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Temperature Measurement · Glossary

What is Response Time?

Symbol: τT63, T90Temperature Measurement

Response time is how fast a temperature sensor follows a step change, given as a time constant τ (63 %) or as T90 (90 %).

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Response Time Explained

A sensor behaves roughly like a first order system. After one time constant it shows 63.2 % of a step change, after about 2.3 time constants 90 %.

Thermowells, air gaps and slow flow increase response time. A slow sensor makes control loops sluggish and hides fast temperature changes.

Response Time Formula

T(t) = Tf + (T₀ − Tf) × e−t/τ   and   T90 ≈ 2.3 τ
T₀, Tfinitial and final temperature
τtime constant (s)
Example: τ = 8 s gives T90 ≈ 18.4 s.
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Calculators for Response Time

CalculatorSensor Response Time CalculatorFree online calculator with formula and worked example.CalculatorCooling Time CalculatorFree online calculator with formula and worked example.

Frequently Asked Questions

What is Response Time?

Response time is how fast a temperature sensor follows a step change, given as a time constant τ (63 %) or as T90 (90 %).

Is response time a fixed property?

No. It depends on fluid, velocity and installation; data sheets state the test conditions.

How can I speed up a sensor?

Use a smaller diameter, a tapered well, spring loading and good thermal contact.

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