.calc-section { background: #fff; padding: 20px; border-radius: 6px; margin-bottom: 20px; box-shadow: 0 2px 4px rgba(0,0,0,0.05); }
.calc-title { color: #2c3e50; font-size: 22px; margin-bottom: 20px; border-bottom: 2px solid #3498db; padding-bottom: 10px; font-weight: bold; }
.input-group { margin-bottom: 15px; }
.input-group label { display: block; margin-bottom: 5px; font-weight: 600; color: #444; }
.input-group input, .input-group select { width: 100%; padding: 10px; border: 1px solid #ccc; border-radius: 4px; box-sizing: border-box; font-size: 16px; }
.calc-btn { background-color: #3498db; color: white; padding: 12px 20px; border: none; border-radius: 4px; cursor: pointer; width: 100%; font-size: 16px; font-weight: bold; transition: background 0.3s; }
.calc-btn:hover { background-color: #2980b9; }
.result-box { margin-top: 20px; padding: 15px; background-color: #e8f4fd; border-left: 5px solid #3498db; display: none; }
.result-title { font-weight: bold; color: #2c3e50; margin-bottom: 5px; }
.result-value { font-size: 24px; color: #2980b9; font-weight: bold; }
.formula-box { font-size: 0.9em; color: #666; font-style: italic; margin-top: 10px; }
.flex-row { display: flex; gap: 15px; }
.flex-row .input-group { flex: 1; }
@media (max-width: 600px) { .flex-row { flex-direction: column; gap: 0; } }
function toggleInputs() {
var type = document.getElementById("conv_type").value;
document.getElementById("ma_input_group").style.display = (type === "ma_to_val") ? "block" : "none";
document.getElementById("val_input_group").style.display = (type === "val_to_ma") ? "block" : "none";
document.getElementById("perc_input_group").style.display = (type === "perc_to_ma") ? "block" : "none";
}
function calculateSignal() {
var lrv = parseFloat(document.getElementById("lrv").value);
var urv = parseFloat(document.getElementById("urv").value);
var units = document.getElementById("unit_label").value;
var type = document.getElementById("conv_type").value;
var resBox = document.getElementById("calc_result");
var resVal = document.getElementById("res_val");
var resTitle = document.getElementById("res_title");
var resForm = document.getElementById("res_formula");
if (isNaN(lrv) || isNaN(urv)) {
alert("Please enter valid range values.");
return;
}
var span = urv – lrv;
var result = 0;
var formulaStr = "";
if (type === "ma_to_val") {
var ma = parseFloat(document.getElementById("input_ma").value);
if (isNaN(ma)) { alert("Enter valid mA value"); return; }
result = ((ma – 4) / 16) * span + lrv;
resTitle.innerHTML = "Calculated Process Value:";
resVal.innerHTML = result.toFixed(4) + " " + units;
formulaStr = "Formula: ((" + ma + "mA – 4) / 16) * " + span + " + " + lrv;
} else if (type === "val_to_ma") {
var val = parseFloat(document.getElementById("input_val").value);
if (isNaN(val)) { alert("Enter valid process value"); return; }
result = ((val – lrv) / span) * 16 + 4;
resTitle.innerHTML = "Calculated Current (mA):";
resVal.innerHTML = result.toFixed(4) + " mA";
formulaStr = "Formula: ((" + val + " – " + lrv + ") / " + span + ") * 16 + 4";
} else if (type === "perc_to_ma") {
var perc = parseFloat(document.getElementById("input_perc").value);
if (isNaN(perc)) { alert("Enter valid percentage"); return; }
result = (perc / 100) * 16 + 4;
resTitle.innerHTML = "Calculated Current (mA):";
resVal.innerHTML = result.toFixed(4) + " mA";
formulaStr = "Formula: (" + perc + "% / 100) * 16 + 4";
}
resForm.innerHTML = formulaStr;
resBox.style.display = "block";
}
4-20mA Signal Scaling Calculator
Current (mA) to Process Value
Process Value to Current (mA)
Percentage (%) to Current (mA)
Result:
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How to Calculate 4-20mA Scaling
In industrial automation and process control, the 4-20mA current loop is the dominant standard for transmitting sensor data. Understanding how to scale these signals into real-world "Engineering Units" (like pressure, temperature, or flow) is essential for PLC programming and instrument calibration.
The Linear Scaling Formula
Because the 4-20mA signal is linear, we use the equation for a straight line (y = mx + b). The standard calculation for converting a current signal to a process value is:
Value = ((mA – 4) / 16) * (URV – LRV) + LRV
Why 4mA and not 0mA?
The "Live Zero" at 4mA is used for safety and diagnostics. If a wire breaks, the current drops to 0mA. Because the transmitter is calibrated to a minimum of 4mA, the controller (PLC/DCS) can immediately distinguish between a process at 0% (4mA) and a broken wire/failed sensor (0mA).
Practical Examples
- Pressure Transmitter: If a sensor is ranged 0 to 100 PSI, a 12mA signal represents 50 PSI (exactly mid-span).
- Temperature Sensor: For a range of -50°C to 150°C, a 20mA signal indicates 150°C, while a 4mA signal indicates -50°C.
- Valve Position: A control valve receiving 12mA from a PLC is typically commanded to be 50% open.
Quick Reference Table
| Percentage | Current (mA) |
|---|---|
| 0% | 4.0 mA |
| 25% | 8.0 mA |
| 50% | 12.0 mA |
| 75% | 16.0 mA |
| 100% | 20.0 mA |