Why it matters
Scaling questions ask you to turn a milliamp reading into a process value, or the reverse. For technicians in Alberta and BC plants, it is daily work and a sure exam topic.
Key points
- 4 mA is 0% of range and 20 mA is 100%. The span is 16 mA.
- Percent = (mA - 4) / 16 x 100.
- Process value = lower range + percent x span of the range.
- A live zero at 4 mA lets you tell a dead loop (0 mA) from a zero reading.
- A 250 ohm resistor turns 4-20 mA into 1-5 V.
Study tip
Always subtract the 4 mA live zero first. Forgetting it is the most common mistake.
Try these
12 mA on a 4-20 mA loop is what percent?
- 25%
- 50%
- 60%
- 75%
Show answer
B. 50% (12 - 4) / 16 = 50%.
A 0-200 C transmitter reads 8 mA. Temperature is:
- 40 C
- 50 C
- 80 C
- 100 C
Show answer
B. 50 C 25% of 200 = 50 C.
0 mA on a 4-20 mA loop most likely means:
- Zero process value
- Open loop
- Over range
- Normal
Show answer
B. Open loop Below 4 mA signals a fault.
Keep going
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