Instruments for pressure measurement

2. Description of laboratory work.
A mercury differential pressure gauge (1) of the DT-50 type, a liquid piezometer (2) and a standard manometer (3) are connected to the collector pipe (4) (Fig.2.5).

Fig.2.5
Water from the pipeline (5) is fed to the collector pipe (4) by means of a valve (6). Draining water from the collector (4) into the drain line (7) is done by means of a valve (8). To increase the pressure in the system, open the valve (6) and close the valve (8). Opening the valve (8), with the valve (6) closed, the pressure decreases. Thus, the installation allows you to record the readings of devices at different pressures.
3.The order of the work.
To determine the pressure in the liquid by a piezometer (2), two readings are made on a millimeter scale: one at the meniscus level in the piezometer, the other at the piezometer connection level, at the point where the pressure is measured. The difference in the counts h₂, multiplied by the specific gravity γ of the liquid in the piezometer, gives the required pressure. When using a differential mercury manometer (1), the mercury levels in the left and right knee of the tube are measured. And the difference in readings is the height of the mercury column h₁. This height is recalculated to the height of the working fluid (water) according to the formula:
Multiplying by the specific gravity of the liquid (water), we find the required pressure. When determining the readings with a standard manometer, the connecting tube must be freed from air bubbles (when measuring excess pressure), for which three-way valves are installed on the tubes. When counting the number of divisions of the scale, the eyes should be set in a position at which it is on the perpendicular to the plane of the dial of the manometer passing through the end of the arrow.
Comparison of the readings of the instruments (piezometer, differential mercury manometer, spring manometer) is performed in the following order:
devices are connected to the pipeline, and a certain pressure is established;
1) the pressure measurements are repeated several times (four or five times) for successively increasing pressures in each instrument;
2) the measurement results are processed:
a) taking as a basis the indication of the piezometer, the correction and the relative error are calculated
where р - indication of the piezometer, N/m²; р₁ , р₂ – pressure on a mercury and spring pressure gauge, N/m².

б) a plot of the relative error of each instrument against the pressure р, .