33% humidity calibration standard used to check and calibrate the accuracy of compatible REED humidity meters, including the R6001, SD-3007, 8706, R6200, R9900, R9905, and R6050SD. Rated to about +/-1% accuracy at 77F (25C) and fits probes up to 0.6 in (15mm) in diameter, housed in a durable molded case with a foam insert.