Emerson Process Management 396PVP Section Start UP and Calibration, ORP Calibration

MODEL 396P and 396PVP	SECTION 5.0
	START UP AND CALIBRATION

SECTION 5.0

START UP AND CALIBRATION

5.1 MODELS 396P and 396PVP pH SENSORS

5.1.1SENSOR PREPARATION. Shake down the sensor to remove any air bubbles that may be present at the tip of the pH glass bulb. In most cases, the pH sensor can simply be installed as shipped and readings with an accuracy of ± 0.6 pH may be obtained. To obtain greater accuracy or to verify proper operation, the sensor must be calibrated as a loop with its compatible analyzer or transmitter.

5.1.2pH CALIBRATION. After a temporary connection is established between the sensor and the instrument, a buffer calibration may be performed. Consult appropriate pH/ORP analyzer or transmitter instruction manual for specific calibra- tion and standardization procedures, or see below for recom- mended two-point buffer calibration procedure.

Recommended two-point buffer calibration procedure:

Select two stable buffer solutions, preferably pH 4.0 and 10.0 (pH buffers other than pH 4.0 and pH 10.0 can be used as long as the pH values are at least two pH units apart).

NOTE

A pH 7.0 buffer solution reads a mV value of approxi- mately zero, and pH buffers read approximately 59.1 mV for each pH unit above or below pH 7.0. Check the pH buffer manufacturer specifications for millivolt values at various temperatures since it may affect the actual value of the buffer solution mV/pH value.

1.Immerse sensor in the first buffer solution. Allow sensor to adjust to the buffer temperature (to avoid errors due to temperature differences between the buffer solution and sensor temperature) and wait for reading to stabilize. Value of buffer can now be acknowledged by analyzer/transmitter.

2.Once the first buffer has been acknowledged by the ana- lyzer/transmitter, rinse the buffer solution off of the sensor with distilled or deionized water.

3.Repeat steps 1 and 2 using the second buffer solution.

4.Once the analyzer/transmitter has acknowledged both buffer solutions, a sensor slope (mV/pH) is established (the slope value can be found within the analyzer/ transmitter).

5.The slope value should read about 59.1 mV/pH for a new sensor and will decrease over time to approximately 47- 49 mV/pH. Once the slope reads below the 47-49 mV/pH range, a new sensor should be installed to maintain accu- rate readings.

Recommended pH Sensor Standardization:

For maximum accuracy, the sensor can be standardized online or with a process grab sample after a buffer calibration has been performed and the sensor has been conditioned to the process. Standardization accounts for the sensor junction potential and other interferences. Standardization will not change the sensor’s slope but will simply adjust the analyzer’s reading to match that of the known process pH.

1.While obtaining a process solution sample (it is recommended that the sample is taken close to the sensor), record the pH value that is shown on the analyzer/transmitter display.

2.Measure and record the pH of the process solution sample with another temperature compensated, calibrated pH instrument. For best results, standardization should be performed at the process temperature.

3.Adjust the analyzer/transmitter value to the standardized value.

5.2 MODEL 396P and 396PVP ORP SENSORS

5.2.1SENSOR PREPARATION. Most industrial applications have a number of ORP reactions occurring in sequence or simultaneously. There can be several components that are oxidized or reduced by the reagents that are used. Theoretically, the ORP potential is absolute because it is the result of the oxidation-reduction equilibri- um. However, the actual measured potential is dependent on many factors, including the condition of the surface of the ORP platinum electrode. Therefore, the sensor should be allowed 1-2 hours to become “conditioned” to the stream when first set-up or after being cleaned.

5.2.2ORP CALIBRATION

1.Make a temporary electrical connection between the sensor and the instrument.

2.Obtain an ORP standard solution, or a standard solution can also be made quite simply by adding a few crystals of quinhydrone to either pH 4 or pH 7 buffer. Quinhydrone is only slightly soluble therefore a few crystals will be required. (Refer to Section 4.3. for an alternate ORP standard solution).

3.Immerse the sensor in the standard solution. Allow 1-2 minutes for the ORP sensor to stabilize.

4.Adjust the standardize control of the instrument to the solution value shown in Table 5-1 (below) or on the label of the standard solution. The resulting potentials, measured with a clean platinum electrode and saturated KCl/AgCl reference electrode, should be within ±20 millivolts of the value. Solution temperature must be noted to ensure accurate interpretation of results. The ORP value of saturated quin- hydrone solution is not stable over long periods of time. Therefore, these standards should be made up fresh each time they are used.

5.Remove the sensor from the buffer, rinse and install in the process.

TABLE 5-1. ORP of Saturated Quinhydrone

Solution (In Millivolts)

	pH 4 Solution			pH 7 Solution
Temp °C	20	25	30	20	25	30
Millivolt Potential	268	264	260	94	87	80

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