GENERAL DESCRIPTIONSensor Systems LLC Pressure Transducers provide a precise electrical voltage output as a function of applied pressure,
with an accuracy previously unattainable. Instruments are available to cover the pressure ranges of 0-0.1 psi through 0-10,000 psi gage,
absolute, and differential with linear or non-linear outputs that are tailored specifically for your application. Why Sensor Systems, LLC Pressure Transducers are the MOST ACCURATESensor Systems LLC was able to substantially eliminate resolution errors by utilizing the special conductive film
resistance element. Thus, the output accuracy is solely determined by the linearity of the sensing element (capsule or Bourdon Tube).
The resistance element is adjusted to compensate even for this error through the use of a special manufacturing technique outlined
below. TECHNICAL DATATypical features & performance of Sensor Systems LLC Pressure Transducers includes:
In all these areas Sensor Systems LLC pressure transducers are superior to LVDT, strain gage, capacitor,
semi-conductor and other types of pressure transducers. Sensor Systems LLC pressure transducers have equivalent accuracy and stability;
the response to high frequency input pressures is faster than that for the LVDT type and slower than that for the other types.
The pressure devices described in this catalog use an elastic pressure-sensing element acting as a prime mover for
positioning an electromechanical transducer, thereby providing an electrical output for a pressure input. In the low pressure ranges the
sensing element is a metal capsule consisting of a pair of symmetrical concentrically-corrugated diaphragms, welded together at their
outer rims to form a hollow flexible member having predictable motion when fluid or gas pressure applied internally or externally. In
the high pressure ranges the sensing element is a metal Bourdon Tube, consisting of a circular tube of oval cross-section, brazed closed
at one end, so that pressure applied internally or externally causes the circular tube to partially straighten and its closed tip to
move in a predictable path. In all cases the electro-mechanical transducer is a variable resistor, in which the moveable contact or
slider is driven by the motion of the capsule or Bourdon Tube. When voltage is applied to the ends of the variable resistor, the
moveable contact position appears on the output terminal as a proportionate fraction of the excitation voltage. Where a corrosive or electrically conductive media is used and enters only into the capsule (gage or Hi side of
Differential) the transducer construction is as shown in Fig. 1 and Fig. 2; where such a media enters the case, the construction is
altered to provide an isolation media for the electro-mechanical transducer. Here the case volume is filled with a neutral fluid and
isolated from the Lo port by a slack flexible membrane as shown in Fig. 3. Pressure caused by such media entering the Lo port is
transmitted to the pressure-sensing element through the neutral isolation fluid. Similarly the dimensions of the capsule and Bourdon Tube pressure elements determine the pressure range, vibration and shock sensitivity, as well as the quality of the operating characteristics listed above. In general, larger diameter thinner metal capsules are used at the lower pressures due to their greater flexibility; however such capsules, while more sensitive to input pressure, are also more sensitive to environmental vibration and shock; internal damping or static and dynamic balancing are provided to minimize these effects. High-pressure transducers using either relatively rigid capsules or Bourdon Tubes are less sensitive to these environmental factors. Capsules and Bourdon Tubes are available in a wide variety of sizes and configurations to permit the optimization of transducer operating characteristics. Likewise, the electromechanical transducer is optimized in the manufacture of each individual transducer through a
semi-automatic calibration process to compensate for pressure element non-linearity or errors induced by electrical loading in the
output and/or to provide an output which follows any desired variation with input pressure. For example, in the Altitude Transducer,
Model 7000, the output is adjusted to be proportional to distance above sea level. In the Model 2000 transducer, the output is adjusted
to be linearly proportional to input pressure.
Ideally, the output of a pressure transducer can be represented by a perfect straight-line relationship between input pressure and output voltage. In practice, this relationship is closely but not exactly achieved due to various inherent characteristics of the two principal components making up the transducer - the pressure sensing element and the electromechanical transducer. PRESSURE ELEMENT PERFORMANCE CHARACTERISTICSThe metal pressure-sensing element is an elastic member which, due to the crystalline structure of metals, does not behave as a perfect spring. In order of magnitude, first, its spring rate or the energy per unit deflection is not a constant but varies somewhat with temperature, depending upon the particular metal alloy, and varies also with the total deflection range; these variations in spring rate produce what is called Temperature Error in output in the former case, and Linearity Error in the latter case. Second, the actual deflection at the same pressure is different depending upon whether the pressure is reached in an increasing direction from a lower pressure or in a decreasing direction from a higher pressure; the difference between "upscale" and "down-scale" deflections at the same pressure is called Hysteresis Error. Third, a certain level of input pressure is required to overcome the inherent internal friction of the metal sensing element before an initial deflection takes place. This reflects itself in the absolute deflection as an uncertainty called Resolution Error. ELECTROMECHANICAL TRANSDUCER PERFORMANCE CHARACTERISTICSThe electromechanical transducer itself does not have a constant output per unit deflection over its entire deflection range, producing a Linearity Error. An effective Linearity Error is also produced by the electrical loading on the resistance element of the electromechanical transducer by the electrical devices outside the pressure transducer to which the moveable slider is connected. The amount of this error is a function of the ratio of resistance element terminal resistance to the effective load resistance. The effective load resistance should be specified when ordering the pressure transducer so that compensation for the loading error can be made during the manufacturing calibration process. Without such compensation the errors in output of Sensor Systems LLC Pressure Transducers due to load ratios greater than 0.001 are shown in Fig. 4. In addition, the moveable contact or slider requires a certain force level from the pressure element to overcome the friction of contact, resulting in a Friction Error. Even though the resistance element in the Sensor Systems LLC electro-mechanical transducer is one continuous smooth-surfaced unit, effects equivalent to Resolution Error and/or Repeatability Error are produced by the finite radius of the moveable contact, which in itself is a spring member. PRESSURE TRANSDUCER ACCURACYAs reflected in the actual measured output of the pressure transducer in a given environment, the maximum effect of all of these errors
in deviation from the perfect straight-line relationship between input pressure and output voltage is called the Static Error Band,
expressed as a % of Excitation Voltage. The Dynamic Error Band is the same as the Static Error Band but measured while the pressure
transducer is subjected to light vibration. In practice, the pressure transducer is frequently surrounded by vibration-producing
equipment, and the Dynamic Error Band is more representative of the actual error under those circumstances.
MEASURING PRESSURE TRANSDUCER ACCURACYThe magnitude of individual errors making up the output Static Error Band can be determined by applying to the individual pressure
transducer a repeated series of known input pressures in both an increasing and a decreasing direction, and recording the output
voltages at each known pressure value; output voltages should be recorded both before and after light tapping on the transducer. Mean
values of voltage output at each pressure point for the repeated runs can be calculated. The Friction Error at any pressure input is the
difference between a tapped and an untapped value of voltage output. The Repeatability Error at any pressure input reached in the same
direction is the difference between the mean value and the extreme value in the repeated series of tapped output voltages. The
Hysteresis error at any pressure input is the difference between the mean values of tapped output voltage when measured in the
increasing and decreasing pressure directions. The Linearity Error at any pressure input is the difference between the mean value of
tapped output measured in the increasing pressure direction and the selected reference line value at that pressure. The Resolution Error
at any pressure input is the smallest change in pressure input which produces an observable change in tapped voltage output, and can be
measured during the course of applying the repeated series known input pressures. Static Error Band is obtained by plotting the untapped
increasing and decreasing pressure values of output voltage for a single series of known input pressures, and observing the maximum
deviation from the straight reference line indicated by the type of linearity.
In addition to the normal specifications such as pressure range, media, environments, etc. used to define a particular transducer for its application, other items relevant to the electromechanical transducer are important and are often defined by system requirements. These include excitation voltage, source loading, power handling, accuracy, output loading, and output signal configuration. EXCITATION VOLTAGE, TERMINAL RESISTANCE POWERThe typical pressure transducer installation is shown schematically in Fig. 6. The power supply provides the Excitation Voltage E, either AC or DC, and the nominal scale factor of pressure transducer becomes Excitation Voltage/Pressure Range, volts/psi.
For example, with 20VDC Excitation Voltage, the output scale factor of a 0-20 psi pressure transducer is nominally 1 VDC/psi. When the nominal scale factor has been decided upon, the Terminal resistance RT of the transducer can be specified. From the power supply point of view only, two considerations enter into the specification of Terminal resistance. First, there is a practical limit to the ability of the resistance element in the transducer to dissipate the heat produced by the electrical energy or power fed to it by the power supply. This Power Rating is listed in the specification table of each transducer model, typically 0.2 watts. The Power applied by the power supply can be calculated from the equation P=EI=E2/RT, and must not exceed the Power Rating of the transducer to avoid damage to it. Using 2OVDC Excitation on a transducer rated at 0.2 watts, the minimum value of Terminal Resistance is calculated to be 2000 ohms. Such a resistance will draw 0.01 amperes of current from the power supply, and the second consideration is whether the power supply can deliver this current. Clearly, for a given Excitation Voltage (a given scale factor), the higher the Terminal Resistance, the cooler the transducer resistance element will run and the lower will be the current demands from the power supply. From the point of view of the input power, then, the Terminal Resistance should be specified as high as possible. TERMINAL RESISTANCE, LOADING LINEARITYWhile the highest Terminal Resistance improves the input power situation, the considerations of electrical loading and linearity call for the lowest Terminal Resistance j the final selection of Terminal Resistance actually becomes therefore, a compromise between these opposing considerations. Electrical loading comes about in the Detector resistance RL (or other following electronic equipment fed by the moveable contact of the pressure transducer) that acts as a shunt on the transducer resistance element and thereby distorts the output of the transducer to an extent determined by the ratio RT/RL.
Referring to the section "Electromechanical Transducer Performance Characteristics", above, it can be seen that for RT/RL
ratios less than 0.005, there is no significant effect of electrical loading on linearity of the output. Thus, for example, when the
effective electrical load resistance RL is 100,000 ohms, any Terminal Resistance less than 500 ohms will result in virtually no
deterioration in output linearity due to electrical loading: if the RL value were 20,000 ohms, any greater Terminal Resistance than 100
ohms will introduce a significant linearity error. In the prior section it was determined in an example that the power rating indicates
a minimum Terminal Resistance of 2000 ohms when the example transducer is excited by 20 VDC: any loading less than 400,000 ohms will
produce a significant linearity error in the output. OUTPUT VOLTAGE AND CURRENTWhen the resistance of the electrical load is infinite in value, no current flows from the pressure transducer
moveable contact, and the transducer output is purely voltage. This occurs in practice where, for example, the pressure transducer
resistance element forms two legs of an electrical bridge network; perhaps the other two legs are made up of a manually set variable
resistor used to establish a control set point, as in Fig. 7. When the voltage output of the pressure transducer exactly matches the
voltage output of the hand set resistor, at the null point, no current flows from the transducer moveable contact so that the effective
load resistance is infinite value. To perform accurately under circumstances of unfavorable electrical loading, it is necessary to interpose an
electrical isolation amplifier between the pressure transducer and the electrical load. Such an amplifier has the characteristics of
high input impedance so that the transducer feeds the equivalent of a high effective load resistance, typically 1 Megohm, and a low
output impedance so that the electrical load, be it pen motor coil, relay or any other current-driven device, is fed by a source of
relatively unlimited current. The Model 4105 P/l Transmitter not shown in this catalog is a self-contained device consisting of pressure
transducer, isolation amplifier power supply, which is intended to provide superior performance at low-cost in such applications. PRACTICAL CONSIDERATION AND PRECAUTIONS
Drawing current through the transducer moveable contact as a result of electrical loading produces distortions in the
output voltage, but drawing excessive current may also permanently damage the transducer. Such excessive currents may be typically drawn
in two ways when using the transducer. First, in testing or installing the pressure transducer, an ohmmeter may be connected between the
moveable contact terminal and the low-end terminal of the transducer variable resistance, as in A of Fig. 9. With no pressure applied
when the wiper is close to the end terminal or when input pressure is applied in such a way as to drive the moveable contact towards the
low-end terminal, the current through the moveable contact increases, to a maximum when the wiper is directly opposite this terminal.
Depending upon the ohmmeter internal voltage and resistance, relatively high currents may be drawn, damaging the resistance element area
close to the low-end terminal. A similar result may be obtained when any form of circuit testing equipment containing its own power
supply, such a circuit continuity testers, is connected to the moveable contact terminal of the pressure transducer. Second, in testing
or installing the transducer, the Excitation Voltage may be inadvertently connected between the moveable contact terminal, as in B of
Fig. 9. Obviously, the full excitation current will pass through the moveable contact and damage due to excessive current flow or
burnout of the resistance element will result. Such a condition of Excitation Voltage connected to the moveable contact terminal may be
artificially or momentarily introduced by circuit anomalies lying outside the pressure transducer. Analogous to electrical overload producing damage is the effect of input overpressure. All pressure transducers shown
in this catalog nominally provide for overpressures 20% greater than the specified pressure range. The general effect of overpressure
beyond the allowance is usually to strain the pressure-sensing element beyond its stable limit and at the same time drive the
electromechanical transducer beyond its range. The effect on transducer output is usually the destruction of its accuracy, if not the
complete disappearance of its output. A greater overpressure factor can be provided, to suit, within the limitations of dimensions and
materials available. The danger of inadvertent overload is greatest in differential pressure transducers where the reversal of Hi and Lo
inputs may occur by accident or failure further up the line. Likewise, in differential units, the absence of one pressure in the face of
the application of the other may produce an overpressure situation. Safety provisions should be made in all pressure circuitry to
minimize the possibility of overpressures.
VIBRATION: As a general rule, when Transducers are subjected to vibration levels above 500 Hz, a vibration isolator is
required to maintain operational accuracy. Vibration levels less than 500 Hz may not require such damping depending on the model, range
and G level.
PRESSURE TRANSDUCERS: To meet requirements of size, performance, environment and the like, special Transducers and/or
custom modification of standard models can be manufactured. Practically any combination of special requirements can be met due to the
versatility of the film resistance element.
NON-LINEAR OUTPUTS: A wide variety of non-linear outputs are available for applications such as airspeed, flow and
altitude measurements. In addition, non-continuous outputs in the form of switches can be supplied. Typical non-linear output functions
are shown in these diagrams. It is important to note that infinite resolution is an inherent property of the film-resistance element and
does not vary with functional outputs.
1. STATIC ERROR BAND represents the maximum deviation from the best straight line drawn through the coordinates of 0%
pressure range, 5% +/- 1% output voltage ratio, and 100% pressure range, 95% +/- 1% output voltage ratio. This band includes the effects
of linearity, friction, hysteresis, resolution, and repeatability, and is expressed as a percentage of Excitation Voltage.
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