Capacitive Pressure Transmitter | Manufacturer’s Guide & Product List

Capacitive Pressure Transmitters

Capacitive pressure transmitter is a type of pressure measuring instrument that measures pressure by measuring changes in capacitance. The deformation of the isolator caused by pressure is used to change the capacitance value. The pressure value is measured by measuring the change in capacitance and converting it into a corresponding electrical signal output.

Capacitive pressure transmitter is suitable for measuring pressure, differential pressure and absolute pressure of various liquids, gases and steams.

Capacitive Pressure Transmitter/Differential Pressure Transmitter Structure

The two pressures of the measured medium pass into the high and low pressure chambers and act on the isolation diaphragms on both sides of the delta element (ie, the sensitive element). The filling liquid in the separator and element is transferred to both sides of the measuring diaphragm.

The capacitive pressure transmitter is a capacitor composed of a measuring diaphragm and electrodes on both sides of the insulating sheet. When the pressure on both sides is inconsistent, the measuring diaphragm will be displaced, and its displacement is proportional to the pressure difference. Therefore, the capacitance on both sides is not equal, and through the oscillation and demodulation links, it is converted into a signal proportional to the pressure.

The working principle of the capacitive pressure transmitter is the same as that of the differential pressure transmitter, except that the pressure in the low pressure chamber is atmospheric pressure or vacuum. The A/D converter of the capacitive pressure transmitter converts the demodulator current into a digital signal, the value of which is used by the microprocessor to determine the input pressure value. A microprocessor controls the operation of the transmitter. Additionally, it performs sensor linearization. Reset measuring range. Engineering unit conversion, damping, square root, sensor trimming and other operations, as well as diagnostics and digital communications.

Below are our Sino-Inst’s Capacitive pressure transducer diagram.

Of course, in addition to capacitive pressure transmitters, we at Sino-Inst also produce other types of pressure transmitters. You can continue to check out other types of pressure transmitters:

Capacitive Pressure Sensor Working Principle

A capacitor consists of two parallel conductive plates with a small gap between them. Capacitance is defined as:

C=er e o A /d

  • e r is the dielectric constant of the material between the plates (1 in vacuum)
  • e 0 is the electrical constant, (equal to 8.854×1012 F/m)
  • A is the area of the board
  • d is the distance between the plates

Changing any variable will cause a corresponding change in capacitance. The easiest thing to control is spacing. This can be achieved by making one or both of the plates into a diaphragm that deflects in response to pressure changes. Typically, one electrode is a pressure-sensitive diaphragm and the other electrode is fixed.

An example of our Sino-Inst capacitive pressure sensor is shown below:

Capacitive Pressure Sensor Working Principle

A simple way to measure changes in capacitance is to make it part of a tuned circuit, usually consisting of a capacitive sensor and an inductor. This can change the frequency of the oscillator or the AC coupling of the resonant circuit.

Featured Capacitive Pressure Transmitters/Differential Pressure Transmitters

capacitive pressure sensor advantages and disadvantages

A capacitive sensor is a device that converts a measured change into a change in capacitance, which itself is a variable capacitor. Because this kind of sensor has the characteristics of simple structure, small size, good dynamic response, high sensitivity, high resolution, and can realize non-contact measurement, it is widely used in detection of displacement, acceleration, vibration, pressure, pressure difference, liquid level, etc. field.

The capacitive pressure transducer produced by our Sino-Inst has the following advantages:

(1) Good temperature stability

The capacitance value of the sensor generally has nothing to do with the electrode material and only depends on the geometric size of the electrode. And the loss of media such as air is very small, so as long as the mechanical properties such as strength and temperature coefficient are considered, the materials and geometric dimensions can be reasonably selected. Other factors (because the heat itself is minimal) have little impact.

The resistive sensor has resistance and generates heat after power supply; the inductive sensor has copper loss, eddy current loss, etc., which causes its own heating and zero drift.

(2) Simple structure and strong adaptability

Capacitive sensors have a simple structure and are easy to manufacture. It can work in various harsh environmental conditions such as high and low temperatures, strong radiation and strong magnetic fields. It has strong adaptability, especially can withstand large temperature changes, and can work normally under high pressure, high impact, overload and other conditions. It can measure ultra-high pressure and low pressure difference. It can also measure magnetic workpieces. In addition, the sensor can be made very small in order to achieve certain special requirements for measurement.

(3) Small electrostatic attraction

There is an electrostatic field between the two plates of the capacitive sensor. Therefore, electrostatic attraction or electrostatic moment acts on the plate. The size of the electrostatic attraction is related to the working voltage, dielectric constant, and distance between the electrode plates.

Generally speaking, this electrostatic attraction is very small. Therefore, only for elastic sensitive components with very small driving force, the measurement error caused by electrostatic attraction must be considered.

(4) Good dynamic response

Since the electrostatic attraction between the plates is very small (about a few 10-5N), the capacitive sensor requires very little energy. And because its movable part can be made very small and thin, that is, its mass is very light, its natural frequency is very high, its dynamic response time is short, and it can work at frequencies of several MHz, making it particularly suitable for dynamic measurements.

And because its dielectric loss is small, it can be powered at a higher frequency, so the system operating frequency is high. It can be used to measure high-speed changing parameters, such as measuring vibration, instantaneous pressure, etc.

(5) Non-contact measurement can be achieved and has an average effect

When the device under test cannot allow contact measurement, capacitive sensors can complete the measurement task. When using non-contact measurement, the capacitive sensor has an averaging effect, which can reduce the impact of the workpiece surface roughness on the measurement.

In addition to the above advantages of capacitive sensors, the electrostatic attraction between the electrode plates is extremely small, so the input energy required is extremely small. Therefore, it is particularly suitable for measurements with low energy input, such as measuring extremely low pressure, force and very small acceleration, displacement, etc. It can be made very sensitive, with very high resolution, and can sense displacements as small as 0.001µm or less.

(1) High output impedance and poor load capacity

The capacity of the capacitive sensor is limited by the geometric size of its electrode, which is generally tens to hundreds of pF, making the output impedance of the sensor very high. Especially when using AC power within the audio range, the output impedance is as high as 106~108Ω. Therefore, the sensor has poor load capacity and is easily affected by external interference, causing instability. In severe cases, it may even fail to work. Shielding measures must be taken, which brings inconvenience to design and use.

Large capacitive reactance also requires the resistance value of the insulating part of the sensor to be extremely high (above tens of MΩ), otherwise the insulating part will act as a bypass resistor and affect the performance of the sensor (such as reduced sensitivity). For this reason, special attention should be paid to the impact of the surrounding environment such as temperature, humidity, cleanliness, etc. on the insulation performance.

Although high-frequency power supply can reduce the output impedance of the sensor, the amplification and transmission are far more complicated than at low frequency, and the influence of parasitic capacitance increases, making it difficult to ensure stable operation.

(2) Parasitic capacitance has a great influence

The initial capacitance of the sensor is very small, but the “parasitic capacitance” such as its lead cable capacitance (1~2m wire can reach 800pF), the stray capacitance of the measurement circuit, and the capacitance formed by the sensor plate and its surrounding conductors is relatively large.

Reduced sensor sensitivity;
These capacitances (such as cable capacitance) often change randomly, which will make the sensor work unstable and affect the measurement accuracy. The amount of change may even exceed the change in capacitance caused by the measurement, causing the sensor to fail to work.

Therefore, there are requirements for cable selection, installation, and connection methods.

(3) Output characteristics are nonlinear

The output characteristics of the variable pole pitch capacitive sensor are nonlinear. Although the differential structure can be used to improve it, it cannot be completely eliminated.

The output characteristics of other types of capacitive sensors are linear only when the edge effects of the electric field are ignored. Otherwise, the additional capacitance generated by the edge effect will directly superimpose with the sensor capacitance, making the output characteristics nonlinear.

More Featured Pressure Measurement Solutions

The capacitive pressure transmitter uses the deformation of the isolator caused by pressure to change the capacitance value. The pressure value is measured by measuring the change in capacitance and converting it into a corresponding electrical signal output.

Capacitive transmitters are mainly used to measure parameters such as pressure, differential pressure, negative pressure, high static pressure, liquid level and absolute pressure of gases, liquids and steam, and then convert them into 4~20mA signal output.

Capacitive transmitters include GP type (gauge pressure), DP type (differential pressure), DR type (micro differential pressure), AP type (absolute pressure), HP type (high static pressure), LT type (flange liquid pressure) position transmitter), DP/GP type (remote differential pressure/pressure transmitter) and other types. The combination of capacitive transmitter and intelligent amplifier board can form an intelligent pressure transmitter, which can communicate with each other through a handheld terminal that complies with the HART protocol for setting and monitoring.

We at Sino-Inst supply various types of capacitive pressure transmitters and differential pressure transmitters. And can be configured with various mounting brackets, flange diaphragms, capillary tubes, etc. If you need to purchase capacitive transmitters, capacitive differential pressure transmitters, or have related technical questions, please feel free to contact us!

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