The Redwood viscometer experiment measures the kinematic viscosity of an oil by timing how long a fixed volume takes to flow through a standard orifice. It is the standard British method for grading lubricating oils and shows how viscosity falls as temperature rises — essential knowledge for lubrication and fuel engineering.
Aim of the experiment
To determine the kinematic viscosity of a given oil at different temperatures using a Redwood viscometer and to study the variation of viscosity with temperature.
Theory
Viscosity is a fluid’s resistance to flow. Dynamic viscosity (μ) measures internal shear resistance, while kinematic viscosity (ν = μ/ρ) is dynamic viscosity divided by density. The Redwood viscometer measures viscosity indirectly as the time (in “Redwood seconds”) for 50 ml of oil to drain through a standardised orifice.
Kinematic viscosity is calculated from the efflux time using:
ν = At − B/t
where ν is kinematic viscosity (centistokes), t is the efflux time in Redwood seconds, and A and B are calibration constants. For a Redwood No. 1 viscometer, A ≈ 0.26 and B ≈ 172 when ν is in centistokes.
Apparatus required
- Redwood viscometer No. 1 (oil cup with agate/jet orifice, surrounded by a water bath)
- 50 ml Kohlrausch flask (receiving flask)
- Thermometers (for oil and water bath)
- Stopwatch
- Heating arrangement and stirrer
Procedure
- Clean the oil cup and orifice; close the orifice with the ball valve.
- Fill the cup with the test oil up to the index mark and fill the surrounding bath with water.
- Heat the bath, stirring continuously so the oil reaches a uniform temperature.
- At the desired oil temperature, lift the ball valve and simultaneously start the stopwatch.
- Record the time taken to collect exactly 50 ml of oil in the flask — this is the Redwood seconds value.
- Repeat at several temperatures and tabulate the efflux time at each.
Observations and calculation
For each temperature, note the efflux time t and compute ν = At − B/t. Plot kinematic viscosity against temperature — the curve falls steeply, confirming that oil thins as it heats. This temperature dependence is exactly why oils are graded by viscosity index.
Precautions
- Keep the orifice scrupulously clean — any deposit changes the flow rate.
- Maintain a uniform bath temperature and stir well before each reading.
- Start the stopwatch at the instant the ball valve is lifted.
Applications
Redwood viscosity grading is used for lubricating oils, fuel oils, and hydraulic fluids, where the right viscosity at operating temperature is critical for film strength and pumping. The concept links directly to what viscosity is and how it is measured and to the Reynolds number experiment, since kinematic viscosity determines whether flow is laminar or turbulent. See the definition in our engineering lab glossary.
Frequently asked questions
What is the difference between Redwood, Saybolt and Engler viscometers?
They are the British, American and Continental-European efflux-type viscometers respectively. All measure efflux time through a standard orifice, but use different orifice sizes and report different units (Redwood seconds, Saybolt seconds, Engler degrees).
Why does viscosity decrease with temperature?
Heating increases molecular energy and reduces the intermolecular forces that resist flow, so the oil thins and drains faster.
What is the difference between dynamic and kinematic viscosity?
Dynamic viscosity (μ) measures shear resistance; kinematic viscosity (ν = μ/ρ) divides it by density and is what the Redwood viscometer reports.
Need a Redwood viscometer for your fluid mechanics or lubrication lab? Request a quote from Scientico India — ISO 9001:2015 certified, CE marked, exporting to 60+ countries, reply within 24 hours.
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