Instructions

UM-0085-B09 DT80 Range User Manual Page 65

Part E – Manipulating Data

Scaling

Most DT80 channel types automatically scale measured values so that the returned values are in appropriate

engineering units. For example, the thermocouple channel types (e.g.

TK) automatically apply the appropriate scaling

polynomial so that the data is returned in °C. However, a number of additional facilities are provided for applying custom

scaling or corrections:

• channel factor

• spans

• polynomials

• thermistor scaling

• intrinsic functions

• expressions

Channel Factor

For many channel types, the channel factor (a channel option consisting of just a floating point number) can be used to

provide a simple multiplication factor. See A Special Channel Option — Channel Factor (P42).

For example, if a high voltage is being measured using an external 12.5:1 voltage divider then the following channel

definition:

1V(12.5)

will multiply the raw reading by 12.5 so that the returned value reflects the actual voltage.

Note: for some channel types the channel factor performs a special function, and therefore cannot be used as a scaling factor. In these

cases a span should be used (see below). For example, if you are measuring a frequency which has passed through a 100:1 prescaler

then you will need to use a span to scale it.

Spans (Sn)

A span transforms a measured signal value (e.g. mV) into the corresponding physical value (e.g. kPa) using a straight

line function:

Figure 17: Span coordinates

A span must be defined before it is applied. This is normally done at the start of a job, before any schedules are defined.

The syntax is as follows:

Sn=a,b,c,d"units"

where:

• n is the poly/span number (1 to 50), which is used simply to distinguish one span from another. Note that a span

and a polynomial cannot have the same number.

• a and b are the physical coordinates of two points on the calibration line (reported or output values)

• c and d are the signal coordinates of two points on the calibration line (measured or input values). If not

specified, 0 and 100 are assumed.

• units replaces the channel's default units text

Spans are particularly suited to 4–20mA current loop inputs. The following defines a span suitable for a current loop

sensor that measures pressure in the range 0-300kPa:

S2=0,300"kPa"