Emerson Process Management 53eA Temp coeff, Operate iso 7.00 pH Sensor iso 7.00 pH

MODEL 54eA	SECTION 5.0
	SOFTWARE CONFIGURATION

5.8 CONFIGURING THE pH MEASUREMENT (continued)

pH diagnostics pH calibration Temperature coeff

ExitEnter

Temp coeff : 0.000

Operate iso: 7.00 pH

Sensor iso: 7.00 pH

ExitEnter

Stabilize pH and Stabilize time: For the controller to accept cal- ibration data, the pH must remain within a specified range for a specified period of time. The default values are 0.01 pH for 10 sec- onds. Using a small pH value and a large time provides the best protection against calibration while the reading is still changing.

8.In many industries pH is used and reported at a reference tempera- ture of 25°C. Correcting a pH reading to 25°C is often referred to as a solution temperature correction. To perform the correction, the solu- tion temperature coefficient must be known.

a. With the cursor on "Temperature coeff", press Enter (F4).

b.Use the é and ê keys to move through the list of items. To make a change press Edit (F4). Use the arrow keys to change the set- ting to the desired value and press Save (F4). For allowed ranges see Table 5-1.

Temp coeff: Enter the temperature coefficient in units of ΔpH/°C. If the pH decreases as temperature increases, the temperature coefficient is negative. The temperature coefficient must usually be determined empirically.

APPLICATION WARNING

If pH is being measured for the purposes of correct- ing a free chlorine reading, do NOT use solution tem- perature correction. Keep the temperature coeffi- cient at 0.000 pH/°C.

Operate iso: Entering a temperature coefficient ALWAYS causes the operating isopotential to change from the normal value of

7.00.When programming the controller to perform solution tem- perature compensation, ALWAYS enter the solution temperature coefficient and allow the transmitter to calculate the operating isopotential pH.

Sensor iso: The isopotential pH is the pH at which the cell volt- age is independent of temperature. Most sensors have an isopo- tential pH fairly close to 7.0, so the default value is 7.00. Certain specialized pH electrodes have isopotential pH significantly dif- ferent from 7.0.

NOTE

Do NOT change the isopotential pH of the con- troller unless you are thoroughly familiar with the role of isopotential points in pH measurements OR the sensor operating instructions specifically state the isopotential pH is a value other than pH 7.

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