Sample lag equals the gas volume in the line, at line pressure, divided by the flow measured at atmospheric pressure. A 10 m line of 4 mm bore at 1 L/min and atmospheric pressure takes about 7.5 seconds; at 6 bar gauge the same line takes about 52 seconds, because the tube holds seven times more gas.
Plug flow is assumed. Real lines mix a little along their length and moisture or sticky gases are slowed further by the tube wall, so treat the result as a minimum.
How it is calculated
The line is a cylinder. Its internal volume is the cross-section times the length. Gas at line pressure is compressed, so a volume of tube holds more gas than the same volume at atmosphere. The analyser draws flow that is quoted at atmospheric pressure, so the delay is the compressed gas content divided by that flow.
volume (mL) = pi / 4 x d(mm)^2 x L(m) lag (s) = volume x (P_abs / 1013.25) / (flow L/min x 1000 / 60)
Worked example
4 mm bore, 10 m, 1 L/min, atmospheric pressure: volume = 125.7 mL, lag = 125.7 / 16.67 = 7.5 s.
The same line fed from a 6 bar gauge tapping: absolute pressure is 7.01 bar, so lag = 7.5 x 6.9 = 52 s. Fitting a pressure reducer at the tapping, so that most of the line runs at low pressure, removes almost all of that delay.
Two cures for a slow reading: shorten or narrow the line, or reduce its pressure close to the tapping. Raising the flow helps too, but only up to the limit the analyser allows.
Limits
- Adds to the analyser's own response, T90. See Response time, T90 and sample lag in gas analysis.
- Moisture, ammonia and other sticky gases adsorb on tube walls and arrive later than the formula says.
- Dead legs and filters add volume; include them as extra tube length.
Frequently asked questions
How long does a gas sample take to reach the analyser?
Line volume divided by flow, with the volume corrected for line pressure. A short, narrow line at low pressure takes seconds; a long line at high pressure can take minutes.
Does pressure affect sample lag?
Yes, strongly. Gas at 7 bar absolute packs seven times as much gas into the same tube, so it takes seven times longer to flush at the same atmospheric-equivalent flow.
How can I reduce sample lag?
Shorten the line, use a smaller bore, reduce the pressure close to the tapping with a regulator, add a fast bypass loop, or raise the flow within the analyser's limits.
Is lag part of response time?
It is added to it. The analyser's T90 describes the instrument; the sample line adds its own delay in front.