The particular TDS/EC meter shown allows a person to take measurements both in TDS ppm (NaCl) mode as well as EC µS mode.
In addition it measures the temperature of hydroponic nutrient solution, or other liquids, in degrees fahrenheit or degrees celsius.
There are those who do not calibrate this type of meter. However, after a year of use this one was off by close to 10%. It is now calibrated every 6-12 months.
The meter is inexpensive and, when calibrated properly, similar in accuracy when compared to more expensive meters.
It will last for years if taken care of. The meter in the images on this page is 5 years old.
To get to the calibration set screw, you have to use something like a slotted precision screwdriver to get under the plastic decal faceplate, and pry it up.
You can calibrate the meter while the plastic faceplate decal is removed. Then after the calibration is complete, the decal can be stuck back in its original location.
For the person who owns the meter, to make calibration quicker and easier, a small hole (about 3/16 inch or 5 mm) was drilled in the plastic faceplate decal when it was removed, directly over the calibration screw.
The 3/16 inch or 5 mm hole shown was made with a drill press. So as not to damage the plastic decal faceplate the hole was started small and made progressively larger.
1/16" - 1/8" - 5/32" - 3/16"
or 1.5 mm - 3.0 mm - 4.0 mm - 5.0 mm
This is not a waterproof meter but it is sealed enough so that if it accidentally dropped into nutrient solution or other liquid and removed and dried off immediately, there is a good chance it will be fine.
By making a hole for calibrating, if the meter is dropped into liquid, the hole will let water in and most likely ruin the meter.
For the owner of the meter it is worth the risk. A piece of waterproof tape is put over the hole to offer some protection.
Calibration Of The Meter
To calibrate you will need:
--- A meter like this.
--- TDS calibration solution 342 ppm (NaCl) .
--- A 3/64 inch (1.8 mm) slotted screwdriver, like one from a precision screwdriver set like this or this.
--- An extremely clean container that is just large enough to accommodate the lower 5/8 inch (16 mm) of the meter, so the meter probes are fully submerged in calibration solution.
Take cap off meter and press the on/off button. The meter will turn on, TDS in ppm and temperature in degrees celsius will be displayed.
Put meter in calibration solution and wait for the reading to stabilize. Keep calibration solution temperature between 24.5-25.5°C or 76-78°F.
Adjust the calibration screw with a 3/64 inch or 1.8 mm slotted screwdriver so the display shows 342 ppm.
That is it, no need to calibrate the µS reading because it is calibrated at the same time you calibrate the ppm reading.
1 --- Calibrate the meter every 6-12 months. More often if you wish to.
2 --- Discard the calibration solution after calibrating. Once poured out, never pour calibration solution back in the container or you will contaminate the entire batch.
3 --- After using the meter, rinse the probes off with distilled or reverse osmosis water. Then shake off any excess water and let the probes dry. When the probes are dry, put the cap on the meter.
4 --- Do the calibrating with calibration solution that is as close to 77°F/25°C as possible or it could throw the measurements off. Make sure the built in thermometer reads between 24.5-25.5°C or 76-78°F when calibrating.
5 --- When calibrating try to get the meter to read somewhere between 340-344 ppm if it is too difficult to get a steady 342 ppm reading. This and even more expensive meters will generally have a tolerance of about +/- 5%.
6 --- If the container the calibration solution is poured into isn't extremely clean it will increase the ppm of the solution and cause inaccurate readings. Wash it thoroughly with distilled or reverse osmosis water, and let it air dry completely, mouth facing down in a dish rack or something, prior to calibrating.
7 --- Calibration set screw tolerance may vary, so an inexpensive precision screwdriver set with various size slotted screwdrivers is a good idea. If the 3/64 inch or 1.8 mm slotted screwdriver doesn't fit you can try the next larger or smaller size, until you find one that works.
8 --- There are 3 buttons on the meter.
1) The top button, marked on/off, turns the meter on or off.
2) The middle button, marked shift, allows you to shift between modes.
3) The bottom button, marked hold, when pressed will retain the meter reading.
9 --- The meter has 4 modes to take readings:
1) TDS in PPM, temp in C° (default).
2) EC in µS, temp in C° (press shift once).
3) TDS in PPM, temp in F° (press shift twice).
4) EC in µS, temp in F° (press shift thrice).