| A McLeod gauge is an absolute
pressure standard to which many other vacuum gauges are calibrated.
It will accurately measure the total pressure of non-condensable
permanent gases (i.e. hydrogen, nitrogen, oxygen, etc.) in a vacuum
system, but will not correctly measure condensable vapors if present.
Many condensable vapors will be condensed during compression of
the gas sample in the capillary tube and not contribute to depression of
the gauge liquid.
The
calibration of the gauge for non-condensables is based upon Boyles’
physical chemistry gas law P1*V1 = P2*V2 and therefore measurement of
the volume of the glass bulb and the volume per unit length or bore of
the capillary tubes is made with high precision.
It is based upon dimensions during manufacture so that once it is
correct, very little can go wrong to change its calibration, and so it
can be reliably used as a reference standard.
If condensable
vapors may be present while calibrating a vacuum gauge against the
McLeod gauge, then a liquid nitrogen cold trap should be used to ensure
that only non-condensables are being measured.
We offer a
number of types of McLeod gauges and they often vary depending on the
pressure range to be measured. |
