Measurement of Pressure with the Manometer
FACTORS AFFECTING MANOMETER PERFORMANCE AND USAGE
CHARACTERISTICS OF INDICATING FLUID
The fluid used also affects the operating range of the manometer. Mercury being 13.6 times the weight of water will move 1/13.6th the distance water will move in response to a given pressure. Dwyer .826 sp. gr. gage oil being lighter than water will move about 1.2 times farther than water in response to a given pressure. This, obviously, expands the scale for easier, more precise reading.
Dwyer U-tube and well-type manometers are furnished with inch scales for use with water or mercury or adjusted scales for use with .826 sp. gr. gage oil. Dwyer solid plastic vertical, inclined and inclined-vertical gages use gage oil.
Dwyer colored gage oil is a stable petroleum base oil with carefully controlled specific gravity which gives an excellent, consistent high visibility meniscus. Dwyer manometers for use with water are furnished with a fluorescein green concentrate which when added to water serves as a setting agent and a dye to improve the consistency and visibility of the meniscus for easier more accurate readability.
Fig. 2-5. Portion of the scale of a Dwyer No. 250.5 solid plastic inclined manometer shown full size. Parallax free reading is made by aligning meniscus with its reflection in the polished scale.
Dwyer solid plastic manometers assure parallax-free readings by the use of silk-screened scales on polished aluminum which reflect the image of the meniscus. When the meniscus and its reflection are aligned, the line of sight is perpendicular to the fluid column at the meniscus and an accurate reading is assured. Smoothly machined bores further enhance the visibility of the meniscus.
FACTORS IN DESIGN AND MANUFACTURE THAT AFFECT INCLINED MANOMETER ACCURACY
2. Indicating Bore Straightness. This is a very real problem with glass tubing. Even with precision bore glass tubing, concentricity between I.D. and O.D. is difficult to control. Additionally, there is the problem of supporting the tube by its O.D. in perfect linearity with its I.D. Magnitude of error from this cause is, therefore, a function of the quality of this particular piece of precision bore tubing and the manner of tube mounting. Variations in accuracy from gage to gage and in a given gage over a period of time can be anticipated as the tube is bumped or bent in use. In contrast, indicating tube bores up to 24" long in Dwyer solid plastic manometers are straight to within .002" over their entire length. It should also be noted that in the Dwyer design with a massive solid block of acrylic plastic, this straightness tolerance is effective for the life of the instrument.