INST 202 Word Problems

Question 1

Range 40° - 140° F
Output signal 4 - 20 mA
Calculate the current output by this transmitter if the measured temperature is 60° For

Answer 1

First we convert the temperature value of 60° into a percentage of range based on the knowledge of the temperature krange span (140° - 40° = 100°) and LRV = 40°

Measured Variable = (span) (P/100%) + LRV
Measured Variable - LRV = (span) (P/100%)
P = (Measured Variable - LRV)/ span x 100%

= (60 - 40)/ 100% = 20%

Next we take this % value and translate it into a 4-20 mA current value using the formula

Current = (16mA) (P/100%) + 4 mA
= (16mA) (20%/100%) + 4 mA
= 7.2 mA

Therefore the transmitter should output a process value signal of 7.2 mA at 60° F

Question 2

An electronic loop controller outputs a signal of 8mA to a direct responding control valve (where 4mA is shut and 20mA is wide open). How far open should the control valve be at this manipulated variable signal level?

Answer 2

We must convert the 8mA signal value into a percentage of value travel. This means determining the percentage value of the 8mA signal on the 4-20mA range. First we need to manipulate the percentage mA formula to solve for percentage (P):

Current = (16mA) (P/100%) + 4mA
P/100% = (Current - 4mA)/16mA
P= [(Current - 4mA)/(16mA)] x 100%

Next we plug in the 8mA signal value and solve for P

Therefore P = [(8mA-4mA)/(16mA)] x 100%
= 25%

Therefore the control valve should be 25% open when the MV signal is at a valve of 8mA

Question 3

An electronic transmitter with an output of 4-20mA is calibrated for a pressure range of 7-10 mPA (g). What pressure is represented by a 12mA signal?

Answer 3

Span of transmitter 20mA-4mA=16mA

Fraction of measurement change
= Output Signal - Live Zero/signal span
= (12mA-4mA)/16mA
= 0.5

Actual Process Change
= (Fractional Change) x (Process Span)
= 0.5 x (10-7mPA)
= 1.5 mPA

Actual Process Value
= Base Point + Process Change
= 7 + 1.5 mPA
= 8.5 mPA (g)

Question 4

A standard electronic transmitter is used to monitor the water temperature in a vessel. The transmitter is calibrated such that it responds to the temperature range of 50°C - 100°C. What is the transmitter output when the water temperature is 85°C?

Answer 4

15.2 mA