Calibration, Care and
TDS & EC Calibration
- HM Digital meters come
factory calibrated (at 342 PPM or 1413 µS, depending on the
product) and are ready to use out of the box. They are designed to stay
consistent. However, after prolonged usage, it may help to recalibrate
your meter using a commercial standard NaCl-based solution (for most HM
Digital meters), which is approximately, 0.5 µS of conductivity.
- Immerse the meter into the
calibration solution. If the meter does not read within 2% of the
calibration solution, adjust the reading by either digital calibration
(for some products) or inserting a mini screwdriver (not included) into
the trimmer pot (the hole on the back of the meter). Turn the trimmer
clockwise to increase the reading and counterclockwise to decrease the
reading. Note that the adjuster is very sensitive. Always refere to
the calibration instructions for your particular product.
Note: HM Digital products are
calibrated with a 342 PPM NaCl or 1413 µS solution. This is suitable for
most applications. However, if you are measuring samples that are consistently
over 1000 PPM, it is recommended to recalibrate the meter for that specific
application. TDS meters are more accurate when calibrated at levels that are as
close as possible to the sample being tested (such as for hydroponics,
aquaculture, tinting and dying or brackish water). HM Digital sells NaCl calibration solution at 342 PPM
and 1000 PPM. Calibration solutions at signifcantly higher levels are
- Do not drop or completely
submerge the unit in water or dip beyond the maximum immersion level. This
unit is not watertight and is not covered under the warranty if water gets
into the unit. (NOTE - The COM-100, PH-200 and ORP-200 are watertight and
completely submersible. Please ensure that the battery compartment and
probe gasket ring are firmly tightened before submersing in water.)
- Do not store the unit in
high temperature or direct sunlight. This will shorten the lifespan of the
- After repeated usage in high
TDS water, it is advised to clean the electrodes to prevent residue
- When necessary, clean the
electrodes by soaking the tip in an acid (e.g., vinegar or diluted
hydrochloric acid (muriatic acid)) and then rinsing well in water. If it
is heavily fouled with organic material, soaking the tip in alcohol or
bleach may help. Gentle wiping with a soft, nonabrasive cloth may also be
- For best results, always
rinse the sensor pins in distilled water and allow to air dry before
replacing the cap. TDS electrodes should be stored dry. (NOTE:
pH and ORP electrodes must be stored in a special storage solution.)
- The batteries may need to be
replaced after extended usage or lifespan. To change the batteries, remove
the top compartment of the meter (on the opposite end of the sensor for
handhelds, on the rear of the unit for monitors) and insert new batteries
according to the polarity of the diagram inside.
ADDITIONAL INFORMATION ABOUT CALIBRATING TDS/EC
METERS & TESTERS
TDS meter can be calibrated to any brand of calibration solution.
issues are 1) what the calibration solution composition is; and 2) what the
value of that calibration solution is.
calibration solution composition should match the conversion factor of the
meter. HM Digital's TDS-3, for example, uses the NaCl conversion factor,
so a NaCl calibration solution should be used. HM Digital's COM-100 uses
one of three selectable conversion factors (NaCl, 442 or KCl), so any of these
three types of calibration solutions can be used for the COM-100.
be noted that calibration compositions have equivalent values in alternate
compositions. Better brands of calibration solutions print the equivalent
values directly on the label or in the instructions. For example, 1060
ppm KCl = 1000 ppm NaCl. Therefore, if you have a 1060 ppm KCl
calibration solution, but a meter with the NaCl conversion factor, such as the
TDS-3, you can still calibrate the TDS-3 with the KCl calibration
solution. In other words, if putting the TDS-3 into a 1060 ppm KCl
solution, the meter should read 1000 ppm.
meters, there is no convesion factor, so the points above are not relevant.
Some brands do not print the calibration solution composition on the
label. Though NaCl is the most common, it is always recommended to
contact the calibration solution manufacturer for verification of the
All of HM
Digital meters can be calibrated to any point within the range of the
meter. For example, the TDS-4 has a range of 0-9990 ppm. Therefore,
it can be calibrated to any point within that range. The COM-100 has
varying ranges depending upon the conversion factor that is selected. On
the TDS side, these ranges are from 0-5000 ppm to 0-8560 ppm. On the EC
side, the COM-100's range is 0-9990 µS. Therefore, the COM-100 can
be calibrated to any value within the selected range.
brands of TDS/EC meters can only be calibrated to specific values that are
pre-programmed into the meter. HM Digital does not do this.
of HM Digital's TDS/EC Calibration Solutions
- Laboratory quality
- Solutions list alternate
composition equivalents on the bottle labels.
- Bottles are a convenient 3
oz. (88 ml) size - neither too big nor too small.
- Bottles have a convenient
wide mouth, allowing a user to simply insert the meter into a stable
bottle (much easier to use than a wobbly Mylar pouch).
Notes about TDS/EC Meters and Calibration
results, a meter should be calibrated as close as possible to what will be
tested. For example, if the water you are testing is always around 150
ppm, then the meter should be calibrated as close to 150 ppm as possible.
are testing a range of values, the meter should be calibrated to the upper
third of that range. For example, if you are testing RO water and tap
water, and the RO water is approximately 0-10 ppm and the tap water is
typically 400-500 ppm, the meter should be calibrated to approximately 350 ppm.
- The average tap water in the
U.S. is approximately 350 ppm (though this varies widely from location to
location and can be anywhere from zero to greater than
1000 ppm). Therefore, HM Digital factory calibrates our meters
to 342 ppm.
- All HM Digital meters are
factory calibrated. Therefore, calibration is not necessary out of
the box. Typically, with proper care and normal usage, HM Digital
TDS and EC meters will retain their calibration for at least a year.
- Only calibrate a meter if
you are sure you need to. As a general rule of thumb, if you are
unsure if you need to calibrate, you probably don't. It is always
safer not to calibrate than to attempt doing so.
- Though calibrating correctly
will improve results, for normal usage and ranges, the difference will not
- Never calibrate a meter to
- Never calibrate a meter to
- Never calibrate a meter to
tap water, or any water or solution that is not certified.
- HM Digital calibration
solutions have a shelf life of approximately 1 year unopened.
- Once opened, HM Digital
calibration solutions have a shelf life of approximately 6 months, if
properly stored and not contaminated.
- All calibration solutions
should be stored in a cool place, out of the heat and sun.
- Contamination can occur if
dipping a meter in water or a second calibration solution with a different
value, and then dipping that meter back into the first calibration
solution. The droplets from the water and other calibration solution
will change the value. If testing between water and a
calibration solution, always let the meter air dry or spray it with a compressed
air can to avoid contamination.