Viscometers - General Theory
Capillary Viscometers
Continuous capillary viscometers are primarily designed to measure the viscosity of Newtonian liquids. Because pressure transducers are used to transmit the measured viscosity, they are readily adaptable to the automatic control of processes. Temperature compensation is not practical with this type of viscometer. Rather, a thermostatic device is used to control the temperature of the sample at a reference temperature before metering through the capillary tube. This type of viscometer has been found successful at viscosities up to 15,000 poises (1,500 Pas) and at temperatures up to 900°F (480°C). The span of the instrument is determined by the bore and length of the capillary, and a large variety of viscosity ranges are possible. The use of large-diameter bores and long capillary tubes is recommended for minimum end effects.
There are basically two different types of viscometers that utilize the continuous capillary principle. One type measures pressure drop across the capillary tube and the other measures upstream pressure as the sample flows through the capillary tube.
Differential Pressure Type To measure the liquid viscosity, the pressure drop across the capillary is measured with a differential pressure transducer connected to the inlet and outlet sides of the capillary. The transducer outlet signal may be pneumatic or electric and is used for indicating, recording or controlling the process.
Backpressure Type The operation of this instrument is quite similar to the differential pressure type except that it measures only upstream pressure to a capillary tube, which discharges to atmosphere or returns the sample to a pressure-regulated process line. The sample fluid is continuously fed to the instrument from the process line or from a vessel. The sample temperature is maintained by flowing through a heat exchanger immersed in a constant-temperature bath. Sample then passes through a pressure regulator to a constant-rate-metering device. Under conditions of constant flow rate and temperature, the sample pressure at the entrance to the measuring capillary tube is proportional to the viscosity of the liquid. The inlet pressure is sensed by a strain gauge. The strain gauge signal is converted to convenient viscosity units to indicate, record, or control the process viscosity. Since it measures only the inlet pressure to the capillary tube, it is extremely important to maintain the outlet side at constant pressure by discharging to atmosphere or to a pressure-regulated vessel or pipeline.
Falling-Piston Viscometer
The working principle of the falling-piston viscometer is quite similar to the falling-ball viscometer. The excellent reproducibility of the falling piston element makes it possible to utilize it for the measurement of both Newtonian and non-Newtonian fluid viscosities as an in-process instrument.
The measuring element of the instrument consists of a piston in a measuring tube. The measuring element can be installed in a tank (open or closed) or in a liquid-filled line, as long as the measuring tube is completely immersed in the fluid. During the filling phase, the piston at the bottom of the tube is automatically raised by an air lifting mechanism or by a motor-cam mechanism. As the piston is raised, a sample of the liquid to be measured is drawn in through openings in the sides of the tube, and fills the measuring tube as the piston is withdrawn.
Falling-Slug or Falling-Ball Viscometers This instrument automatically measures the time required for a cylindrical slug of a specific density to fall a given distance in a vertical tube filled with the process liquid at a known, constant temperature.
The sample pump introduces the fresh sample and purges the system of the previous sample. Two separate thermostats in the temperature well control the purge and recirculation cycle by activating a three-way valve. The flow velocity raises the slug to the top of the fall tube, and when the sample temperature is reached, the pump and the sample stop, thereby permitting the slug to fall. As it does, it actuates a magnetic switch attached to the side of the fall tube, which in turn starts the recorder motor. The slug then actuates a second magnetic switch located at an adjustable distance (1 to 20 in., or 25 to 500 mm) below the first. This switch stops the recorder motor. The resultant time measurement is directly proportional to the viscosity of the sample. Actuation of the lower switch also initiates the system to the purging phase.
Float Viscometers
These viscometers are used industrially and in the pilot plant to measure the viscosity of process fluids and to continuously indicate, record, or control the process. This type of viscometer is used in a closed flow system. The operating principle is similar to that of the variable area flow meter (rotameter), where the viscous drag force on a float is proportional to the orifice opening required (between float and tapered tube) to move the fluid through that orifice at a constant flow rate.
In a rotameter-type flow meter the flow rate, float and liquid specific gravity, and viscosity of the fluid being metered affect the forces acting on the float. For flow meter applications, the floats are designed so that the viscous drag area is relatively small and the float is viscosity insensitive. Thus the float is sensitive only to flow rate and density changes of the fluid. In the viscometer, the flow rate through the variable area meter is held constant. Therefore, the position of the float is a measure only of fluid viscosity and density or of kinematic viscosity. To increase its sensitivity, the viscometer float is designed with a relatively large viscous drag area. For float-type viscometers, accurate viscosity measurement demands a carefully controlled flow rate. Based on their flow controller requirements, there are three different types: single float, two float, and concentric.
Single Float The single-float viscometer is a direct reading viscosity instrument for continuous measurement. A positive displacement pump (other flow control devices can also be used) provides the constant sample flow rate through the instrument. The recommended flow rate is between 0.75 and 2.0 GPM (2.9 and 7.6 l/m). Temperature compensation units are not available for this type of viscometer. If a metering pump is used to provide a constant flow-rate, the temperature rise through the pump should be known for proper correction by the use of a viscosity vs. temperature curve. Transmission of the float position is possible for recording or controlling purposes by the use of an armature attached to the float extension rod with a magnetic sensing device around its outer periphery.
Two-Float This is a low cost viscometer designed to provide intermittent viscosity measurement in the laboratory, on the test bench, or in industry. It is only for local indication and viscosity signal transmission or automatic control is not possible. It incorporates two floats. The upper float. is sensitive to fluid flow rate and the lower to viscosity. In operation, the fluid flow rate is manually adjusted to a predetermined value as indicated by the position of the upper float. By maintaining a constant reference flow rate as indicated by the flow rate float, the position of the other float indicates the viscosity of the fluid on a direct reading scale.
Concentric The concentric viscometer consists of a differential pressure regulator that maintains a constant pressure drop across the meter, and a variable area flowmeter with a viscosity-sensitive float. As the fluid enters the instrument, it splits into two streams. The portion of the fluid that flows upward around the differential pressure float is used to control the pressure drop across the meter. The upper end of the differential pressure float acts as a control valve, and when flow rate changes, it throttles the flow of fluid to maintain a constant pressure drop. The pressure drop across this portion of the meter is determined by the weight of the float. The portion of the fluid that flows downward enters the viscometer tube by way of an orifice and then passes the viscosity-sensitive float. Constant flow rate through the viscometer tube is maintained since the fluid flows through an orifice at a fixed pressure drop. Thus, measurement of viscosity is possible under constant flow rate. An extension attached to the viscosity float transmits its movement through a magnetic coupling to a receiver for display, recording or automatic process control, as desired.
Oscillating-Blade Viscometers
The blade moves between the fixed side walls and in that process discharges the process fluid through the openings of one wall of the measuring chamber, while pulling in a fresh sample through the other wall. The blade is rotated around its fulcrum by a plunger/solenoid mechanism, as its tip travels the distance between points A and B. When the stroke is completed, the polarity of the current to the plunger coil is reversed and the blade is returned to its original position. Adjustable mechanical stops are provided to limit the stroke. The time of travel between points A and B is detected optically, as a beam interrupter blocks a light beam upon the completion of the stroke. The total stroke time is about 0.2 seconds, the blade makes a stroke every two seconds, and the average time of several strokes (adjustable) is measured as an indication of viscosity.
Rotating Cone, Disk, Sphere or Spindle Viscometer
Rotating cone viscometers are designed to operate in industrial environments on a continuous basis. Viscosity is measured by sensing the torque required to rotate a spindle continuously in a liquid. In a process viscometer application, the sample is continuously replaced and is subjected to a constant shear rate, thus measurement of non-Newtonian apparent viscosity is possible, as well as absolute viscosity measurement of Newtonian fluids.
In operation, a synchronous induction motor (for safety) drives a cage coupled through a calibrated spring to a spindle arm, which supports the spindle or cylinder in the fluid being measured. During measurement, the spring tends to wind up until its force equals the viscous drag on the spindle. At this point, the cage and spindle both rotate at the same speed but with a definite angular relationship to each other proportional to the torque on the spring. Two methods are used to convert the angular relationship into a viscosity reading. One side of a variable capacitor is attached to the cage and the other to the spindle arm. A capacitance is thus made proportional to the angular relationship between cage and spindle.
The other method of signaling is based on a potentiometer mounted in the cage with its free member connected to the spindle arm. A resistance is made directly proportional to the angular relationship between cage and spindle. In either case, the measured signal is transmitted to a suitable receiver as an electrical impulse, and is converted into viscosity units. The variable capacitor type is preferred for most applications in the low viscosity ranges.
Magnetically Coupled Viscometer In one design of the rotating cone viscometer a magnetic coupling is provided between the electronic detector at atmospheric pressure and the rotating sensor, which is exposed to the process pressure. With this separation between atmospheric and pressurized areas, no purging is required to keep the liquid away from the measuring instrument, and operating conditions at the levels of 2850 PSIG (19.7 MPa) at 20~C or 1620 PSIG (11.2 MPa) at 300°C can be tolerated. The magnetically coupled viscometer should not be used to measure fluids containing fiber, ferrite, or abrasive materials as they interfere with the operation of the magnetic coupling or damage the stainless steel-sapphire bearings.
Gyrating-Element Viscometer The viscometer differs from the previously described designs because its element gyrates instead of rotates. This gyrating motion is the result of the drive shaft not being straight. The drive and the torque-sensing mechanism are isolated from the process by stainless steel bellows, which are suited for 1000 PSIG (70 barg) operating pressures and 572°F (300°C) operating temperatures. The drive motor speed is adjustable from 15 to 120 RPM, and the unit can detect viscosities between 10 and 10 million cps (the overall range is subdivided into several working ranges).
Agitator Power Operation of this instrument is the same as that of the rotating cone viscometer except that the torque exerted on the process agitator blade is measured by a transmitting wattmeter (thermal converter). It measures the power consumed in driving an agitator in a mixing tank. Since most of the industrial agitator motors are oversized, the torque response is poor at low viscosities. The size of the motor or impeller size should be selected to operate in a region where the viscosity versus torque relationship is linear. The motor, reducer, bearing assembly, and pressure seal are all part of the viscometer, and any change in characteristics of these components would affect the power consumption of the motor and, in turn, the viscosity reading. This instrument is very simple and easy to install, but has a low ratio between changes in agitator power consumption and changes in viscosity of the fluid. Many different designs of impellers are available to improve the sensitivity and rangeability.
Double-Cylinder Viscometer It consists of a stationary outer and a rotating inner cylinder and provides self-cleaning operation and a fast measurement. During operation the process fluid is drawn into the measuring chamber by the propeller through the apertures in the stationary outer cylinder. The propeller pumps the process fluid downward into the measurement zone, where the measurement cylinder is rotated inside the stationary external cylinder. Some of the process fluid enters the measuring zone, which is the clearance space between the two cylinders. The torque required to rotate the measuring cylinder is measured as a signal that is proportional to viscosity.
Vibrating Reed Viscometer
The vibrating reed viscometer is designed for continuous measurement of viscosity in process streams. It can be installed directly in process vessels or pipelines. It consists of a frequency generator, vibrating spring rod, probe, and a pickup unit to complete a measurement loop through the process material. The drive coil is excited at a frequency of 120 cycles per second (cps) from a 60 cps input frequency. This produces a pulsating magnetic field, which causes the drive armature to vibrate at the same frequency.
Mechanical vibration of the drive armature is transmitted to the probe through an all-welded pressure sealed node, where amplitude of vibration is zero. The fundamental principle of operation is that the amplitude of probe vibration depends upon the viscosity of the process media. The resistance to the shearing action caused by the probe vibration increases with increase in the process media viscosity. The amplitude of probe vibration is transferred through a second welded node point along the upper spring rod to the pickup end of the detector. The pickup end is similar to that of the driver end, except that a permanent magnet is used to induce a 120 cps AC voltage in the pickup coil from the vibration of the pickup armature. The magnitude of the voltage generated in the pickup coil is a measure of the process viscosity since it is proportional to the pickup armature vibration. The 120 cps output signal from the detector is converted into a 0 to 10 mV DC signal to indicate, record, or control viscosity of Newtonian or non-Newtonian fluids.
