Endress+Hauser CPS97D Specifications

TI01405C/07/EN/01.18
71425018
2018-06-15

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Technical Information

Memosens CPS97D

ISFET sensor for pH measurement with long-term
stability in media with high dirt loads
Digital with Memosens technology

Application

• Contaminated media:
– Solids
– Emulsions
– Precipitation reactions

• Process applications with:
– Rapidly changing pH values
– Varying temperatures and pressures

• Water treatment and wastewater

Your benefits

• Break-resistant
– Sensor body made entirely of PEEK
– Can be installed directly in the process, saving time and cost for sampling and

laboratory analysis

• Reference system:
– Open aperture
– Stabilized, hard gel

• Can be used in particle-laden media with a high dirt load

• Operation at low temperatures
– Short response time
– Consistently high accuracy

• Longer calibration intervals than with glass electrodes
– Shorter hysteresis in event of temperature change
– Smaller measuring error following exposure to high temperatures
– Virtually no acid and alkaline errors

• Integrated temperature sensor for effective temperature compensation

Advantages offered by Memosens technology

• Maximum process safety thanks to non-contact, inductive signal transmission

• Data security thanks to digital data transmission

• Very easy to use as sensor data saved in the sensor

• Predictive maintenance possible as sensor load data are recorded in the sensor

• Heartbeat

Function and system design

U

D

U

GS

1

2

S

D

Si (n)

Si (p)

Si (p)

I

D

U

D

U

GS

1

3

4

5

6

2

D

Si (n)

Si (p)

Si (p)

I

D

S

Memosens CPS97D

Measuring principle

Ion-selective, or more generally, ion-sensitive field effect transistors (ISFET) were developed in the
1970s as an alternative to glass electrodes for pH measurement.

General principles

Ion-selective field effect transistors are based on an MOS

1)

transistor arrangement →  1,  2.
Unlike the MOS, however, the ISFET sensor does not have a metal gate (item 1) as the control
electrode. Instead, in the ISFET sensor →  2,  2 the medium (item 3) is in direct contact with
the gate insulator layer (item 2). Two highly p-conducting regions are diffused into the n-conducting
substrate material (item 5) of the semi-conductor (Si). They act as the charge-supplying electrode
("Source", S) and the charge-accepting electrode ("Drain", D). The metal gate electrode (in the case of
the MOSFET) and the medium (in the case of the ISFET) forms a capacitor with the underlying
substrate. A difference in potential (voltage) between the gate and substrate (UGS) increases the
electron density in the area between the "Source" and "Drain". A conductive channel forms
→  2,  2(item 4), such that a current ID flows when a voltage UD is applied.

A0036074

 1 MOSFET principle

1 Metal gate
2 Conductive channel (N-conducting)

 2 ISFET principle

1 Reference electrode
2 Gate insulator layer
3 Medium
4 Conductive channel (N-conducting)
5 N-doped silicon substrate
6 Sensor shaft

A0003856

With the ISFET, ions that are in the medium and located in the boundary layer between the
medium/gate insulator generate the electric field (gate potential). The effect described above causes
a conductive channel to form in the silicon semi-conductor substrate between the "Source" and
"Drain", and causes current to flow between the "Source" and "Drain".

Suitable sensor circuits use the dependence of the ion-selective gate potential to generate an output
signal that is proportional to the concentration of the ion type.

pH-selective ISFET

The gate insulator acts as an ion-selective layer for H+ ions. While the gate insulator is also
impermeable to these ions (insulator effect), it allows reversible surface reactions with H+ ions.
Depending on the acidic or alkaline character of the medium, functional groups in the insulator
surface either accept or donate H+ ions (amphoteric character of the functional groups). This results
in positive charging at the insulator surface (H+ ions accepted in the acidic medium) or negative
charging at the insulator surface (H+ ions donated in the alkaline medium). Depending on the pH
value, a defined surface charge can be used to control the field effect in the channel between the

1) Metal Oxide Semiconductor

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Memosens CPS97D

1

2

3

"Source" and "Drain". The processes which lead to the creation of a charge potential and therefore to a
control voltage UGS between the "Gate" and "Source" are described by the Nernst equation:

Measuring system

UGS = U0 +

U

GS

U

0

R Gas constant (8.3143 J/molK) 2.3 . RT
T Temperature [K]
n Valency (1/mol)

2.3 . RT

Potential between gate and source F Faraday constant (26.803 Ah)
Offset voltage a

nF

.

lg a

ion

ion

Activity of ion type (H+)

Nernst factor

nF

At 25 °C (77 °F) the Nerst factor of the pH measurement has the value -59.16 mV/pH.

The complete measuring system comprises at least:

• ISFET sensor

• Memosens data cable: CYK10 (Memosens, digital sensor)

• Transmitter, e.g. Liquiline CM44, Liquiline CM42

• Assembly
– Immersion assembly, e.g. Dipfit CPA111
– Flow assembly, e.g. Flowfit CPA250
– Retractable assembly, e.g. Cleanfit CPA871
– Permanent installation assembly, e.g. Unifit CPA842

Additional options are available depending on the application:
Automatic cleaning and calibration system, e.g. Liquiline Control CDC90

Chemicals and process engineering

 3 Measuring system

1 ISFET sensor
2 Retractable assembly CPA871
3 Liquiline M CM42 transmitter

There is only a small range involving high pH values in combination with high temperatures where
the sensor's long-term stability is somewhat compromised. Media with such properties reduce the
insulator oxide of the ISFET chip. As this is the pH and temperature range of CIP cleaning media, the
ISFET sensor is only used here in combination with an automatic retractable assembly.

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A0036024

Memosens CPS97D

1

2

3

4

Advantages of the CDC90 fully automated cleaning and calibration system:

• Cleaning in place (CIP):
The sensor in the retractable assembly is automatically retracted from the medium for the
duration of the alkaline phase or for the entire CIP process. The sensor is then rinsed with a
suitable cleaning agent in the rinse chamber.

• Calibration cycles can be set individually

• Reduced maintenance thanks to fully automated cleaning and calibration

• Optimum reproducibility of the measurement results

• Very low individual value tolerances thanks to automatic calibration

Wastewater

A0024535

 4 Wastewater measuring system

1 ISFET sensor
2 Dipfit CPA111 immersion assembly
3 Memosens data cable CYK10
4 Liquiline CM42 transmitter

Properties

Acid or alkaline errors
Another important advantage over the glass electrode is the lower acid or alkaline errors in extreme
pH ranges. In contrast to the glass electrode, almost no foreign ions can build up at the ISFET gate.
Between pH 1 and pH 13, the measured error averages Δ pH 0.02 (at 25 °C (77 °F)) and is therefore
at the detection limit. The following graphic shows the average measured error of the ISFET sensor
in the pH 1 to 13 range compared with two glass electrodes (2 different pH glasses) at the extreme
values of pH 0.09 (1 M HCl) and 13.86 (1 M NaOH).

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Memosens CPS97D

Δ

pH

0

-0,05

-0,1

-0,15

0,05

0,1

0,15

0,2

0,25

0,3

0,35

1 M HCI

pH 1...3

1 M NaOH

ISFET CPSx7D

glass 1

glass 2

 5 Measured errors of the ISFET sensor compared to two different pH glass electrodes

• Resistance to breaking
The sensor's resistance to breaking is its most distinctive external feature. The entire sensor
technology is embedded in a PEEK shaft. Only the highly durable ISFET chip and the reference are
in direct contact with the medium.

• Measurement stability and sensor response time
The ISFET response times are extremely short over the entire temperature range. With the ISFET
sensor, there is no (temperature-dependent) equilibrium setting as in the gel layer of the pH glass
in a glass electrode. This means it can also be used at low temperatures without a deceleration in
the response time. The effect of large and fast temperature and pH value fluctuations on the
measured error (hysteresis) is smaller than with a glass electrode, as the stress on the pH glass
does not apply here.

Communication and data processing

Communication with the transmitter

Always connect digital sensors with Memosens technology to a transmitter with Memosens
technology. Data transmission to a transmitter for analog sensors is not possible.

Digital sensors can store measuring system data in the sensor. These include the following:

• Manufacturer data
– Serial number
– Order code
– Date of manufacture

• Calibration data
– Calibration date
– Calibration values
– Number of calibrations
– Serial number of the transmitter used to perform the last calibration

• Operating data
– Temperature application range
– Date of initial commissioning
– Hours of operation under extreme conditions
– Number of sterilizations
– Sensor monitoring data

Dependability Maintainability

Easy handling

Sensors with Memosens technology have integrated electronics that store calibration data and other
information (e. g. total hours of operation or operating hours under extreme measuring conditions).

A0038183-EN

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