SF6 Leak Detection: What Is the Difference Between ppm and Leak Rate?
An SF6 detector shows 10 ppm near a valve. Does that tell you how many grams of gas the equipment loses each year?
The reading alone cannot answer that question. Understanding the difference between gas concentration and leak rate helps users interpret results and select the right testing method.
What Does a ppm Reading Mean?
When an instrument reports ppm by volume, it measures the proportion of SF6 in the sampled gas.
A reading of 10 ppmv means approximately 10 parts of SF6 per million parts of the gas sample by volume. It describes the sample reaching the sensor at that moment.
It does not directly measure the total quantity escaping from the valve.
What Does Leak Rate Mean?
Leak rate describes the amount of gas escaping per unit time under specified conditions.
Common units include g/year for mass loss and mbar·L/s or Pa·m³/s for gas throughput.
A result expressed in g/year may come from a short measurement converted to an annual equivalent. It does not necessarily mean the equipment was monitored for a full year or that leakage will remain constant throughout that year.
Why Can the Same Leak Give Different ppm Readings?
A sniffer probe draws in escaping gas mixed with surrounding air. The measured concentration depends on how that gas reaches the probe.
Moving the probe farther away may allow more dilution. Ventilation can move gas away from the sampling point. Scanning too quickly may not allow enough time for gas transport and instrument response. Background SF6 can also affect the result.
For this reason, ppm readings taken under different conditions should not automatically be treated as a comparison of leak rates.
How KSTONE Detectors Fit into the Testing Process
Locating a Leak
For leak location, the operator scans potential leakage points and observes changes in concentration.
The KSTONE LF-1D+ SF6 leak detector is designed for micro-leak location in GIS and high-voltage switchgear. Its published sensitivity is 0.01 ppm.
This concentration sensitivity helps users detect small changes in sampled SF6. It should not be interpreted as a direct annual mass-loss value. Probe positioning, background conditions and the inspection procedure still matter.
Evaluating Leakage with an Accumulation Method
After locating a suspected leak, a factory may need to quantify leakage using a defined test procedure.
In a wrapping or accumulation method, gas collects within an enclosure around the test area. The assessment uses the concentration increase over a measured period, together with the effective enclosure volume and relevant test conditions.
KSTONE identifies LF-300, LF-302 and LF-303 as models for annual leak-rate calculation using the wrapping method. The result depends on the complete test arrangement, including background measurement, gas mixing, accumulation time and calibration.
The detector supplies concentration data. The defined measurement procedure provides the basis for calculating leakage.
See KSTONE’s leak detector selection guidance
Can ppm Be Converted Directly into g/year?
There is no universal conversion factor.
A single concentration reading beside a valve lacks information about gas collection, dilution and elapsed time. Quantitative leakage assessment requires a suitable calibrated method and its associated measurements.
Before selecting an instrument, define whether the task is to locate leakage, measure concentration or evaluate leakage against an acceptance limit. Then confirm the instrument configuration, calibration and test procedure for that task.