Calibrating the Cardy Potassium Meter

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Calibrating the Cardy isn't complicated. It's basically wash and rinse, wash and rinse.  

To clean the sensor you can use distilled water, which is a lot less expensive than buying refills of the "de-ionized" water that comes with the kit. And don't use their testing tape to clean the sensor. Buy Kleenex brand facial tissues in those little pocket packs. They are potassium-free, and work just fine.

To calibrate the meter first:

1. Wash your hands and dry them on a clean towel or paper towel.
2. Remember that your breath contains potassium ions, so do not breathe on the sensor plate.
3. The air also contains potassium ions, so when you test the meter will stabilize at a reading only briefly -  a few seconds - before it starts to rise again. It's this first stable reading you use.

"Calibrating" it is essentially washing the sensor (using distilled water) until it has no potassium ions on it (i.e. registers 0). To do this drop one or two drops of distilled water onto the sensor, then dab it off with a clean dry tissue. You may have to repeat this several times, being careful to avoid breathing on the sensor while you're doing this.

Once you have the number down to 0 you drip a couple of drops of the 20 ppm "standard" solution on the sensor.  This solution contains 20 ppm (parts per million) K+ on it,  then you turn the small dial at the top until the readout registers 20 x 100. This will take some fiddling.

Once you have the 20 x 100 reading you wash the sensor plate again, until it reads 0, and dry it off. Add the 15 ppm "slope" solution and if needed adjust the little screw on the faceplate until it reads 15 x 10. (Lift the little rubber stopper on the face plate to adjust the screw.)

Once it reads 15 x 10 you wash the sensor off with deionized water until it reads 0. You dry it and it's calibrated. 

If I am calibrating the meter and plan to use it to test immediately I've found that once I get the Cardy calibrated it helps to lay a piece of clean tissue over the sensor and drip a single drop of water on it, so it adheres to the sensor plate. This keeps potassium ions from falling on the sensor. Before I test I dab off this tiny bit of moisture.  If I am just calibrating the meter to put it away I skip this step. 

They send you everything you need to do this except something to clean the sensor with. I use Kleenex brand tissues which come in the small wrapped purse packs to dry the sensor. They are ion-free. I just keep moving to a dry area on the tissue.

You need calibrate the meter this way only once a month. It takes about five minutes once you've done it a few times, but you really need to do it when you are feeling well.

They suggest doing a "short" calibration on a daily basis, but most of the people who use the Cardy use it dozens of times a day in greenhouse operations. I do a short calibration every fifth or sixth time I use it. To do the short calibration you clean the sensor until it registers 0, dry it, then put a couple of drops of the 20 ppm solution on it. If it registers 18 or 22 or any number except 20 you turn the small dial until the readout says 20 x 100. Then you wash it with deionized water again until it says 0. Then it's ready to use.

When I do a test I simply wash the sensor with deionized water until it reads 0, dry it and test. I do not even do the two-step calibration first. I have tested it in the ER using this "quick" method without any loss of accuracy.

The Cardy comes with a little roll of deionized tape. To do the test you cut off a 1/2" long piece of tape and lay it on your tongue, using the tweezers provided. Don't touch it with your hands. Once the tape is wet with saliva you take it off your tongue with the tweezers and lay it on the sensor. In one or two seconds the readout will stabilize. Note the number, and whether it's x 1, x 10, or x 100. Write it down and use our on-line calculator if you are at the computer, or 

Saliva has a different concentration of K+ ions than blood serum, but it reflects the level of blood serum very accurately. I have used mine in the ER alongside blood draws, and my readings were much finer than the blood draws. They read a bit lower, because blood samples always hemolize to a certain extent. The ER staff and EMTs are always fascinated by my Cardy and immediately say they are going to see if they can't get one.

The Cardy is an excellent tool and is not difficult to use, once you get over your initial nervousness.