What is meant by Scaling of Control values and how is it done?

 

Nag,

Scaling is when you change the control system input signal to equal the signal seen by the field transmitter. For example, you have a transmitter in the field measuring flow from 0 - 3,000 gpm. The transmitter sends a signal of 4mA at 0 flow and 20 mA at 3,000 gpm (12mA at 1,500 gpm).

All the control system sees is a signal that varies from 4 - 20 mA so you need to "Scale" the signal back again. How it's done depends on the control system, but for example in a crude system you would subtract 4mA to give a span of 16mA then multiply the remainder by 3000/16 (Example: 12ma - 4 = 8, 8 x 3,000/16 = 1,500)

Usually this task is done by the control system. When you assign the inputs to the field devices, a form will ask what 4mA is equivalent to and what 20 mA is equal to and that's it. 4 - 20mA is often referred to as 0 - 100%.

In the control system you might see something other than 4-20mA, for example an early Modicon would convert the 4-20 signal with a 12 bit A-D converter into a count range of 0 - 4095 counts, but the idea is the same.

Roy