Thermocouple loop uncertainty budget
Combine the five contributors that actually set a thermocouple loop's uncertainty: wire tolerance, cold-junction error, extension-junction error, drift and shield noise: into one realistic figure and one worst case.
Budget
Enter the five contributions to see the combined loop uncertainty. Leave a term blank only if it genuinely does not apply to your loop.
Why the budget matters more than the wire class
The dominance table above usually settles an argument in one line: in most loops one contributor carries the majority of the variance, and spending on the others buys almost nothing. Tightening the wire class while the cold junction drifts, or calibrating more often while interference dominates the signal, is effort in the wrong place. Budget first, then buy.
Frequently Asked Questions (FAQ)
Why is loop uncertainty larger than the wire tolerance alone?
Because the wire is only one contributor. The total is the sum of wire tolerance, cold-junction error, extension-junction error, drift, and shield noise, which is why a loop quoting ±1.5 °C because the wire is ±1.5 °C is quietly wrong. Each term has to be budgeted against the accuracy class, not assumed away.
What is the difference between the two totals this tool shows?
The root-sum-square total assumes the five contributions are independent and can partially cancel, so it is the realistic figure for a healthy loop. The arithmetic total assumes every term goes the same way at once, which is the worst case, useful for checking a safety margin, pessimistic for normal operation.
Where do I get the numbers to put in?
From your own loop: the wire certificate or datasheet for tolerance, the instrument specification for cold-junction accuracy, the extension conductor's class for the extension junction, the calibration interval history for drift, and a measurement or an upper bound for the noise term. The tool deliberately ships no defaults, because a default would be a figure we have not measured.
Does this replace a calibration?
No. A budget tells you whether the loop can hold its class and which term is worth attacking; a calibration tells you where the loop currently sits. Use the budget to decide the calibration interval, and the calibration to confirm the budget was honest.
Want the loop budgeted for your process?
Tell us the type, class and calibration interval, we supply the wire and the batch EMF certificate the budget is built on.