## Handy Formula for Transmitter / Calibration (x = %Span + MNR)

This is a Handy Formula for Transmitter to Control Valve Signal [Current to Pneumatic Converter (I/P)] or when you do Calibration, that I learned back when I am still studying at the Meralco Foundation Institute (MFI), and I find it easy & useful when dealing such cases like below. Hopefully, this will also help others to do the computation a lot faster and easier.

Below is just a sample temperature loop diagram and we'll be applying this to our "Handy Formula", and with that note kindly examine below diagram.

Below is just a sample temperature loop diagram and we'll be applying this to our "Handy Formula", and with that note kindly examine below diagram.

Probably, for the most of the training's and seminars that we have attended, below table is the commonly used illustration to show the relationship of Transmitter Milliampere (4 - 20 mA) output to Current to Pneumatic Converter (I/P or sometimes E/P), and below table is very simple and easy to understand.

But what if we are required to calculate the

**Transmitter mA output , I/P output and provide the percentage opening of the Control Valve**based only to the actual process measurement value. See below table.Actually, this can be solve by means of interpolation, but we can use below formula to make our calculation simple:

where:

Observe below diagram:

**x = % Span + MNR**where:

**x**= Given value / Measured value / Available value**%**= Percentage**Span**= Maximum Range - Minimum Range**MNR**= Minimum RangeObserve below diagram:

Solution:

using

where:

We will look first the Percentage Value of 64.5 Deg. C to its temperature range of 0 - 100 Deg. C based on the above table.

where:

using

64.5 = % (100 - 0) + 0

64.5 - 0 = % 100

64.5 = % 100

64.5 / 100 = %

We are now going to use this percentage value to compute for the actual transmitter mA output, range is 4 - 20 mA.

using

where:

x = 0.645 (20 - 4) + 4

x = 0.645 (16) + 4

Now, to compute for the Current to Pneumatic Output (3 - 15 psig), we will still be using the Percentage Value of the above temperature measurement (64.5 Deg. C).

using

where:

x = 0.645 (15 - 3) + 3

x = 0.645 (12) + 3

Lastly, to compute for the Percentage Valve Opening, just copy or refer to the Percentage Value of the above temperature measurement (64.5 Deg. C) since range is also from 0 to 100 Deg. C. This is due to the fact that once the actual temperature measurement varies, the Control Valve opening will also varies from 0% (fully close) to 100% (fully open).

% = 0.645 x 100

% = 64.5

Therefore, the Valve Opening for the above temperature measurement is

using

**x = % Span + MNR**where:

**x = 64.5**We will look first the Percentage Value of 64.5 Deg. C to its temperature range of 0 - 100 Deg. C based on the above table.

where:

**Span = Maximum Temperature - Minimum Temperature (based on the above Temperature Range from table)****MNR = 0**using

**x = % Span + MNR**64.5 = % (100 - 0) + 0

64.5 - 0 = % 100

64.5 = % 100

64.5 / 100 = %

**0.645 = %**We are now going to use this percentage value to compute for the actual transmitter mA output, range is 4 - 20 mA.

using

**x = % Span + MNR**where:

**x = actual mA transmitter output****% = 0.645****Span = Maximum mA - Minimum mA (based on the above mA Range from table)****MNR = 4**x = 0.645 (20 - 4) + 4

x = 0.645 (16) + 4

**x = 14.32 mA**is the Transmitter and Controller output going to our I/P or E/P converterNow, to compute for the Current to Pneumatic Output (3 - 15 psig), we will still be using the Percentage Value of the above temperature measurement (64.5 Deg. C).

using

**x = % Span + MNR**where:

**x = actual psig of Current to Pneumatic output****% = 0.645****Span = Maximum psig - Minimum psig (based on the above psig Range from table)****MNR = 3**x = 0.645 (15 - 3) + 3

x = 0.645 (12) + 3

**x = 10.74 psig**is the Current to Pneumatic output going to our Control Valve or Final ElementLastly, to compute for the Percentage Valve Opening, just copy or refer to the Percentage Value of the above temperature measurement (64.5 Deg. C) since range is also from 0 to 100 Deg. C. This is due to the fact that once the actual temperature measurement varies, the Control Valve opening will also varies from 0% (fully close) to 100% (fully open).

% = 0.645 x 100

% = 64.5

Therefore, the Valve Opening for the above temperature measurement is

**64.5 %**.The above formula is not only limited to Temperature but also applicable to all other process variables (Pressure, Flow and Level) and other application similar to this one.

You will appreciate more of this formula for complex ranges like 30 - 985 deg C or 0 to 25 deg C and just a reminder, practice more to master this technique.

You will appreciate more of this formula for complex ranges like 30 - 985 deg C or 0 to 25 deg C and just a reminder, practice more to master this technique.

Date Published: March 8, 2019

Source: Leo - MFI Alumni - Batch 23

Source: Leo - MFI Alumni - Batch 23