RTU Scale Factor

The scale factor in this case has nothing to do with the input signal. It refers to the ratio between a stored digital value and the engineering units that value represents.

The RTU uses a 2-step process to convert the transduced (0-1 mA) input to the engineering units that operators can use. The first step samples the input signal and converts it to a digital value, then stores that value in a register. The scale factor of the analog-to-digital conversion is fixed by the size of the register. You should be able to find the register size in the RTU manual, but a typical RTU storage register has 1 bit for sign (+/-) and 11 bits for magnitude. This equates to 2^11 = 2048 in base 10. To allow for a zero value, the register span is +/-0 to 2047. The size of the register is based upon the chipset and overall design of the RTU, and is not adjustable.

The second step is to convert the digital value to the desired engineering units. Depending on the RTU's purpose, the 0-1 mA input may represent amperes, volts, flow rate, pressure, or any other parameter which can be measured by a transducer. An adjustable scale factor, representing the ratio between the desired engineering span and the register value, is provided to allow the end user to change the ratio and provide the proper indication to the operator.

The scale factor in this case is 400/2047, or 0.195407914. The programming software usually only allows scale factors of 3 to 4 digits, so the actual value is rounded to 0.195.

You can change the scale factor depending upon what value you want the 0-1 mA input signal to represent. For example, if you wanted to measure the voltage of a bus, you might use a transducer which produces 0-1 mADC for a 0-5000 volts. The RTU would still convert the 0-1 mA input signal to +/- 0-2047 for digital storage. However, that 2047 would now represent an engineering span of 5000 volts. Thus the scale factor would now be 5000/2047, or 2.443.

Thank you for the explanation, I do understand it well.