While you can use any constant read meter to test the solution or waste strength, this meter is inexpensive and pretty durable. It has lab quality electrodes and they are easy to calibrate. The large back lit display is really nice. I use these exclusively for my clients.
Excerpts for their literature:
The 24–7 Nutrient Monitor™ from CWP Instruments™ is a great all around monitor for nutrient applications needing constant monitoring. It measures PPM, pH and Temperature. Its large illuminated display allows easy reading from a distance. Calibration screws are conveniently located on the front for easy access. CWP Instruments™ has priced the 24-7 Nutrient Monitor™ to have the lowest MSRP of any quality constant read meter currently on the market. This low price will allow more first time growers to afford the quality and no hassle features of a constant read meter. Along with the meter, the box includes 1382 PPM, pH 4.0, and pH 7.0 calibration solutions to get you going quick and easy.