Pressure measuring instruments with flexible elements

Pressure measuring instruments with flexible el-ements are the most common pressure measur-ing devices used today. They combine a high grade of measuring technology, simple operation, ruggedness and flexibility, with the advantages of industrial and therefore cost-effective production.

Figure 1.17 Piston with clearance compensation

This design can also be used in combination with the differential piston principle.

Deadweight testers are often used together with testing pumps.

1. Cylinder 2. Piston 3. Weight support 4. Calibration weight 5. Filling connection 6. Testing pump 7. Instrument to be tested 8. Handwheel

Figure 1.18 Testing pump with deadweight piston

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Figure 1.19 Flow chart of signal ouput, analog and digital, for instruments with flexible elements Needing no external power supply, they are the

best choice for most applications. The applications for pressure measuring instruments with a flexible measuring element range from highly automated chemical processes, i.e. in refineries or in plastics, pharmaceuticals and fertilizer production, to hy-draulic and pneumatic installations in mechanical engineering, and even pressure cookers. These types of pressure measuring instruments can also be found at all the critical process monitoring and safety points of today's highly important energy installations - from exploration wells to power sta-tions - as well as in environmental protection.

The principle behind these instruments is simple:

the pressure to be measured is channeled into the chamber of a measuring element where one or more of its walls are flexed in a certain direction by an amount proportional to the pressure. The amount of the flexure is small, usually from just a few hundredths of a inch to a maximum of one-half inch. This flexure is then usually converted into a rotational motion by a movement. The pressure is then read off a graduated scale. A case protects the complete measuring system against external forces and damage.

In many applications the movement of the pointer is also used to show the measurement signal in analog or digital form with electric or pneumatic measuring outputs. Figure 1.19 shows the input to output flow chart for pressure measuring instru-ments with flexible eleinstru-ments.

Various accessories allow these instruments to fit into the process or to adapt them for special mea-surement problems, i.e. high temperature process fluids.

The case and its components are not only used to hold the measuring system in position but also serve to protect the user in the event of a leak in the gauge.

Because pressure measuring instruments with flexible elements are so widely used, they will be described separately in Section 1.4.

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I 1.3.2.2 Electrical pressure sensors and

pressure measuring instruments Today many measuring principles are used in electrical pressure measuring instruments. Most methods are based on the measurement of a dis-placement or force. In other words, the physical variable "pressure" has to be converted into an electrically quantifiable variable. Unlike mechani-cal pressure measuring methods, this conversion requires an external power source for the pres-sure sensor.

This pressure sensor is the basis of electrical pressure measuring systems. While mechanical gauge element displacements of between 0.004 and 0.012 inches are standard, the deformations in electrical pressure sensors amount to no more than a few microns.

Thanks to this minimal deformation, electrical pressure measuring instruments have excellent dynamic characteristics and low material strain resulting in high resistance to alternating loads and long-term durability. It is also possible to manufacture electrical pressure measuring in-struments in very small overall sizes, i.e. by using semiconductor materials.

1 Pressure sensor element 2 Pressure sensing 3 Pressure transducer 4 Pressure measuring

instrument

Figure 1.20 Basic design of all electrical pressure measuring instruments

∆R = ρ • ∆ ( ) + ∆ρ • ( ) ^(1-19)

ρ = Specific electrical resistance l = Length of the resistor wire

s = Cross-sectional area of the resistor wire

S I S

I

Indicating and evaluating equipment such as mea-suring amplifiers, analog and digital displays, log-gers, controllers, etc. are only described in this book where necessary for better understanding of sensor principles.

Sensor types with strain gauges Semiconductor strain gauges (piezoresistive effect)

Semiconductor materials have been used for elec-trical pressure measuring purposes since the middle of the nineteen-sixties. Pressure sensors based on semiconductor materials (mainly silicon) are continuously being improved in parallel to de-velopments in microelectronics.

The principle behind these instruments is de-scribed by the following equation, which defines the change of resistance in a tensioned wire.

If this fixed-length wire is subjected to a force from all sides, its resistance changes as described.

Figure 1.21 Pressure transmitter with freely suspended wires

The first part of the equation:

ρ • ∆ ( ) ^(1-20)

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S I S

I

Figure 1.22 Wafers

Further key steps include the production of dia-phragms by etching or boring. The resistors are attached (doped) into these diaphragms. Normally the resistors are attached on the edges of the dia-phragm because this is where the greatest changes of stress - and therefore the biggest changes of resistance - occur when pressure is applied. Resistance changes equal to around 10%

of the nominal resistance value occur under pres-sure.

This silicon wafer (also known as the system wa-fer) is then attached to a carrier wafer in an alloy-ing process. This carrier wafer is made of the same material as the system wafer.

Finally, the carrier wafer is drilled with a hole for relative pressure measurements and then split into chips. The silicon chip is an elementary sen-sor with a very small overall size.

describes the change of electrical resistance caused by the change of conductor geometry. In the elastic range the elongation of the wire is ac-companied by a corresponding reduction of its cross-sectional area. Close examination of the change of resistance in an electrical conductor makes this effect clearer:

R = ρ • ^(1-21)

If the length l increases, it is clear that there will be a reduction of the cross-sectional area, result-ing in an increase of the resistance R.

The second part of the equation

∆ ρ • ( ) ^(1-22)

describes the piezoresistive effect, which leads to a considerable change of resistance when sub-jected to mechanical loading. In semi-conductor materials this change of resistance is due to the changed mobility of electrons in the crystalline structure. With semiconductor materials (mostly silicon) the change of resistance is about 100 times greater than with metallic materials, ing very small pressure sensors while still allow-ing very small measurallow-ing ranges into the "H₂O range on the other.

In the production of silicon sensors the base material is a specially grown silicon monocrystal.

This monocrystal is then cut into wafers, paying attention to the orientation of the crystal structure because monocrystals display various properties in different directions (anisotropic).

The wafer is then polished. All the other process-ing steps, i.e. ion implantation, dopprocess-ing, etc., are common to the processing of silicon in electron-ics production and are not explained in this book.

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15 Figure 1.24 View of a silicon chip

Figure 1.25 Basic design of a Wheatstone bridge circuit

In practice the bridge is connected to more resis-tors for balancing, for temperature compensation and for setting the nominal sensitivity.

The bridge is said to be balanced when the out-put signal is U_M = zero. This is the case when the ratio of the resistors R₁:R₃ is the same as the

ra-1 Base plate of the housing 2 Glass enclosure 3 Kovar^R center tube 4 Gold-Tin solder

5 Relative pressure vent hole 6 Silicon carrier wafer 7 Metal alloy 8 Silicon system wafer 9 Cavity

10 Silicon epoxy layer (corresponds to a pressure-sensitive diaphragm)

11 Aluminum bond wires

Figure 1.23 Basic design of a pressure sensor

To reduce the great temperature effects inherent with semiconductors, four resistors are joined together on the chip to form a Wheatstone bridge.

Figure 1.25 shows the basic design of a Wheat-stone full bridge.

← 1.5 x 1.5 mm →

← 4 x 4 mm →

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From this equation it is clear that the measured variable P and the change of resistance (∆R) are linearly proporational.

Strain foil gauges

For a long time, strain foil gauges were the most popular sensors for pressure measurement. Their main advantage is that they can be easily applied to any deformable body using adhesive materials.

The strain foil gauge usually consists of a carrier foil typically made of phenolic resin and measures between 0.2 and 0.6 thousandths of an inch (5 and 15 mm) thick. Since this foil is nearly always applied to metallic base materials, it also acts as an insulator. This carrier foil holds the metallic strain gauge, which consists of an approximately 0.2 thousandths (5 mm) thin winding-shaped measuring grid. The strain gauge material is usu-ally Constantan.

Figure 1.26 Different designs of strain foil gauge

(1-23) tio of the resistors R₂:R₄, i.e. the drop in voltage over R₁ is the same as the drop in voltage over R₂. As the result of the deformation caused by pres-surization, the resistors R₁ and R₄ become bigger and R₂ and R₃ smaller (active full bridge). The measurement signal is therefore the bridge cross voltage U_M3, which can then be processed into a standard industrial signal in the series-con-nected direct voltage amplifiers or carrier fre-quency amplifiers.

Resistive sensors are the most widespread sen-sors in industrial use.

U_S = U_B • + U₀

U_s = Signal voltage U_B = Supply voltage U₀ = Offset voltage

R (p) = Pressure-dependent resistors

Metallic strain gauge

The principle of the metallic strain gauge was dis-covered in 1843 by the physicist Wheatstone.

Since then it has been used in various applica-tions, including electrical pressure measurement.

Pressure sensors with metallic strain gauges gen-erally differ in the way they are applied to the de-formable bodies. All these strain gauges are gov-erned by the following equation:

= k • ^(1-24)

R, ∆R = Resistance, change of resistance K = Constant proportionality factor

E = Modulus of elasticity of the spring material F = Mechanical force (in our case F = p · a

proportional to the pressure) A = Pressurized area

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17 a corruption of the output signal. These effects can only be minimized with elaborate compensation measures.

Furthermore, the strain foil gauge tends to creep under load due to the elasticity of the necessary bonding material. Modern strain gauge production techniques and specially formulated adhesives help to compensate for these negative factors but cannot eliminate them completely.

Thick-film strain gauges

Thick-film technology has been successfully used for several years in the production of electronic circuits. It represents a cross between an inte-grated circuit and an SMD chip (SMD = surface mounted device). The thick-film circuit (also called a hybrid circuit) is applied to a carrier material, which is usually made of an aluminium oxide ce-ramic (Al₂O₃) or a stainless steel diaphragm.

Thick-film technology allows resistors, insulating layers, conductors, and to a limited extent, even capacitors to be manufactured in an additive print-ing process.

Figure 1.29 Hybrids for sensor applica-tions

Different paste materials exist for the various lev-els of resistance. Strain gauges can be printed with these pastes on a substrate (carrier material).

These resistors are then baked onto the substrate in an oven at process temperatures of between around 1550°F and 1750°F (850°C and 950°C).

Figure 1.27 Pressure transmitter with a strain gauge glued into position

Figure 1.28 Pressure sensing with laminate-type sensor

To protect the thin strain gauge layer, an additional plastic film is applied over the strain gauge. Strain foil gauges are relatively easy to use and they can also be applied to curved surfaces (see Figure 1.28). They are still a popular choice, therefore, especially for the simple measurement of forces.

One of their big drawbacks, however, is the actual bonding, because this necessitates applying an additional inorganic binding agent between the deformable body and the strain gauges. Differ-ences between the coefficients of expansion of the materials used result in unequal elongation when the temperature changes, and this leads to

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Substrate materials such as Al₂O₃, which are also used as a diaphragm material for capacitive pres-sure sensors, have very good elastic properties and are virtually free of hysteresis. Another advan-tage of this technology is that it allows the possi-bility of installing the complete compensation and evaluation electronics on a substrate by the SMD method in a single operation.

Figure 1.30 Thick-film pressure sensor

Figure 1.30 shows that the diaphragm with the printed resistors is joined to the ceramic carrier using a glass solder. As with all deformable dia-phragm bodies, the force-related measuring range can be varied by altering the diaphragm thickness or size.

For many industrial pressure measuring applica-tions, ceramic is usually not compatible with pro-cess media. It is possible, however, to apply the strain gauge to stainless steel diaphragms. These metallic diaphragms require an additional insulat-ing layer between the resistors and the diaphragm.

When choosing the materials for the metallic dia-phragm it is important to select steels which have only slight scaling at the high process tempera-tures of up to 1750°F (950°C).

Very high production capacities have been devel-oped over the past few years for this technology.

Therefore, for applications requiring large num-bers of gauges, thick-film technology is an eco-nomical alternative to the strain foil gauge.

Thin-film strain gauges

The most modern strain gauge production pro-cess is based on thin-film technology. It combines all the advantages of the conventional strain foil gauge without any of its disadvantages. The main advantages are very low temperature sensitivity and excellent long-term stability.

In most cases the deformable body is a dia-phragm with a simple circular shape.

Figure 1.31 Cross section of a circular diaphragm

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∆ρρ

σr = Radial stress σt = Tangential stress

Figure 1.32 Stress characteristic on the surface of a circular diaphragm

On thin-film strain gauges, four resistors are again connected together to form a Wheatstone bridge.

The resistors are positioned in those areas of the diaphragm where the greatest changes of stress occur. When pressure is ap-plied, the resistors experience the greatest elon-gation at the center of the diaphragm and the greatest compression along the edges. This re-sults in the following equation:

U_S = U_B • ( + k • ε) ^(1-25)

U_B = Voltage supply U_S = Signal voltage

ρ = Specific resistance of the bridge resistors Ri

∆ρ = Change of ρ with the elongation ε = Elongation

k = k factor

These strain gauges are made of exotic materials, i.e. NiCr and semiconductor materials such as sili-con. The main differences lie in their k values.

k values of various materials:

Material k value Application

NiCr approx. 2 Metallic strain gauge Si approx. 100 Semiconductor strain

gauge Ruthenium oxide approx. 15-20 Thick-film strain gauge

In the next section we will take a detailed look at the technology and production of pressure sen-sors with metallic strain gauges. The base mate-rials for the diaphragms consist of metals which have a low deformation hysteresis. CrNi steels are normally used in order to achieve a high degree of compatibility with the process media. Special materials such as Elgeloy or Hastelloy C4 are also used but only in specific applications due to their difficulty to process.

As for the diaphragm shape, a distinction is drawn between circular and annular diaphragms. The advantage of an annular diaphragm is that there is no balloon effect (additional elongation) acting on all four resistors and resulting in a linearity deviation.

Figure 1.33 Cross section of an annular diaphragm

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After the resistance layer has been applied in a thickness of 50 to 200 nm, the actual strain gauges are produced using photolithography in a wet etching process.

Figure 1.35 Sensor with thin-film strain gauges

Further insulating, passivation and contacting lay-ers are added, as in thin-film technology. It is pos-sible to include temperature compensating resis-tors in the sensor layout in addition to the strain gauges.

Figure 1.36 7 mm thin-film sensor element

Thin-film sensors are becoming increasingly im-portant, particularly for high-pressure measure-ment.

On an annular diaphragm, the strain gauges are positioned over the inner and outer bending edge, as shown in Figure 1.33. This is where the dia-phragm experiences the greatest changes of stress.

The production of thin-film pressure sensors is a combination of the high-precision mechanical fab-rication of a deformable body and the covering of this body with strain gauges in a variety of pro-cesses.

First, the thickness of the diaphragm must be kept to very close tolerances, mainly by lapping. The surface of the diaphragm is then prepared for the actual coating process by polishing to a maximum peak-to-valley height of approx. 0.1 µm. The next step is to apply an insulating layer to the pol-ished stainless steel membrane. This can be ac-complished, for example, by the PECVD process (plasma enhanced chemical vapor deposition).

In this process a coat of SiO₂ is applied to the dia-phragm surface. SiO₂ is similar to glass in its in-sulating properties. This layer is around 4 to 6 microns thick and has an insulating resistance of at least 2 mega-ohms.

The actual resistance layer is then applied by a sputtering process (cathode ray sputtering).

Figure 1.34 Sputtering process

This process is a controlled glow discharge and is performed under ultra high vacuum. The dia-phragm material and the insulating layer form a molecular bond which is required for a break-free compound that guarantees very good long-term stability.

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F

Strain gauge transmission principles

As mentioned in the previous section, the many types of strain gauges are used for electrical pres-sure meapres-surement. Strain gauges themselves only convert a deformation (elongation or com-pression) into a change of resistance, so they must be applied to a deformable body. From the equation for pressure:

p = (1-26)

it is clear that a specific defined area A is required for the creation of a deformation in order to deter-mine the force via pressure. Materials with very good elastic properties are used for these deform-able bodies. Stainless steels are used as a rule because of their elastic properties and good com-patibility with process media. Ceramic materials are also being used more and more often due to their good diaphragm properties. It is a character-istic of these materials to produce a strictly linear elongation (conforming with Hooke's law) on their surface when pressure is applied. This effect is used in the following conversion principles.

Diaphragm conversion

In most cases of diaphragm-based conversion, circular diaphragms or annular diaphragms are used. These diaphragms can be calculated and manufactured relatively easily.

The stress characteristic in a circular diaphragm is shown in Figure 1.32. The strain gauge is ap-plied to the side facing away from the medium.

One advantage of these diaphragms is that the measuring range can be adjusted by changing the diaphragm diameter or the diaphragm thickness.

When selecting the diameter and the thickness, it is normal to choose a diaphragm that has a maxi-mum elongation of around 1.1 to 1.3 inches/foot (900 to 1100 mm/m). This equals an elongation of around 0.1%.

The pressure ranges in which these circular and annular diaphragms are used are between around 15 PSI and approximately 60,000 PSI (1 to 4000 bar).

A different type of diaphragm-based conversion is applied in piezoresistive sensors based on semi-conductor materials. Since the diaphragm mate-rial (silicon) and electrical connection of the actual pressure sensor are very sensitive and incompat-ible with most media, the pressure must be di-rected onto the silicon diaphragm using a separat-ing diaphragm and a pressure transfer liquid. Sili-cone oil is mainly used as pressure transfer me-dium. Convoluted diaphragms made of stainless steel are then used on the side facing the me-dium.

Figure 1.37 An encapsulated

Figure 1.37 An encapsulated

In document BR RF Handbook en Us 18447 (Page 28-46)