Yokogawa PH18 Differential pH Sensor - Manual (Page 6)

6
IM 12B6J4-E-A
1. InTRoDUCTIon
The PH18 is not a conventional electrode. It relies on
two dissimilar ion sensitive enamel membranes to
generate the signal. The measuring element is like a pH
electrode, and the reference like a Sodium ion
electrode.
Therefore the PH18 can only be used in
combination with pH meters that feature:
1 Dual high impedance inputs
2 Adjustable setting for Isothermal Point
3 Adjustable temperature coefficient
The Yokogawa models PH202 and
PH402 satisfy these requirements.
In both cases the membrane enamels are bonded
directly to the metal substrate with no filling solution.
The response curves Fig. 1 (pH), Fig. 2 (reference) and
Fig. 3 (application range) show the way in which the
potential is generated at each element and combined
in the output of the sensor.
Fig. 1 Above pH11 the effect of sodium on the pH
membrane starts to cause non-linearity (Sodium
error)
Fig. 2 Below pH3 the high Hydrogen ion content
causes a change in the reference response
Fig. 3 A linear response to pH is obtained between 3
and 11 pH
Fig. 4 A plot of reference voltage against Sodium
content.
The revolutionary measuring principle has some big
advantages. The absence of filling solutions and
reference junctions virtually eliminates the problems
caused by aging and pollution of the reference sensor.
Regular cleaning of the sensor virtually eliminates drift,
and the sensor benefits from a very long working life. It
is vital, however, to fit the sensor to the application
correctly. The special nature of the reference element
dictates that there must be a certain Sodium level in
the process.
Fig. 4 shows a plot of reference voltage against
Sodium content. Because of the exponential nature of the response, it
is plain that above about 0.5 N Na+ (30 g NaCl/l) in the
solution, the reference output remains virtually constant.
Of course when the Sodium concentration remains
constant in a process the reference voltage will also be
constant at much lower levels of Sodium.
It is because of the need to evaluate the chemistry
of the process, that it is necessary to have an
Application Data Sheet (page 16) completed before
approval for this sensor can be made.
The mechanical construction of the sensor also means
that it may be used in processes involving both high
temperatures and pressures. By eliminating the filling
solutions, the sensor is truly robust and can even
withstand severe thermal shocks that would ruin most
systems.
The stainless steel mounting adapter forms the liquid
earth (solution ground) connection needed to ensure
best stability of measurement. EXA also uses this
connection in the diagnostic circuit.
The PH18 is a differential pH sensor. It does not
measure absolute pH except in limited applications.
It does, however, measure a single control
point accurately, repeatably and with minimum
maintenance.