Pressure Transducers

Pressure is not, as sometimes mistakenly thought, a force but a force per unit area (pressure = force/area). This is a fine distinction as far as a pressure transducer is concerned, for any device capable of converting applied mechanical force into some other form of energy is also, in fact, a pressure transducer converting a force per unit area into another form of energy. Thus, force transducers can also be used for pressure measurement, provided the area over which the force is applied is restricted and known. Also, because the application of pressure is commonly associated with resulting mechanical movement or displacement of something to which it's applied, a displacement transducer is also capable of pressure measurement.

The basis for all pressure transducers is a force-collecting member, which is some form of elastic element. The force resulting from applied pressure is then converted into some other form of energy, usually electrical energy. The main exception to this rule is the Baudon tube, or metallic bellows, which converts pressure energy directly into mechanical movement. These types of mechanical transducers are widely used as pressure gauges, less so as specified pressure transducers.

As an illustration of this distinction, a Baudon tube pressure gauge was once the common form of oil pressure gauge used with automobile engines. To work as a gauge it needed to be piped directly to the oil supply. The usual modern alternative is the pressure transducer mounted directly on the engine block at a suitable point exposed to oil pressure. The converts the oil pressure level into a proportional electrical signal, which is taken to an electric meter (millimeter) by wiring to indicate the pressure. Such a system has greater integrity because it's “fail safe.” In the event of a broken lead (or broken instrument), there is no possible loss of oil, only a loss of instrument reading. Also it's more readily adaptable to electrical warning signaling (e.g., tripping an electric current to operate a low-level warning light when the indicating instrument itself can be dispensed with).

However, a movable or “elastic” element remains a necessary feature of a pressure transducer; the usual form is a plain diaphragm, often of metal but sometimes of other materials, such as silicon. This forms the force-collecting member, the mechanical movement of which is then convertible into an electrical signal by a true transducer, such as a strain gauge, piezoelectric crystal, variable capacitance, or variable reluctance. The earlier forms of microphones using packed carbon particles behind a diaphragm were another (movable resistance) type of pressure transducer for sound-pressure-level conversion to electrical signals; these have been replaced with condenser and piezoelectric types.

Before we dismiss the Baudon tube entirely as a pressure transducer, though, it should be said that it does have certain useful applications. it's particularly suitable for accurate high-pressure measurement, e.g., up to about 100,000 lb/in Also, its mechanical movement can be connected to another type of transducer, such as a potentiometric device, strain gauge, or linear variable-differential transformer, to give an electrical signal out put. This is also the case with metallic bellows, which are far more sensitive than the Baudon tube for sensing low pressure (think of the aneroid capsules used in barometers, barographs, and altimeters). The primary disadvantages of the Baudon tube and metallic bellows as elastic elements in a pressure transducer are the relatively large volumes involved and their very limited frequency response because of their bulk and inertia. Thus, simple diaphragms are normally preferred.

DIAPHRAGM PRESSURE TRANSDUCERS

The most widely used type of elastic element in a pressure transducer is the flat circular diaphragm clamped all around its edge. Under applied pressure this produces an immediate deflection of the diaphragm, which can be sensed in a number of ways, such as with the center of the diaphragm mechanically coupled to the sensory element by a short pushrod (ill. 18-1), or deflection of the diaphragm monitored directly. The four main types of sensory elements used are resistance strain gauges, piezoelectric crystal, capacitance, and inductance or reluctance.

ill. 18-1. Edge-on diagram of the basic configuration for a pressure transducer.

STRAIN-GAUGE PRESSURE TRANSDUCERS

These are the most widely used types, with strain gauges bonded to, or into, the diaphragm. The trend is towards using semiconductor strain gauges, which provide high sensitivities in smaller sizes with lower input voltages and higher frequency capabilities. Sometimes the actual diaphragm is a silicon chip into which strain gauges are inorganically bonded and automatically diffused.

This solid-state integrated-circuit technique permits formation of micro-miniature strain gauges within the silicon diaphragm, enabling transducer diaphragms as small as 0.050 in. (1.27 mm) with active diameters as small as 0.028 in. (0.71 mm). This is not achievable with stainless steel diaphragms and precludes their use in applications requiring transducers smaller than 0.125 in. (3.18 mm) diameters. Other secondary advantages of silicon diaphragms are that they tend toward better stability of zero offset and long- term drift.

The major disadvantages of silicon diaphragms are their difficulties of providing water and chemical media protection and their tendency to shatter under particle impingement. Silicon is a brittle material, crystalline in structure, and can crack or shatter on impact. Protective screens are available to minimize this property, but they can't always be used conveniently.

Stainless steel diaphragms have the obvious advantages of ruggedness and the ability to maintain pressure seals even after electrical failure due to accidental overload. They are easy to waterproof and can be beam welded to resist a variety of corrosive media. How ever, they are not available in diameters less than 0.125 in. (3.18 mm) with suitable performance specifications.

Semiconductor transducers provide measurements from as low as 110 dB up to 50,000 psi and natural frequencies up to 1 MHz and higher. They are offered in gauge, sealed gauge, and absolute and differential modes and have the advantage of simplicity of application. All that is required is a stable power supply, and their inherently high output is sufficient to drive a variety of indicating and recording devices.

A pressure transducer may incorporate a reference pressure supply. This can have four options:

- Gauge-psig: Transducer is referenced to ambient pressure through an open reference tube.

- Sealed-psis: Transducer is referenced to 1 atmosphere pressure sealed within the transducer.

- Absolute-psis: Transducer is referenced to absolute zero pressure either by sealing a vacuum within the transducer cavity (true absolute) or electrically referencing to absolute zero within the compensation module.

LI Differential-psid: Transducer is referenced to second pressure source through the reference tube. Differential units must use a nonconductive non-corrosive media that will not affect the construction; e.g., water and media containing water may not be permissible. The reference port is the low-pressure side in all differential measurements.

CIRCUITRY

Strain-gauge pressure transducers may have a half-active bridge of two strain gauges or a fully active bridge with four strain gauges. In the former case the bridge circuit needs to be completed externally with two additional resistors or resistor decades. A typical half-bridge circuit is shown in ill. 18-2, R1 and R2 being the external resistors. Resistors Rc are calibration adjustment resistors, normally an integral part of the transducer.

Color coding is not always the same with different makes of transducers, although green is normally + out and white - out. Excitation input is red and black, which may be + and - or - and +, respectively.

ill. 18-2. A half-bridge circuit for use with pressure transducers (X).

FREQUENCY RESPONSE

Standard pressure transducers are normally single-degree-of freedom systems. The useful frequency range is linear within ± 5-20 percent of the resonant frequency. From 20 percent of resonance to resonance the pressure transducer increases sensitivity to a maximum. Above resonance the sensitivity decreases to a point at which the pressure transducer no longer responds to a pressure input (ill. 18-3). Most semiconductor pressure transducers operate in a dc or static mode (0 Hz). This allows the pressure transducer to read static base pressure as well as the dynamic pressure changes. Ex citing the transducer above its linear range (20 percent of resonant frequency) can be dangerous and should be done with extreme care.

ill. 18-3. Typical resonance curve for a pressure transducer.

INPUT and OUTPUT IMPEDANCES

In the case of Entran pressure transducers, the nominal resistance of the bridge (and its individual strain gauges) is the output impedance. All units that are compensated for thermal sensitivity shift have a voltage-dropping resistor on the bridge input side (usually in the + In Voltage wire, but it can be — In Voltage wire or both). The input impedance is the nominal value of the drop ping resistor (in series with the bridge) plus the bridge impedance. For best results the readout instrumentation input impedance should be at least 1 M. For an impedance of 20 times the output impedance of the bridge, the sensitivity is reduced by 5 percent. With an input impedance of 50 times, the reduction is 2 percent; an input impedance of 100 times yields a 1-percent reduction. For application requiring the most stable zero offset drift, the high value of Entran’s custom option bridge impedance should be chosen. Zero offset drifting is a result of self-heating effects of the gauges. As the gauge impedance increases, the power dissipated for a given excitation voltage decreases.

When selecting optional transducer impedances, note that the actual resistance of semiconductor strain-gauge bridges varies greatly with temperature and , in some cases, may be as high as 20 percent per 100 degrees Fahrenheit. Therefore, bridge impedance can't be used as a form of impedance matching if tight matching is required. Optional input/output impedances will have similar ratios as the standard input/output impedance offered with the transducer in question. Therefore, if you select a specific optional input impedance, the output impedance will be dictated by the required specifications.

Typical nominal bridge impedance ranges that are available are 120 U, 250 U, 350 U, 500 U, 1000 U, 1500 U. Actual values of input and output impedances are recorded on each calibration sheet.

ZERO OFFSET

All transducers have some “null pressure” or “baseline” off set. When the transducer is powered, the “null pressure” output will not be exactly 0 mV. If the zero offset value is not specified in the data sheet, it's typically between ±10 mV, but in some cases it can be as high as ±15 mV. This usually represents a moderate percentage of full-scale output.

Most recording devices have their own zero-balance circuit to null out transducer off sets. If you zero the transducer yourself, don't shunt one of the transducers legs externally. This procedure will change the thermal zero shift compensation provided with the unit. don't shunt the pressure transducer to reduce zero offset. This will alter the thermal zero shift performance.

Adjustable zero offsets are also available from Entran as an option. A blackbox module can be provided with an adjustable zero- offset trim pot, or a no-charge five-wire output is available for you to install your own zero-offset trim pot. A tight zero trimming with fixed resistors can be expensive; the five-wire output is an ideal way of achieving zero offset control at no additional cost. On five wire versions, connecting two wires will revert to a standard four- wire system when adjustability is not needed.

Zero offset can also be affected adversely by transducer mounting. Any stresses placed on or near the diaphragm will automatically result in changes in the zero offset. These changes can also be thermally sensitive. Over-torquing of threaded transducers can have the same effect. For threaded devices a recommended installation torque is indicated on the calibration sheet. Zero offsets are trimmed at Entran with the indicated torque applied.

The zero offset will move to its final value while the pressure transducer is being “warmed up.” Typical warm-up times can vary from five minutes to several hours, depending upon the transducer and desired level of stability. For critical dc measurements, where ultimate stability is required, a 4-hour warm-up may be advisable. Once the zero reaches equilibrium, it will then exhibit a small drift with time. When minimal drifting is required, the following four alternatives yield better results:

- Choose high-impedance options. Because drifting is a function of power dissipated, the higher gauge resistance is more stable for a given input voltage.

- Operate the transducer with a lower input voltage. Power dissipated is proportional to the square of the input voltage. Using one-half the typical input voltage dissipates one-fourth the power.

- Specify that your pressure transducer is to be temperature compensated in the medium in which the device will be used.

- When making dynamic measurements, the output of the pressure transducer can be ac coupled. This completely eliminates the zero offset and all its effects may be ignored.

EXCITATION VOLTAGE

For a standard (Entran) unit the sensitivity is directly proportional to the input voltage. By lowering the voltage, with no other changes in circuitry, the sensitivity decreases, but all other specifications remain the same. If it's necessary to lower the in put voltage with either no reduction or a partial reduction in sensitivity, the value of the thermal sensitivity resistor (Rs) can be reduced. This allows more of the applied voltage to be directed to ward the bridge but, in turn, reduces the effectiveness of the thermal sensitivity compensation. The compensating resistor Rs is expressed as the difference between the input impedance (Rj) and the output impedance (R). With no resistor a typical thermal sensitivity shift (tss) might be approximately -15 percent per 100 degrees Fahrenheit.

The following relationships are approximate and indicate the effect of sensitivity change versus input voltage and thermal sensitivity shift for one particular series of transducers. Check the Entran directly for the particular series in which you desire this adjustment for specific values. By selecting the maximum tss you can accept and the desired input voltage, you can calculate approximate sensitivities.

Vo = 2 (V1/Vids) (Ro/(Rs+Ro)) Vods

Rs = Ro – [(tss) Ro/(-15%/100 deg. F)] for tss between 0 and -15%/ 100 deg. F.

Vids = Input voltage from data sheet

Vi = Desired input voltage

Vods = Sensitivity from data sheet

Vo = New sensitivity

tss = Thermal sensitivity shift (0 to -15%/100 deg. F.)

Ro = Output impedance form data sheet

Example: Entran’s EPN-300-100 pressure transducers

Vids = 15V

V = 3.5 mV/psi

R = 500 ohm

Would like to operate 10 Vdc with maximum thermal sensitivity shift of -5%/100° F.

V = 2.8 mV/psi

For maximum stability of zero offset, lower input voltages are recommended. Sensitivity can also be increased by placing a higher voltage on the pressure transducer.

PIEZOELECTRIC PRESSURE TRANSDUCERS

A cross section of a basic piezoelectric pressure transducer is shown in ill. 18-4. The quartz crystal is contained in a subassembly called the element. The element typically contains at least one but usually several quartz crystals, an insulator, a metal end piece, an electrode to collect the charge, and a fine wire that conducts the charge to the connector pin. The element is held together by a preload sleeve, which is a very thin metal cylinder. During assembly the sleeve is stretched slightly, then welded in place in the elongated condition. This preloads or clamps the components of the element tightly together, ensuring the most rigid structure by removing all relative motion between these components.

18-4. Construction of a piezoelectric pressure transducer.

The element is contained in a steel outer housing or case, which provides for the mounting and sealing of the transducer, contains the electrical connector assembly, and most importantly, supports the outer edge of the diaphragm.

The diaphragm is a thin metal disc that has two main functions. It seals and protects the quartz element and , as previously mentioned, converts the pressure to a compressive force that deforms the element, producing the charge output.

In a typical quartz pressure transducer the composite modulus of elasticity of the element is slightly less than that of the steel housing, because a small portion of its length is made up of quartz, whose modulus is one-third that of steel. This means that the element will deflect slightly more than the outer housing when subjected to pressure.

This presents no real problem for low-pressure transducers, and thin relatively flexible diaphragms can be used because deflections are very slight.

When it comes to higher-pressure measurement, however, more rigid construction may be necessary. In fact, spherical constructions based on a piston movement instead of a diaphragm, or thick flush-diaphragms have been developed for measuring pressure above 50,000 lb/in. Whatever the construction, application of pressure to a piezoelectric pressure transducer produces a change in stress across the crystal element, resulting in a change of charge. This charge is converted into a usable output voltage signal through a charge amplifier.

It is an inherent property of a piezoelectric pressure transducer that it only responds to changes in applied pressure and is thus only accurate for dynamic pressure inputs. There is no output when a steady pressure is applied.

Some manufacturers simulate a steady-state output by incorporating a long time constant circuit in the charge amplifier, but this is subject to low-frequency rolloff. The major advantages of the piezoelectric device are the relatively high inherent output and the very-high-frequency response that can be achieved. However, piezoelectric pressure transducers are not easy to install and use because of the charge amplifier, which also makes their initial cost high.

ill. 18-5. A capacitive pressure transducer can be connected into the resonant circuit of an oscillator, resulting in an output frequency that varies with the applied pressure.

CAPACITANCE PRESSURE TRANSDUCERS

Capacitance-type transducers employ a diaphragm and a fixed plate (ill. 18-5). Application of pressure to the diaphragm changes the dielectric strength between the plates and , hence, the capacitance of the device. This capacitance is normally used as part of a tuned circuit, where pressure applied will result in a change in frequency of output.

ill. 18-6. An inductive pressure transducer can be connected into the resonant circuit of an oscillator, resulting in an output frequency that varies with the applied pressure.

This type of transducer can be employed for pressure ranges from 5 to 50,000 psi with moderate frequency response. The capacitive device always requires associated electronic circuitry, which is a disadvantage in terms of cost per measurement and simplicity of application but does allow control of the output, such as 0-5 V or 4-20 mA for pressure transmitters. Capacitance pressure transducers are widely used as sound pressure transducers.

Capacitance pressure transducers can offer advantages where high accuracy and stability are required from small-size transducers and are suitable for interfacing with a wide variety of instrumentation.

INDUCTIVEIRELUCTANCE PRESSURE TRANSDUCERS

There are various types of pressure transducers working on the principle of variable inductance or variable reluctance. Their application is, however, limited by the relatively large mass of the moving iron element (an iron core attached to a diaphragm) and /or the fact that relatively large displacements are involved (as in the linear variable-differential transformer). Figure 18-6 illustrates the basic principle. Some of the more compact types use the diaphragm itself as the iron element, but they are still bulkier, more complicated, and less sensitive than piezoelectric or strain-gauge transducers.

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