**GMP 3**

**Good Measurement Practice**

**for**

**Method of Reading A Meniscus Using Water or Other Wetting Liquid**

Two common methods are used for setting a meniscus. The method used in
calibration should be consistent with the intended use of the volumetric
standard. For interlaboratory comparisons, the method to be used should
be defined during the planning stages of the comparison.

In all apparatus where the volume is defined by a concave meniscus, the
reading or setting is made on the lowest point of the meniscus. In order
that the lowest point may be observed, it is necessary to place a shade
of some dark material immediately below the meniscus, which renders the
profile of the meniscus dark and clearly visible against a light
background. A convenient device for this purpose is a collar‑shaped
section of thick black rubber tubing, cut open at one side and of such
size as to clasp the tube firmly.

Two common types of meniscus readers are available. These include
black/white meniscus card readers or magnifying glasses with
cross-hairs. The width of the graduation will affect the readability of
the meniscus and should be estimated to the nearest 1/10 of a division.
Black/ white meniscus card readers are simple to create as shown in the
figure below. More elaborate card readers may be purchased that allow
placement around the neck of a flask.

This type of meniscus reader is generally preferred for Option A method
of reading the meniscus.

Another type of meniscus reader consists of a clear lens (plastic or
glass) with etched lines on the front and back that are aligned to
prevent parallax errors in reading. This type of reader is preferred for
Option B.

The meniscus of most liquids used in volumetric standards is concave
with the lowest point in the center used to determine the reading. The
meniscus formed by a non-wetting liquid, such as mercury (Hg), is convex
with the highest point in the center. The highest point of such a
meniscus is used to make the reading. The reading of a mercurial
barometer is a classical example of this kind. In making the reading,
the observer\'s eye should be normal to and in the same horizontal plane
as the meniscus. The illumination is adjusted to get a sharp definition
of the meniscus. Elimination of parallax error is very important and can
be judged by slight fluctuations of eye level that do not affect the
reading.

The curvature of a meniscus is related to the surface tension of the
liquid and inversely related to the diameter of the tubing in which it
is formed. When reading any meniscus, it is important to ascertain that
it is in an equilibrium position. Tapping of sight glasses and/or small
motions of containers may be used to induce slight displacements of the
meniscus. Return to the same reading is evidence of a stable meniscus.

**Option A -- (Suitable only when graduation lines extend more than 75
percent of the circumference of the sight gage area.)**

The position of the lowest point of the meniscus with reference to the
graduation line is such that it is in the plane of the middle of the
graduation line. This position of the meniscus is obtained by making the
setting in the center of the ellipse formed by the graduation line on
the front and the back of the tube as observed by having the eye
slightly below the plane of the graduation line. This is illustrated
below. The setting is accurate if, as the eye is raised and the ellipse
narrows, the lowest point of the meniscus remains midway between the
front and rear portions of the graduation line. By this method it is
possible to observe the approach of the meniscus from either above or
below the line to its proper setting.

1

**Option B**

The position of the lowest point of the meniscus with reference to the
graduation line is horizontally tangent to the plane of the upper edge
of the graduation line. The position of the meniscus is obtained by
having the eye in the same plane of the upper edge of the graduation
line as shown below.

**Comparison of Option A and Option B Methods**

For most practical applications the difference between these two methods
is insignificant compared to the tolerances of the volumetric standards.
However, a component for measurement uncertainty should be included as
appropriate. When performing calibrations, using the glassware as
precision standards with clear liquids, or when comparing results among
laboratories the difference in meniscus setting is directly related to
the visible thickness of the meniscus and the volume of liquid contained
in the neck between the top of a graduation line and the middle of the
graduation line as shown in the following diagram as 1) which results
with Option B when clear liquids are used and 2) which results with
Option A and is only possible when clear liquids are used.

When opaque liquids are used, an additional error or correction can be
estimated based on the volume contained between the upper and lower
edges of the meniscus. The curve of the meniscus and impact of this
error is dependent on the diameter of the meniscus and surface tension
of the liquid being used.

Where possible, the method of reading the meniscus during calibrations
should be performed in the same way in which the meniscus will be read
during normal use.