Analyzer Sample Line Lag Time Calculator
Find how long a sample takes to travel from the process tap to the analyzer, for gas or liquid sample lines, so the total analyzer response time can be checked.
- V = π/4 × d² × L + extra = 0.5921 L
- Gas is compressed: factor = (p + 1.013) ÷ 1.013 = 3.961
- t = V × factor ÷ Q = 70.35 s
Sample Lag Formulas
t = V ÷ Q (liquid)
V = (π/4) × d² × L + Vcomponents
V is the internal volume of the tube and components, d the tube inside diameter, L the length, Qstd the gas flow at standard conditions and Pabs the absolute line pressure. Gas at higher pressure holds more standard volume, so lag rises with pressure. 1/4" OD tube with 0.035" wall has an ID of about 4.57 mm.
Worked example
A 30 m run of 1/4" tube (4.57 mm ID) plus 0.1 L of filters, carrying 2 L/min of gas at 3 barg, holds 0.59 L and has a lag of about 70 s.
Frequently Asked Questions
What is sample lag time?
The time a sample takes to travel from the process to the analyzer. It adds directly to the analyzer response time seen by the control system.
Why does pressure increase lag for gas?
At higher pressure the same tube holds more gas, so at the same standard flow it takes longer to replace it.
What is a fast loop?
A high flow bypass that brings fresh sample close to the analyzer quickly, with only a small slipstream going through the analyzer.
Learn more on the blog
Instrumentation and control standards
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
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