The falling ball viscometer is an instrument commonly used in laboratories to measure the viscosity of transparent solutions, with a simple structure. Put the solution to be tested in a glass viscosity measuring tube, put it in a heating constant temperature bath, and keep it at a constant temperature. Then put a stainless steel ball into the tube, let it fall freely, record the time t' required for the ball to fall at a constant speed for a certain distance S, and calculate the viscosity of the solution. Depending on how the ball falls, the
Divided into vertical falling ball test method and eccentric fall ball test method.
1. Vertical falling ball test method
(1) Scope and description
This method is suitable for the determination of transparent coating products with high viscosity, and the measured viscosity is the conditional viscosity. That is, at a certain temperature, the time it takes for a steel ball of a certain specification to fall vertically through the upper and lower scale lines of the glass tube containing the sample, expressed in seconds (s).
(2) Instruments and materials
① Falling ball viscometer. Specifications and dimensions are shown in Figure 1-15. The viscometer consists of two parts: a glass tube and a steel ball. The length of the glass tube is 350mm, the inner diameter is (25±0.25)mm, and there are scale lines 50mm away from the sides of the tube mouth at both ends, and the distance between the two lines is 250mm. There are cork stoppers on the upper and lower ends of the nozzle, and an iron nail in the middle of the cork stopper at the upper end. The glass tube is fixed vertically on a stand (determined by a plumb bob). The diameter of the steel ball is (8±0.03) mm, and its specification should meet the requirements in the GB/T 308-2002 standard.
② stopwatch. Graduation 0.2s.
③ permanent magnet.
3) Measurement method
①Pour the transparent sample into the glass tube so that the sample is 40mm above the upper scale line. Put in steel balls and plug in cork with iron nails. Place the permanent magnet on the cork with iron nails.
②Turn the pipe upside down so that the iron nails attract the steel ball, then turn it over and fix it on the frame. Using a plumb bob, adjust the glass tube so that it is vertical. Take away the permanent magnet, let the steel ball fall freely, and start the stopwatch immediately when the steel ball just falls to the scale line. Stop the stopwatch timing when the steel ball falls to the lower scale line. The viscosity of the sample is represented by the time (s) for the steel ball to pass through the two scale lines.
(4) The result indicates that the average value of the two measurements is taken as the measurement result, and the difference between the two measured values should not be greater than 3% of the average value.
(5) Refer to the national standard GB/T1723-1993.
2. Eccentric falling ball method
(1) Scope and description
The eccentric falling ball viscometer is the Hoeppler viscometer, also known as the rolling falling ball viscometer. The structure is shown in Figure 1-16. Its characteristic is that the tube is inclined at a certain angle, so that the ball rolls along the tube wall and slides down stably, which can avoid the measurement error caused by the deviation of the vertical line when the ball falls vertically; in addition, when the ball slides down the tube wall, Silver gray spots can be reflected on the tube wall, so the viscosity of opaque liquids can also be measured.
(2) Instruments and materials
The inner diameter of the sample tube of the eccentric falling ball viscometer is 16mm, and there are two annular measurement lines m1 and m2 engraved on it, and the distance between the upper engraved line m1 and the lower engraved line m2 is (50±l)mm [or 100±l) mm], m1 is about 60mm from the top of the measuring tube, m2 is about 40mm from the bottom surface, and the viscometer is equipped with a Super Constant Temperature Bath with circulating water supply to control the test temperature.
3) Measurement method
①The viscometer test tube, measuring ball, etc. are first washed several times with absolute ethanol or other organic solvents until they are clean, and then dried with a hair dryer.
②Inject the sample along the inner wall of the measuring tube so that the liquid level is about 15mm lower than the top of the measuring tube. Carefully put the measuring ball into the sample tube with a tweezer, add a vent plug, and tighten the screw cap of the sealing cap. The measurement can only be performed after the air bubbles in the measuring solution disappear.
③Use a rubber tube to connect the glass outer cylinder with the Super Constant Temperature Bath and circulate water supply to control the temperature of the glass outer cylinder at ±0.1 "C, and keep the constant temperature for no less than 20 minutes.
④ Drop the measuring ball back and forth in the sample tube for 2-3 times until it reaches the top of the sample tube, then rotate the glass outer cylinder 180° and lock the viscometer with the positioning pin to prepare for measurement.
⑤ Start timing when the measuring ball falls to the engraved line mi, and stop timing when it reaches the engraved line m2. The moment of starting and stopping the stopwatch should be the moment when the lower edge of the measuring ball is tangent to the engraved line. The time (s) required for the measuring ball to descend through the two marked lines represents the viscosity of the liquid.
(4) The result means two parallel measurements, the arithmetic mean of the two measured values is taken, and the difference between the two measured values should not be greater than 3% of the average value. The measured seconds can be converted into absolute viscosity by the following formula:
