Ceramic diaphragm
The ceramic diaphragm uses the porosity of unglazed ceramic. It's KCl outflow rate is approximately 0.2 ml / 24 h (p = 1m water column). Its electrical resistance is relatively high at 1 kΩ. In measurement solutions with greater ionic strength, the concentration gradient at the diaphragm is very large, meaning diffusion potentials are very easily created. At lower ionic strengths the resistance of the test material may be too high for exact measurements. Both effects are amplified by low outflow rates, and so ceramic diaphragms are less suitable in such cases. Due to the high risk of blockage of its pores, it is also not suitable for solutions containing suspended particles. Only in measurement solutions that contain oxidizing substances is it clearly superior to the platinum diaphragm.
Ground-joint diaphragm
The ground-joint diaphragm works with the thin gap of the unlubricated ground glass as an outfl ow opening for the electrolyte. The outfl ow rate is 3 ml/24 h (p = 1m water column) and greater. Its electrical resistance is very low at 0.1 kΩ. It is suitable for measurements in contaminated solutions, as it is easy to clean. Due to the high outfl ow rate, it is suitable for both high and lowion solutions. In versions without a screw connection, the ground gap must be manually adjusted in order to set a consistent fl ow rate.
KunststoffdiaphragmaPlastic diaphragm
For special applications there
are also diaphragms made
from plastic fi bers. For example,
single-rod measuring chains with
a plastic shaft often have diaphragms
made from nylon fi bers
so as to avoid contamination of
the connection hole. For process
measurements in solutions that
contain fl uoride, electrolyte keys
with PTFE diaphragms are used.
Platinum diaphragm
The platinum diaphragm is an
SI Analytics® development. It
consists of fi ne, twisted platinum
fi laments between which the
electrolyte fl ows out along
precisely defi ned channels. The
platinum diaphragm does not
easily become blocked and
therefore features a very constant
outfl ow. With approximately 1 ml /
24 h (p = 1m water column) and
approximately 0.5 kΩ electrical
resistance, it has advantages over
ceramic diaphragms. However it
is more sensitive to mechanical
stress. It is also less than optimal
for strongly oxidizing or reducing
solutions due to the occurrence of
disruptive potentials.
Hole or annular gap diaphragm
With polymer electrolyte electrodes
a conventional diaphragm
becomes superfl uous, as the solid
surface serves as an interface.
In combination electrodes, this is
utilized e.g. in the form of an annular
gap diaphragm. It consists
of an annular interface drawn
around the sensor between the
membrane and the outer tube. A
relatively low resistance is achieved
due to the relatively large
electrolyte/measuring medium
interface and its small distance
from the sensor. The ring-shaped
arrangement around the sensor
eliminates geometrically induced
interference effects.