Chapter 15 [p243] suggests the use of a 12mm diameter reinforcing rod.  It can be more convenient to use lengths of half-inch galvanized steel pipe, as these can be threaded and easily coupled together on site.  As the top of the tube may not be exactly level, when readings are taken, a ball bearing of about 25mm diameter can be rested on top of the tube; the top of the ball bearing will then provide an accurately definable point on which the staff can be rested.