Emerson Rosemount 385+ -02, Reference Manual, Page: 23 / 42

3.1 Startup
To obtain best accuracy, the sensor must be calibrated as a loop with the transmitter. Please refer
to the transmitter manual for proper calibration and setup procedures. Example of 1056 start-up
shown below.
3.2 pH Calibration Using Buffer Solutions or Grab
Samples
The loop may be calibrated with the sensor’s measuring tip submersed in standard pH buffer
solutions (two point calibration) or with a process grab sample of a known pH value (one point
standardization). Please refer to the corresponding transmitter manuals for proper procedures.
3.3 ORP Calibration
An ORP loop is best calibrated using an ORP standard solution.
3.3.1 Quinhydrone Solution
A commonly used ORP standard solution is a saturated quinhydrone solution. This can be made by
simply adding a few quinhydrone crystals to either a 4 pH or a 7 pH buffer. Quinhydrone is only
sightly soluble so only a few crystals will be required. The solution will have a yellow color. The
resulting potentials should be within ±20 millivolts of the value shown in Table 3-1. The ORP value
of saturated quinhydrone solution is not stable over long periods of time and therefore new
solutions should be made each time they are used.
Section 3: Startup and Calibration
pH 4 pH 7
Temperature °C 20 25 30 20 25 30
Millivolt Potential 268 264 260 94 87 80
Table 3-1: ORP of Saturated Quinhydrone Solution
Reference Manual Startup and Calibration
00809-0100-3386 February 2019
Startup and Calibration 17