Design And Calibration Of The Urotensiometer

In order to obtain direct readings of the urinary surface tension values with a maximum error of ± 1 dyne/cm., the previously discussed factors were taken into consideration in designing and calibrating the urotensiometer which we conceived. (Fig. 217) The glass capillary tube has a bore diameter of 0.5 mm. and is approximately 14 cm. in length. It is calibrated to indicate the surface tension of a fluid with a specific gravity of 1.015 directly in terms of dynes/cm. in the following manner: a continuous column of distilled water at 18°C is drawn up to about three fourths of the height of the tube and allowed to descend with the tube maintained vertically. The point at which the top of the column stops is marked. It represents a surface tension of 73 dynes/cm. for water having a specific gravity of 1.000. In order to make the necessary correction for a fluid having a specific gravity of 1.015, the distance between this point and the tip of the tube is divided into 74 (instead of 73) equal parts. The tube is calibrated down to 50 dynes/cm. since lower values have not been encountered. In the tubes manufactured by Clay Adams, New York, the markings are permanent. The split line feature of the scale permits easy visualization of the meniscus. The encircling lines help in maintaining the tube in the vertical position.


Fig. 217. Urotensiometerócalibrated to indicate by direct readings in dyne/cm the surface tension of fluids having a specific gravity of 1.015.

The Measurement Of Urinary Surface Tension With The Urotensiometer

To determine surface tension by means of the Urotensiometer, the tapered end of the tube is introduced into the bulk of the urine specimen.

The fluid is drawn slightly above the highest mark by mouth suction and evacuated several times by positive pressure. The tube is again filled to the same point, care being taken this time that no air bubbles interrupt the continuity of the fluid column. The tube is removed from between the lips, and the tip of the capillary is then gradually raised toward the surface of the fluid. When the top of the column descends to the top line (T) of the scale, the tip of the tube is removed from the fluid and maintained in a vertical position at eye level. The descent of the top of the column can best be observed by viewing the meniscus between the ends of the split line calibration markings. The top of the column descends within one or two seconds to an initial point (Pi) where it comes to a temporary halt or its rate of descent suddenly slows perceptibly. The column again slowly descends, coming to rest after several minutes at a second point (P2). After some time, the descent may again be resumed at a much slower rate until a third and final stopping point (P3) is reached after more than fifteen minutes. For routine measurements, the first reading (Pi) is considered as the surface tension value of the urine. This corresponds roughly to the surface tension value of the specimen before any important secondary redistribution of molecules has taken place.

The capillary tube should be thoroughly cleaned with distilled water after use. It is well to check the tube before each series of measurements, using distilled water at room temperature. If the check readings are above 74 or below 73 on the scale, the tube must be carefully flushed through with distilled water by means of a suction pump. Occasionally, water alone may not be sufficient and it will be necessary to clean the tube with sul furicochromic cleaning solution, followed by thorough flushing with water, in order to obtain correct check readings. When the tube is not in use, it is best left standing in a glass beaker containing distilled water.

Surface Tension In Clinical And Experimental Research

The Urotensiometer for the first time makes possible determinations of the surface tension of urine and other physiological solutions as a routine laboratory procedure. The highest surface tension value for urine encountered clinically is 73 dynes/cm., and this is correlated with a minimal quantity of surface active substances. The lower the surface tension of the urine in dynes/cm., the greater the amount of tensio active agents present in the specimen. A surface tension of 52 dynes/cm. is the lowest clinical value that we have found by this method in more than 100,000 measurements made during the last 12 years.

The first problem concerning the meaning of the different values of urinary surface tension arose when it was observed that usually the urines with low specific gravity have high surface tension, while those with high specific gravity have low surface tension. The direct correlation between the values of surface tension and specific gravity of the samples thus had to be investigated with the supposition that the amount of water in the urine will have a great influence, by itself, on surface tension. While a correlation between surface tension and water content is often observed, it is not a cause and effect one. Urinary samples with a specific gravity as low as 1.003 were seen with a surface tension of 58 dynes/cm. while samples with a specific gravity as high as 1.030 had a surface tension of 70. Although very seldom encountered, these values have invalidated the supposition that it is the amount of water in the urine which determines the value of the surface tension, so that from the analytical point of view determination cannot be substituted.