How do you set up Analog Scaling?
To proceed with Alarm Configuration when you are using channels 9 and 10 for analog values, click on the Analog Scaling tab.
This page is used to specify analog scaling information for channels 9 and 10 when they are configured for 4-20 ma analog as opposed to digital input monitoring. For Channels configured as analog, it is necessary to specify how the values will be translated into final readings, as well as in relation to analog set point entries that you may make on the Alarm Configuration page.
Scaling settings established on this page affect the range of possible entries for analog alarm set points on the Alarm Configuration page, even if your chosen Service Level and/or Application Template selection does not provide for actual analog readings to be delivered.
THERE ARE FIVE SCALING METHOD CHOICES:
The first three choices are simplest to enter, because they are pre-rolled. The last two choices offer more powerful custom flexibility in translating the input signal values to corresponding physical values.
1. Percentage (no units): Readings and set point entries will range from 0% at 4 ma input, to 100% at 20 ma input.
2. Raw Counts: Readings and set point entries will range from a count of 205 at 4 ma input, to 1023 at 20 ma input. These counts are the output of Analog to Digital Converters, with the counts representing the analog value at the input.
3. Milliamps: Readings and set point entries will directly reflect the number of milliamps input (4 ma to 20 ma)
4. Gain/Offset Scaling Specification Method: This scaling method can be used if you already know the specific gain/offset relationship between the input signal value translated into a count range of 205 to 1,023 (based upon a total count range of 0 to 1,023 for the 10-bit A/D conversion) and the corresponding physical values. Given that there is a raw count range of 205 to 1023, choosing Gain/Offset allows you to enter scaling and offset values which will apply to this range of counts. For example, if you enter a gain factor of 2 and an offset of -100, then a raw count of 205 would give a reading of 205 x 2 - 100 = 310. A Raw count of 1023 would give a reading of 1023 x 2 - 100 = 1,946.
5. Two-Point Scaling Specification Method: When the gain/offset scaling parameters are not already independently known, one practical way to establish the proper scaling is to visit the RTU site and observe the correlation between the observed physical quantity being measured (such as tank level) and the analog raw count reading at that same time.
4-20 milliamps is used extensively throughout the process control industry. When this type of transducer is used, the two-point scaling factor should be selected. Click on the "Custom Scaling Method: Two Point" radio button. From the pulldown, select "milliamps". Next the "real-world" reading should be entered for the "units measured" textboxes for Point 1 and Point 2. When the input signal is at the low end of the scale, enter the physical reading for Point 1. When the input signal is at the high end of the scale, enter the physical reading for Point 2. To identify the type of physical reading, enter the monitored reading type, such as water level in feet, volts, or psi, in the Message 2 textbox of the corresponding analog channel, found on the Alarm Config tab page.
To determine the value of raw counts, navigate to the Analog Scaling tab for the RTU to be configured. Set the scaling method for the channel to be configured to "Raw Counts" and click the Save button.