ATI Technologies B12-70 Manuale O & M - Pagina 15

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Model B12-70 Wet Acid Gas Transmitter

Transmitter calibration requires adjustment of both zero and span. Zero is adjusted when the

sensor is exposed to air containing no acid gases. Span is adjusted when a gas stream containing a

known concentration of acid gas is passed through the low volume flowcell.

As previously mentioned, adjusting the transmitter zero requires that the sensor be exposed to

acid gas free air. The simplest approach is to simply expose the sensor to ambient air that is free of

gases to which the sensor will respond. Threading the low volume flowcell onto the sensor will isolate

the sensor from surrounding air for zeroing purposes.

If the area in which the sensor is operating is known to be gas free, then the transmitter can be

zeroed without further equipment. If not, use of "zero air" from a gas cylinder is recommended. Zero air

is available as part of all ATI calibration kits, or may be obtained from any specialty gas supplier. When

zero air is to be used, a low volume flowcell (part no. 00-0422) must be used. The low volume flowcell

provides a confined space around the sensor into which the zero air can flow. Low volume flowcells

provide tube fittings at the bottom to connect air tubing as shown in Figure 2.

To zero the transmitter, remove the cover and connect a DVM to the test points shown in Figure

6. Observe the DVM value to be sure that it is no longer declining, and that it is stable ± 4 mv. Use the

zero potentiometer (Figure 6) to adjust the test point voltage to 0.040 VDC (40 mv.). If the transmitter is

supplied with the LCD option, no DVM is required.

Once the zero has been set, thread the low volume flowcell onto the sensor (if it is not already

there from zeroing). Attach a source of gas with a know acid gas concentration and adjust the sample

flowrate to 500 cc/min. For convenience, chlorine gas may be used as a simulant for calibration. A typical

chlorine gas source is the electrochemical chlorine generator manufactured by Advanced Calibration

Designs in Tucson, AZ. Allow sample to flow for 10 minutes.

Observe the DVM value. The reading should be relatively stable ±4 mv. Adjust the span

potentiometer (Figure 6) until the proper test point voltage is obtained. The proper voltage to set when

adjusting transmitter span will vary depending on the range of the transmitter. The following formula is

used to calculate the proper span voltage for any transmitter.

V = 0.040 V + [0.160 X (Measured Concentration ÷ Transmitter Range)]

As an example, suppose you are calibrating a transmitter with a range of 0-10 PPM, and the

chlorine gas standard used for calibration has a value of 5 PPM. The above formula would then become:

Therefore, adjusting the transmitter to a reading of 120 mv. at the test point would properly

calibrate the transmitter.

O & M Manual

-Release E, 7/15

CALIBRATION

V = 0.040 V + [0.160 X (5 ÷ 10)] = 0.120 V.

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