I've obtained this hydrostatic level in 2008. The instrument is probably of Dutch origin, but unsigned. It is slightly less basic than the water bottle level; two communicating containers of water connected by a hose, allowing to bridge larger distances between the bottles than with the water bottle level. In addition to that both bottles have a scale on them for direct reading instead of using a line of sight.

The level was used at Tata Steel Netherlands (at that time 'Hoogovens') for shaft alignment and was already in use some 40 years ago. Roughly 20 years ago they were discarded and subsequently taken home by one of the employees. It was from the latter that I got hold of them.

Both bottles have a scale with a 14 centimetres range, divided in whole millimetres. When properly used it should be no problem to reach (sub)millimetre accuracy at any given distance as long as the hose allows to bridge it.

Hydrostatic levels can and have achieved extremely high accuracies. For measurements of the highest accuracy they were supplied with micrometers to read the water level at a micrometer level. Under favourite circumstances a repetition can be achieved between 10 and 40 micrometres, while under less favourite circumstance this will be between 40 and 300 micrometres.¹

External factors can influence the quality of the readings. The temperature, if not equal along the whole hose, may affect the level as the density of water varies with it. It is therefore recommended to try to do the levelling as close as to 4°C as possible as around that temperature its density changes the least with temperature. Alternatively heavy water (H₂O₂) could was used as its density is stable around 11°C.²

Wind too may affect the observations. In most cases the bottles have an open connection to the air. Due to the venturi effect the level in the bottles may be affected by as much as 0.5 millimetres.² In order to overcome this a fully closed system has been used where the bottles were connected with a air hose as well.²

Wind is the effect of difference in air pressure between two geographical locations. When using a hydrostatic over extreme large distances (i.e. more than a kilometre), the difference in air pressure should be taken into account.

Here in the Netherlands several vessels have been used to lay these extreme long hoses between various locations. The first hoses (made of lead with steel armour) were 1800, 2230 and 3240 metres.³ Later these hoses could be combined to achieve routes up to 7 kilometres.⁴ When levelling with their system, barometric pressure was recorded and corrected for at both ends of the hose.⁵

The instrument came without a case, but still is in good working order. Both glass bottles survived in good order and so have the scales.

[1]: K. Dreszler, "Zu einigen Problemen bei Messungen mit der Schlaugwaage", in: Vermessungstechnik, Zeitschrift des Vermessungs- und Kartenwesens für Wissenschaft und Praxis, 13 Jahrgang, Heft 10, (Berlin, 1965), p.385.

[2]: idem, p.384.

[3]: A. Waalewijn, "Toepassing van Hydrostatische Waterpassing in Nederland", in: Maandblad van de Stichting Nederlands Genoodschap voor Landmeetkunde, Geodesia, 3e jaargang, nr. 2, p.29.

[4]: idem, p.33.

[5]: idem, p.32.