Relationship between Input and Output Ranges
In Table 2 and Figure 31 we see the
relationship between input range and output range for a transmitter. The graph of Figure 31 shows that the relationship
is linear.
Input Range
100  500ÂșC

%

Output Range
3 – 15 psi

Output Range
4 – 20 mA

100

0

3

4

200

25

6

8

300

50

9

12

400

75

12

16

500

100

15

20

Table 2
Refer to Figure 32 as you follow
the examples. This figure shows a
temperature control loop
Figure
32 A Temperature Control Loop
Figure
32 A Temperature Control Loop
What if the temperature was 250ÂșC? How can we find out what the output should be?
First, find the temperature as a
percentage of spans.
Use the following equation for the
calculation of percentage input or output span values.
Therefore:
Next find the value of output signal for
a 37.5% input value. Remember that a
transmitter is linear. A 37.5% input
produces a 37.5% output.
To convert a percentage to an actual
output signal value, first calculate the percentage span and then add the zero
value using the equation below.
Therefore, to find 37.5% of 3 to 15psi:
Or in the case of a 4 to 20mA signal
system, we have:
If the output signal is not 7.5 psi in
the case of a pneumatic transmitter or 10mA for an electronic transmitter; then
the transmitter needs calibrating.
Let’s convert from a known output value
to a process variable value for the same temperature transmitter. We know the transmitter range is 100ÂșC 
500ÂșC.
A pressure gauge on the output indicates
10 psi for a pressure transmitter and 13.3 mA for an electronic transmitter.
What should the input temperature be?
Find 10 psi as a percentage of spans.
Next find 58.33% of 500ÂșC  100ÂșC
The actual temperature should be
333.32ÂșC at the input. If it is not,
then the transmitter requires calibration.
Let’s look at another example with a
pneumatic transmitter. The transmitter
has a range of 0 to 600 psi. The actual
pressure is 450 psi. What should the
value of the output signal be?
First find 450 psi as a percentage of
span.
Next find 75% of 3 to 15 psi
If the output signal is not 12 psi, the
transmitter requires calibration.
When working with an electronic
transmitter the Span in the equation becomes
16 mA and the Live Zero is 4 mA.
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