Humidity Measurement Systems

• Humidity - Presence of water vapours in a gas or air
• Measured in terms of absolute humidity, relative humidity or the dew point temperature
• Absolute humidity is the mass of water vapour present in the unit mass of the moist air or gas
• The relative humidity is the ratio of the water vapour pressure of the in a mixture of gas to the water vapour pressure in a saturated mixture the same temperature.
• The dew point temperature is the saturation temperature of the gas water mixture.
• At dew point the relative humidity is 100%.

• The instrument which measures the humidity directly is known as hygrometer.
• Can be calibrated to indicate the absolute or relative humidity
• Sling psychrometer which uses dry bulb and dry bulb temperatures
• The dry bulb thermometer measures the ambient temperature where as the wet bulb thermometer measures the temperature of saturated air i.e. the reduction in temperature due to evaporative cooling.
• The relative humidity is determined from the two temperatures readings with the help of psychrometric chart.

The conducting film of the resistive hygrometer is made by the lithium chloride and the carbon.

The conducting film places between the metal electrodes.

The resistance of the conducting film varies with the change in the value of humidity present in the surrounding air.

The moisture absorbs by the lithium chloride will depend on the relative humidity. If the relative humidity is high, the lithium chloride will absorb more moisture and their resistance decreases.

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The change in the value of resistance is measured by applying the alternating current to the bridge circuit.

Direct current is not used in the bridge as they breakdowns the layer of lithium chloride.

Resistive Hygrometer

• The change in capacitance of the capacitor shows the surrounding humidity.
• Capacitive hygrometer gives the very accurate result.
• It is made by placing the hygroscopic material between the metal electrodes.

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• The hygroscopic material can quickly absorb the water.
• The material absorbs water because of which the capacitance of the capacitor decreases.
• The electronic circuit measures the change in capacitance.

Capacitive Hygrometer

Impedance Type Hygrometer

• In this, impedance is a function of the water vapours present in the air or gas
• The probe consists of an aluminum strip, which is anodized to form porous layer of aluminum oxide.
• A thin coat of gold is applied over the aluminum oxide. Leads from the gold and aluminum electrodes connect the sensing element to the external circuitry. Water vapours penetrate the gold layer and equilibrate over the aluminum oxide.
• The number of water molecules absorbed over the aluminum oxide is a function of water vapour present in the sample.
• This contributes towards an increase in the conductivity of the aluminum oxide.
• The total probe impedance is the reciprocal of the probe conductivity.

Viscosity measurements

Viscosity is a measure of a fluid’s resistance to flow, or more precisely, it is the ratio of the force required to overcome internal friction between layers of fluid (shearing stress) to the change in speed between layers of fluid (velocity gradient).

A viscometer, viscosity meter or rheometer is a measuring instrument used to determine a fluid's internal flow resistance or viscosity.

The viscosity of the product with the help of the measurement describes its flow behavior.

Many food stuffs have inherently got a high viscosity. Therefore, the exact knowledge is very important for the quality control of the raw material, food and precursors as well as for the food plant engineering (flow behavior in the manufacturing process), because it states about the consistence and strength of food

Gravimetric capillaries and flow cups

Gravimetric capillaries – as well as flow cups – rely on gravitational force as the drive. The resulting quantity is kinematic viscosity.

Flow or efflux cups and gravimetric capillaries should only be used for measuring the viscosity of ideally viscous liquids

Take a capillary with precisely specified dimensions (inner diameter, length) and an equally precise distance given by two marks. Let a known quantity of liquid flow through this capillary and measure the time the liquid level takes to travel from one mark to the other. The measured time is an indicator for viscosity (due to the velocity of flow depending on this quantity). To obtain kinematic viscosity (v = ny), multiply the measured flow time (t_f) by the so-called capillary constant (K_C). This constant needs to be determined for each capillary by calibrating the capillary, i.e. by measuring a reference liquid of known viscosity.

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V = K_C ⋅ t_f

Typical representatives of glass capillaries

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1. Ostwald capillary (named after Wilhelm Ostwald, 1853 – 1932, German chemist[2])
2. Ubbelohde capillary (named after Leo Ubbelohde, 1877 – 1964, German chemist[3])
3. Cannon-Fenske capillary (a modified Ostwald capillary, designed by Dr. Michael R. Cannon, American scientist, inventor, and educator, and Merrell Robert Fenske, 1904 – 1971[4]
4. Houillon capillary (Standard ASTM D7279 describes measurement using this capillary type)

Types of flow cups

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Flow cups are used for testing all kinds of coatings, but also ceramic suspensions, drilling fluids, and even hot bitumen. Depending on the specific application, a different flow cup is utilized

Some types of flow cups.

1. ISO 2431
2. DIN 53211
3. Ford
4. Zahn
5. Engler
6. Shell

Pressurized capillary viscometers

• Capillary viscometers utilizing a weight
• High-pressure capillary viscometers with electric drive
• Capillary viscometers utilizing gas pressure

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1. Weight
2. Steel piston
3. Steel cylinder
4. Heating and insulation
5. Polymer melt
6. Die
7. Extrudate

Falling-ball and Rolling-ball viscometers

A falling-ball / rolling-ball viscometer does not measure a liquid’s flow time, but the rolling or falling time of a ball. Gravity acts as the driving force.

Rolling-ball principle

A ball of known dimensions rolls or falls through a closed capillary, which contains the sample liquid. A preset angle determines the inclination of the capillary. The time it takes the ball to descend a defined distance within the fluid is directly related to the fluid’s viscosity.

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Other forces than the ones named above are negligible, provided there is laminar flow.

A portion of the gravitational force, which depends on the angle, drives the ball downwards.

FG … Effective portion of gravitational force

FB … Effective portion of buoyancy force

FV … Viscous force

Viscosity equation:

Rolling time of the ball multiplied by the density difference of ball and sample and the adjustment constant K gives dynamic viscosity.

η=K⋅(ρb−ρs)⋅tr

η ... dynamic viscosity [mPa, s]

K ... proportionality constant [1]

ρb ... ball density [g/cm³]

ρs ... sample density [g/cm³]

tr ... ball rolling time [s]

F_G=m⋅g=ρ⋅V⋅g

m ... mass [kg]

g ... acceleration of gravity [m/s2]

ρ ... density [kg/m3]

Density and dimensions of the ball influence the gravitational force acting on the ball.

Devices operating at inclination angles between 10° and 80° are usually defined as rolling-ball viscometers. If the inclination angle exceeds 80°, the instrument in question is a falling-ball viscometer.

• The basic concept is to measure the elapsed time required for the ball to fall under gravity through a sample-filled tube inclined at an angle.
• The tube is mounted on a pivot bearing which quickly allows rotation of the tube 180 degrees, thereby allowing a repeat test to run immediately.
• Three measurements are taken and the average time it takes for the ball to fall is the result.
• A conversion formula turns the time reading into a final viscosity value.

Falling Ball Viscometer

APPLICATIONS

Beverages

Coatings

Cosmetics

Detergents

Food

Paint

Petroleum Products

Pharmaceuticals

Polymers

Soap

Rotational Viscometer

Rotational viscometers measure the viscosity of the sample by turning a spindle in a cup. The viscosity is determined through the measurement of the torque on a vertical shaft that rotates a spindle. Two different setups are used when measuring viscosity using a rotational viscometer:

Rotational viscometers push the upper limit of the potential measuring range further than gravity-based devices. They use a motor drive, which is significantly stronger than the earth’s gravitational force. Therefore, they are suited for measuring more highly viscous substances. The resulting quantity is dynamic viscosity

Depending on which part is driven, there are two principles:

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Couette principle

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Searle principle

The motor drives the cup while the bob is stationary. Such a system minimizes the risk of turbulent flow. The crucial point in this setup is how to implement the cup drive: the driving shaft needs to be leak-tight against liquid used for temperature control (if any). Due to that difficulty, there are only few classic Couette instruments on the market.

The motor turns a measuring bob or spindle in a container filled with sample fluid. While the driving speed is preset, the torque required for turning the measuring bob against the fluid’s viscous forces is measured.

Types of rotational instruments by the way the torque is measured

Spring instruments

A spiral spring connected to both the motor and the rotating bob registers the motor torque as a measure of viscosity.

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1 … Motor

2 … Slotted disc on motor side

3 … Position sensor on motor side

4 … Spiral spring

5 … Pivot

6 … Position sensor on bob side

7 … Slotted disc on bob side

8 … Rotating measuring bob

Servo motor instrument

The motor current is proportional to the motor torque, which represents the viscosity of the sample.

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1 … Servo motor providing highly precise measurement of the motor current

2 … High-resolution optical encoder counting the rotational speed

3 … Encoder disc

4 … Rotating measuring bob

Measuring bobs for rotational viscometers

An extensive quantity of different measuring bobs serves to test various types of substances. Generally, the relation between bob size and sample viscosity is inversely proportional: The lower the viscosity is, the more voluminous the bob should be. However, most of these bobs come without defined geometry, which gives relative viscosity value[10] and reduces comparability of measurement results

Disc spindles

Cylindrical spindles

1. T-bar spindle for paste-like substances
2. Krebs spindle, used in the paint and coatings industry
3. Paste spindle
4. Vane spindle for gel structures that are extremely shear-sensitive (e.g. dairy desserts and sauces)

Coaxial cylinder geometry used with rotational viscometers. Precisely defined dimensions allow for calculating shear rate, shear stress, and absolute dynamic viscosity.

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Rc ... radius of container [m]

Rb ... radius of bob [m]

L ... length of bob [m]

γ˙ ... shear rate [s-1]

ω ... angular velocity [rad/s]

τ ... shear stress [N/m2]

M ... measured torque [Nm]

η ... dynamic viscosity [Pa, s]

Coaxial cylinder systems

Cone-plate and parallel-plate measuring systems

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• Cone-plate and parallel-plate systems shear the sample under test in a defined gap between the fixed plate and the rotating bob.
• The bob is shaped as a cone or as a plate.
• For cones, an angle of 1° is recommended.
• The cone radius should be between 10 mm and 100 mm.
• For parallel plates, the standard demands that H << R. So, depending on the plate radius and the sample under test, H can range from 0.5 mm to 3 mm. Due to the narrow gaps, only a small amount of sample is required. However, a gap is open to the side so it follows that low-viscosity liquids may simply flow away, especially at higher rotational speeds, when turbulent flow and centrifugal forces occur.

1 … Cone-plate system, radius R and cone angle α

2 … Cone-plate system with truncated cone, radius R, cone angle α and gap a

3 … Parallel plate system, radius R, and gap H

Ostwald Viscometer

https://www.youtube.com/watch?v=Gs3gfwG9a7k

https://www.youtube.com/watch?v=HDUYO9kDkB4

Saybolt Viscometer

https://www.youtube.com/watch?v=OAJ6CDerrjA

Rheometer

https://www.youtube.com/watch?v=ONoV98kMkYA

Viscometer

https://www.youtube.com/watch?v=Iw_B_uU562w