Orion’s Portable Meters are compact, battery operated and microprocessor controlled for all measurement needs. All meters feature a custom digital LCD display, which simultaneously displays temperature along with measurement results. Ideal for field, plant, or laboratory use, each meter is lightweight and designed to fit comfortably in the hand. A flip stand allows easy use in the lab. The Orion 210Aplus is a basic pH meter that features 2 point autocalibration. The Orion 230Aplus is a pH meter that features autocalibration and automatic temperature compensation, millivolt, relative millivolt and (EH) ORP mode. The Orion 250Aplus has all the features of the 230Aplus plus 3 point auto calibration and RS232-C output for use with the Orion 900A printer or other serial peripherals. The Orion 290Aplus adds concentration measurements and an internal datalogging function to make a truly versatile meter for pH or ISE analysis. This manual contains instructions for all four meters. See the calibration and measurement section for details on your particular meter. The general information section contains descriptions of hardware which pertain to all meters. The Temperature Compensation and Troubleshooting sections contain information applicable to all meters.