function calculateValueFromMA() {
var min = parseFloat(document.getElementById('pvMin').value);
var max = parseFloat(document.getElementById('pvMax').value);
var ma = parseFloat(document.getElementById('inputMA').value);
var resultDiv = document.getElementById('calcResult');
if (isNaN(min) || isNaN(max) || isNaN(ma)) {
resultDiv.innerHTML = "Please enter valid numeric values.";
resultDiv.style.display = "block";
resultDiv.style.color = "#d9534f";
return;
}
var range = max – min;
var percentage = (ma – 4) / 16;
var val = min + (range * percentage);
var pctDisplay = (percentage * 100).toFixed(2);
resultDiv.style.display = "block";
resultDiv.style.color = "#333";
resultDiv.innerHTML = "Process Value: " + val.toFixed(4) + "Signal Level: " + pctDisplay + "%";
}
function calculateMAFromValue() {
var min = parseFloat(document.getElementById('pvMin').value);
var max = parseFloat(document.getElementById('pvMax').value);
var val = parseFloat(document.getElementById('inputPV').value);
var resultDiv = document.getElementById('calcResult');
if (isNaN(min) || isNaN(max) || isNaN(val)) {
resultDiv.innerHTML = "Please enter valid numeric values.";
resultDiv.style.display = "block";
resultDiv.style.color = "#d9534f";
return;
}
var range = max – min;
if (range === 0) {
resultDiv.innerHTML = "Range cannot be zero.";
resultDiv.style.display = "block";
return;
}
var percentage = (val – min) / range;
var ma = 4 + (16 * percentage);
var pctDisplay = (percentage * 100).toFixed(2);
resultDiv.style.display = "block";
resultDiv.style.color = "#333";
resultDiv.innerHTML = "Loop Current: " + ma.toFixed(4) + " mASignal Level: " + pctDisplay + "%";
}
Understanding the 4-20 mA Current Loop
In industrial instrumentation and control systems, the 4-20 mA current loop is the prevailing standard for transmitting sensor information. Unlike voltage signals, current signals are immune to electrical noise over long distances and are not affected by voltage drops caused by the resistance of the wiring.
Why 4 mA and not 0 mA?
The "Live Zero" (4 mA) allows the system to distinguish between a zero-level process measurement and a dead wire or sensor failure. If the signal drops to 0 mA, the PLC or controller immediately knows there is a "Loop Break" or hardware fault.
Mathematical Formulas Used
To calculate the process value ($PV$) from a current ($I$):
PV = PV_min + ((I – 4) / 16) * (PV_max – PV_min)
To calculate the current ($I$) from a process value ($PV$):
I = 4 + (16 * (PV – PV_min) / (PV_max – PV_min))
Calculation Example
Imagine a pressure transmitter calibrated from 0 to 150 psi. If your multimeter reads 12 mA, what is the pressure?
Step 1: Calculate the percentage of the signal. (12mA – 4mA) / 16mA = 0.5 (or 50%).
Step 2: Apply the percentage to the span. 0.5 * (150 – 0) = 75.
Step 3: Add to the minimum value. 75 + 0 = 75 psi.
Common Applications
This calculator is essential for technicians and engineers working with:
Pressure and Level Transmitters
Temperature Sensors (RTDs and Thermocouples with Transmitters)