A calculation becomes useful when it answers a decision. Follow these three jobs from the first measurement to the next check.
01
A pressure display stops at 8 bar
The transmitter covers 0–10 bar over 4–20 mA. The PLC display appears to plateau near the top. Do not change the scaling just to make the display climb.
Predict the signal first. At 8 bar, the expected current is 16.8 mA. Use reverse scaling to prepare the expected values before touching the loop.
2
Check the electrical limit. A 20 V supply, a 12 V transmitter minimum and 470 Ω total resistance leave −1.4 V headroom at 20 mA. Under this simple model the maximum sustainable current is about 17.02 mA, close to the observed plateau.
3
Confirm with measurements. Measure the voltage at the transmitter near the failing point, check the actual minimum-voltage specification, and identify every series burden. Correct the supply or burden within the equipment ratings; then repeat the five-point scaling check.
A correct scaling equation cannot recover a current the electrical loop cannot deliver.
A conveyor has a 1,500 rpm motor, a 30:1 reducer and a 100 mm roller. An encoder mounted on the roller reports 1,000 channel cycles per revolution. The counter is configured for x4.
Follow the mechanics. The roller turns at 50 rpm and moves about 15.708 metres of belt per minute, assuming no slip. Its travel per revolution is π × 100 ≈ 314.159 mm.
2
Use the encoder’s shaft, not the motor’s. Enter 314.159265 mm per encoder revolution, 1,000 cycles and x4. The resolution is about 0.07854 mm per count. A 500 mm move needs about 6,366.198 counts, so the program must choose and document a rounding rule.
3
Make one measured move. If the error is nearly a factor of four, check whether the scale used pulses while the counter reports decoded counts. If the error grows or changes with load, inspect slip, coupling and working diameter instead of tuning a magic correction factor.
The travel scale belongs to the shaft where the encoder is installed.
The manual lists a float at holding reference 400101 using the conventional six-digit scheme. A known test value is 1.0. The returned register bytes are 00 00 3F 80.
Remove address ambiguity. The reference translates to wire offset 100 (00 64), read function 03. A two-register request to unit 1 starts with 01 03 00 64 00 02. Calculate its CRC only after those bytes are correct.
2
Check the data layout separately. Decode 00 00 3F 80 using CDAB and the stored float is 1.0. ABCD produces a tiny subnormal value. A valid CRC would accept either payload; it cannot choose the interpretation for you.
3
Confirm the register map and repeat at another known value. Keep the documented offset, type and word order together in the tag configuration so the next person does not have to rediscover them.
Address selection, transport integrity and data interpretation are three separate checks.