M
easuring pH value is the most important analytical measure-
ment when it comes to determining the quality and control-
ling processes. Approximately 50 % of all measuring points
in analytical measuring are pH measurements. Every year more than
onemillion pH sensors aremanufactured for process instrumentation.
With so many measuring points, it pays to take a look at the factors
influencing the safety of a pHmeasurement andwhat actionsmanufac-
turers of analytical measuring technology have taken to reduce risks.
Safety factor 1: Signal transmission
A typical pH measuring point consists of a pH glass electrode with
a PG 13.5 sensor head or a ¾“ NPT electrode in plastic housing.
pH measurement is a very sensitive, high-resistance measurement
and, depending on the pH value, generates voltage in the range of
-400…400 mV. These high resistance signals must be conducted
to the converter partially via double shielded cables. The analogue
voltage value is digitally converted there and then converted into
the pH value using the Nernst equation. In many cases, the tem-
perature compensation is conducted in parallel using temperature
sensors integrated into the electrode as Pt100, Pt1000 or NTC. Pure
Analytical
sensors
with a
transmitter in the sensor head
By W Babel, Krohne
analogue signals come from the sensor and the entire intelligent
signal processing must take place in the transmitter (converter). For
years there have been attempts to solve the pH-specific problem of
high resistance with connectors (sixplug, VP, Top68, etc) and cable
designs as well as circuitry in the transmitter but there has never
been a decisive breakthrough.
Safety factor 2: Calibration
Owing to various influences such as medium, temperature, humidity,
environmental conditions and pollution, and depending on the ap-
plication, pH measurements lose their accuracy over time and need
to be calibrated, cleaned, regenerated and ultimately replaced. The
calibration cycles for all electrochemical sensors vary from daily to
once a year. Until 2004, because of the analogue sensors, calibration,
cleaning and regeneration had to take place at the measuring site.
Analytical sensors that feature the transmitter
in the sensor head considerably reduce error
sources in the measuring chain.
FLOW MEASUREMENT + INSTRUMENTATION
Digital analytical sensors with integrated transmitter technology make external transmitters superfluous, effectively eliminating one of the
greatest sources of error in the measuring chain.
Electricity+Control
July ‘14
26
