Endress+Hauser CPS491D Specifications

Technical Information

Tophit CPS491 and CPS491D

ISFET Sensor for long-term stable pH measurement in media with high dirt loads Analog or digital sensors with Memosens technology

Application

• Process applications with: – Quickly changing pH values – Alternating temperatures and pressures

• Water purification and wastewater

• Media with high dirt loads: – Solids – Emulsions – Precipitation processes

With ATEX, FM and CSA approval for application in hazardous areas

Your benefits

• Resistant to breaking – Sensor body made completely of PEEK – Direct installation into the process, reduces effort and costs

for sampling and laboratory analysis

• Double-chamber reference system: – poisoning resistant – polyacrylamide free gel

• Application possible in heavily soiled media

• Application possible at low temperatures – Short response time – Constantly high accuracy

• Longer calibration intervals than glass electrodes – Lower hysteresis with alternating temperatures – Low measuring error after high-temperature loading – Almost no acid and alkaline errors

• With built-in temperature sensor for effective temperature compensation

Further benefits offered by Memosens technology

• Maximum process safety through contactless inductive signal transmission

• Data safety through digital data transmission

• Easy handling thanks to storage of sensor-specific data in the sensor

• Predictive maintenance possible thanks to registration of sensor load data in the sensor

TI377C/24/EN/05.10

Tophit CPS491/CPS491D

Si (p)

Si (n)

Si (p)

Si (n)

U =U+

GS 0

lg a

ion

2.3

Function and system design

Measuring principle Ion-selective, or more generally ion-sensitive field effect transistors (ISFET) were developed in the 1970s as

an alternative to the glass electrode for pH measurement.

Basics

Ion-selective field effect transistors use an MOS (pos. 1) is not a control electrode. Instead, the medium (see Fig 2, pos. 3) in the ISFET is in direct contact with the gate isolator layer (pos. 2).Two strongly N-conducting areas are diffused in P-conducting substrate (see Fig 2, pos. 5) of the semiconductor material (Si). These N-conducting areas are current supplying ("Source", S) and current accepting ("Drain", D) electrodes. The metallic gate electrode (in case of the MOSFET) resp. the medium (in case of the ISFET) forms a capacitor with the substrate below. A potential difference between gate and substrate (U

) causes a higher electron density between "Source" and "Drain". A N-conducting channel

(pos. 2) is formed, i.e. a drain current (I

) is induced.

transistor arrangement (see Fig 1) where the metallic gate

Fig. 1: Principle MOSFET

1 Metallic gate 2 N-conducting channel

With the ISFET, the medium is in direct contact with the gate isolator layer. Therefore, H medium, which are located in the medium / gate isolator boundary layer, create the electric field (gate potential). Depending on the effect described above, a N-conducting channel is formed and a current between "Source" and "Drain" is induced. Suitable sensor circuits use the dependence on the ion-selective gate potential to create an output signal proportional to the concentration of the ion type.

pH selective IsFET The gate isolator serves as an ion-selective layer for H

well (isolator effect) but allows reversible surface reactions with the H Depending on the acidic or alkaline character of the measurement solutions, functional groups in the isolator surface accept or reject H

ions (amphoteric character of the functional groups). This leads to a positive (H+ acceptance in the acidic medium) or negative (H surface. Depending on the pH value, a defined surface charge can be used to control the field effect in the channel between "Source" and "Drain".The processes which lead to the creation of a charge potential and therefore to a control voltage U

UGS ... U R ... T ... n ...

Potential between gate and source

...

Offset voltage

Gas constant (8.3143 J/molK) Temperature [K] electrochemical valueability (1/mol)

between "Gate" and "Source" are described with the Nernst equation:

a0003855

Fig. 2: Principle ISFET

1 Reference electrode 2 N-conducting channel 3 Gate isolator layer 4 Medium 5 P-doped silicon substrate 6 Sensor shaft

ions available in the

ions. The gate isolator is impermeable to the ions as

rejection in the alkaline medium) charging of the isolator

F ...

...

ion

ions.

Faraday constant (26.803 Ah) Activity of ion kind (H+)

Nernst factor

a0003856

1) Metal Oxide Semiconductor

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At 25 °C (77 °F), the Nernst factor is –59.16 mV/pH.

Tophit CPS491/CPS491D

pH (25 °C)

ΔpH

1 M NaOH1 M HCl

E+H ISFET

pH 1 … 13

Glass 1

Glass 2

-0.2

0.0

0.2

0.4

0.6

0.8

Important characteristics of Tophit CPS491

• Resistance to breaking This is the most obvious feature of the sensor. The complete sensor technology is embedded in a PEEK shaft. Only the highly resistant isolator layer and the reference have direct contact with the medium.

• Acid or alkaline errors A further, important benefit compared with the glass electrode is the considerably reduced number of acid or alkaline errors in extreme pH ranges. In contrast to glass electrodes, practically no foreign ions can build up at the ISFET gate. The measuring error of < 0.01 pH (between pH 1 and 13) at 25°C (77 °F) is near by the detection limit. The figure below shows the acid or alkaline error of the ISFET between pH 1 and 13 and the comparison to the glass electrode (two different pH glasses) at pH values 0.09 and 13.86.

Fig. 3: Comparison of acid and alkaline errors

a0003867-en

• Measurement stability and sensor response time The ISFET response times are very short over the whole temperature range. With the ISFET sensor, there is no (temperature-dependent) equilibrium setting as in the source layer of a pH glass of a glass electrode. They can also be used at low temperatures without a deceleration in response time.Large and fast temperature and pH value fluctuations have a smaller effect on the measuring error (hysteresis) than with a glass electrode, as there is no stress exerted on the pH glass.

• Reference system The integrated reference electrode of the sensor is a double-chamber reference system with a bridge electrolyte. The benefits are an efficient and stable contact between the diaphragm and the reference lead, and the extremely long poisoning path. The bridge electrolyte is highly resistant to temperature and pressure changes.

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Tophit CPS491/CPS491D

500

300

100

-100

-300

-500 0246 8101214

b c

• Isothermic curves – The Nernst equation defines the dependence of the measuring voltage on the hydrogen ion content (pH

value) and the temperature. It is the basis of pH measuring technology and for ISFET sensors too. A temperature-dependent value for the potential change per pH value can be worked out from this equation (isothermic curve, potential change per pH value at a defined temperature).

– The isothermic curves of the ISFET sensor are very close to the theoretical values (see Fig 4). This is further

proof for the high pH measurement precision of the sensor.

Fig. 4: Isothermic curves

a0003868-de

a Isothermic curve at 8 °C (46 °F), slope –55.8 mV/pH b Isothermic curve at 37 °C (99 °F), slope –61.5 mV/pH c Isothermic curve at 61 °C (142 °F), slope –66.3 mV/pH

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Tophit CPS491/CPS491D

Memosens (CPS491D) Maximum process safety

The inductive and non-contacting measured value transmission of Memosens guarantees maximum process safety and offers the following benefits:

• All problems caused by moisture are eliminated. – The plug-in connection is free from corrosion. – Measured value distortion from moisture is not possible. – The plug-in system can even be connected under water.

• The transmitter is galvanically decoupled from the medium. The result: No more need to ask about "symmetrically high-impedance" or "unsymmetrical" (for pH/ORP measurement) or an impedance converter.

• EMC safety is guaranteed by screening measures for the digital measured value transmission.

• Application in explosion-hazardous areas is unproblematic; the integrated electronics are intrinsically safe.

Data safety through digital data transfer

The Memosens technology digitalizes the measured values in the sensor and transfers them to the transmitter contactlessly and free from interference potential. The result:

• An automatic error message is generated if the sensor fails or the connection between sensor and transmitter is interrupted.

• The availability of the measuring point is dramatically increased by immediate error detection.

Easy handling

Sensors with Memosens technology have integrated electronics that allow for saving calibration data and further information such as total hours of operation and operating hours under extreme measuring conditions. When the sensor is mounted, the calibration data are automatically transferred to the transmitter and used to calculate the current measured value. Storing the calibration data in the sensor allows for calibration away from the measuring point. The result:

• Sensors can be calibrated under optimum external conditions in the measuring lab. Wind and weather do neither affect the calibration quality nor the operator.

• The measuring point availability is dramatically increased by the quick and easy replacement of precalibrated sensors.

• The transmitter does not need to be installed close to the measuring point but can be placed in the control room.

• Maintenance intervals can be defined based on all stored sensor load and calibration data and predictive maintenance is possible.

• The sensor history can be documented on external data carriers and evaluation programs at any time. Thus, the current application of the sensors can be made to depend on their previous history.

Communication with the transmitter

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

The sensor is connected to the cable connection (CYK10) without contact. The power and data are transferred inductively Once connected to the transmitter, the data saved in the sensor are read digitally. You can call up these data using the corresponding DIAG menu.

Data that digital sensors save 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

• Operational data – Date of commissioning – Hours of operation under extreme conditions – Number of sterilizations – Data for sensor monitoring.

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