The single tube manometer
The U-gage requires the reading of two levels since it is not advisable to measure the displacement of one meniscus from zero, assuming that this is half the difference in height, unless the inside diameter of the tube is perfectly constant.
This difficulty can be avoided by building the manometer with one arm (reservoir) with an area many times the area of the other one (Figure 1.2):
Single tube manometer
in this way, it is sufficient to read the level in the narrow tube, the change in level in the reservoir being negligible. On the other hand, the height of the reservoir can be adjusted in order to adjust the zero of the scale before making the reading. However, the single-tube manometer requires the taps to be switched to change from positive to negative differentials while either can be read without tap changes on the U-tube instrument.
Since the internal diameters of the two arms in this case are very different, strictly speaking, the reading should be corrected for the different effects of surface tension in the two arms. Since the magnitude of the correction depends very much on the cleanliness of the tube manometer (the presence of grease in particular has a significant effect), it would be better to reduce the effects of surface tension by adopting a pipe diameter that is not too small.