often interested in finding

the conductivity of a solution. Conductivity, C, is found using the following formula

C = G• kc

where G is the conductance, and kc is the cell constant. The cell constant is determined for a

probe using the following formula

kc = d/A

where d is the distance between the two electrodes, and A is the area

of the electrode surface. For example, the cell in the figure shown

here has a cell constant of

kc = d/A = 1.0 cm/ 1.0 cm2



= 1.0 cm-1

The conductivity value is found by multiplying conductance and the

cell constant. Since the Vernier Conductivity Probe also has a cell constant of 1.0 cm-1

, its

conductivity and conductance have the same numerical value. For a solution with a conductance

value of 1000 µS, the conductivity, C, would be

C = G• kc = (1000 µS)5(1.0 cm-1

) = 1000 µS/cm

A potential difference is applied to the two probe electrodes in the Conductivity Probe. The

resulting current is proportional to the conductivity or TDS value of the solution. This current is

converted into a voltage to be read by Logger Pro.

Alternating current is supplied to prevent

the complete ion migration to the two

electrodes. As shown in the figure below,

with each cycle of the alternating current,

the polarity of the electrodes is reversed,

which in turn reverses the direction of ion

flow. This very important feature of the

Conductivity Probe prevents most

electrolysis and polarization from

occurring at the electrodes. Thus, the

solutions that are being measured for

conductivity are not fouled. It also greatly

reduces redox products from forming on

the relatively inert graphite electrodes.

The Vernier Conductivity Probe has three sensitivity range settings.

• 0 to 200 µS/cm (0 to 100 mg/L TDS)



• 0 to 2000 µS/cm (0 to 1000 mg/L TDS)

• 0 to 20,000 µS/cm (0 to 10,000 mg/L TDS)

These ranges are selected using a toggle switch on the end of the amplifier box attached to the

probe. It is very important to consider this setting when loading or performing a calibration; no

single calibration can be used for all three settings.

1 cm

1 cm

d = 1 cm

graphite electrodes

Epoxy body

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