With Pressure Sensors, changes in pressure
can be measured to confirm suction, verify mounting, manage source
pressures, and test for leaking. Differential Pressure Flow Sensors are
also available.
What Are Pressure Sensors?
A pressure sensor is a device equipped with a pressuresensitive element
that measures the pressure of a gas or a liquid against a diaphragm made
of stainless steel, silicon, etc., and converts the measured value into an
electrical signal as an output.
Operating Principles
A semiconductor piezoresistance dispersion pressure sensor has a
semiconductor distortion gauge formed on the surface of the diaphragm, and
it converts changes in electrical resistance into an electrical signal by
means of the piezoresistance effect that occurs when the diaphragm is
distorted due to an external force (pressure).
A static capacitance pressure sensor has a capacitor that is formed by a
static glass electrode and an opposing movable silicon electrode, and it
converts changes in static capacitance that occur when the movable
electrode is distorted due to an external force (pressure) into an
electrical signal. (The E8Y uses the static capacitance method, and other
models use the semiconductor method.)
Semiconductor Distortion Gauge Construction
Piezoresistance Effect
The electrical resistance of the above conductor is expressed by the
following formula:
R = ? × L/S.
When this conductor is pulled to the right or left as shown below, the
length increases and the crosssectional area decreases.
The electrical resistance of the above conductor is expressed by the
following formula:
R' = ? ×(L+1)/Ss.
Accordingly,
R' > R.
This shows how the application of a mechanical force changes the
electrical resistance.
Explanation of Terms
Gauge Pressure
The amount of pressure is expressed in terms of atmospheric pressure. It
is referred to as "positive pressure" when it is greater than one
atmosphere, and "negative pressure" when it is less than one atmosphere.
Absolute Pressure
This is the amount of pressure expressed in relation to an absolute
vacuum.
Pressure Difference (Relative Pressure)
This is the amount of pressure compared to any particular pressure (the
reference pressure).
Atmospheric pressure
The pressure of the atmosphere. The standard atmospheric pressure (1 atm)
is equal to the pressure of a column of mercury with a height of 760 mm.
Vacuum
A pressure less than one atmosphere.
Pressure Detection Range
The range of pressure that can be detected by the Sensor.
Withstand Pressure
The pressure that must be withstood without degraded performance after
returning to the pressure detection range.
Repeat Accuracy (ON/OFF Output)
Repeat accuracy refers to the deviation in the operating point when the
output inverts while pressure is increased or decreased at a temperature
of 23°C, divided by the full scale of the pressure detection range.
Accuracy (linear)
This is the deviation from the rated output current (4 mA, 20 mA) when
zero pressure and the rated output are applied at a temperature of 23°C,
divided by the fullscale value.
It is expressed in units of %F.S.
Linearity
The analog output changes in an approximately linear fashion with respect
to the detected pressure. The change, however, deviates slightly from an
ideal straight line. This deviation is expressed as a percentage of the
full scale.
Linear Hysteresis
An ideal straight line is drawn between the output current (or voltage) at
zero pressure and the rated current (or voltage), and the difference
between the measured current (or voltage) and the ideal current (voltage)
is obtained as an error. The error as the pressure rises and the error as
the pressure falls are obtained, and the maximum value of the absolute
value of the difference between the rising error and falling error is
divided by the full scale current (or voltage). This is the linear
hysteresis, and it is expressed in units of %FS.
Hysteresis (ON/OFF Output)
The difference between the output ON pressure and OFF pressure is divided
by the full pressure scale.
Noncorrosive Gas
Substances contained in the air (nitrogen, carbon dioxide, etc.) and inert
gases (argon, neon, etc.).
Measurement Method
A new Measurement Law was enacted in Japan on November 1, 1993. This law
prohibits the use of Torr for anything except internal human body pressure
measurements. From September 30, 1999, the use of kgf/m², mHg (except for
blood pressure measurements), and mH_{2}O was prohibited.
Pressure Unit Conversion Table

kgf/cm² 
mmHg 
mmH_{2}O 
Pa 
1kgf/cm² 
1 
735.559 
1.000028 x 10^{4} 
0.0980665M 
1mmHg 
1.3595
x 10^{3} 
1 
1.3595
x 10 
0.133322k 
1mmH_{2}O 
0.99997
x 10^{4} 
7.356 x
10² 
1 
0.00980665k 
1Pa(N/m_{2}) 
1.0197
x 10^{5} 
7.5006
x 10^{3} 
0.10197 
1 
Output Impedance
1. Measuring the Output Impedance of Voltage Output
Models
Figure 1
Ro : Output impedance
Rx : Load resistance
Eo : Output voltage (terminals open)
Ex : Output voltage (with load Zx connected)
Ix : Load current (with load Zx connected)
In Figure 1, the current (Ix) that flows when the load resistance (Rx) is
connected is calculated as follows:
The output impedance (Ro) in Equation (1) is calculated as follows:
The voltage (Eo) is measured when the output is open, followed by the
voltage (Ex) when a load resistance (for example, the minimum value of the
permitted load resistance of a transducer) is connected.
The measured values Eo and Ex and the connected load resistance (Rx) are
inserted into Equation 2 to calculate the output impedance (Ro) of the
transducer.
2. Measuring the Output Impedance of Current Output
Models
In Figure 2, the voltage (Ex) of the output terminals when the load
resistance (Rx) is connected is calculated as follows:
The output impedance in Equation (3) is calculated as follows:
Here, the current (Io) is measured with the output shortcircuited.
Figure 2
Ro : Output impedance
Rx : Load resistance
Io : Output current (output terminal shortcircuited)
Ix : Output current (with load Rx connected)
Ex : Output voltage (with load Rx connected)
Next, the output current (Ix) is measured when a load resistance (for
example, the maximum value of the permitted load resistance of a
transducer) is connected. The measured values Io and Ix and the value of
the connected load resistance (Rx) are inserted into Equation 4, and the
output impedance (Ro) of the transducer is calculated. The output
impedance of the transducer introduced here is the value for normal
operation.
3. Desirable Output Impedance
In general, it is best to make the output impedance of a voltage output
transducer as small as possible, i.e., as close to 0 W as possible, to
minimize the effects of load fluctuations on the transducer. For a current
output transducer, the opposite is true: the higher the impedance (the
closer to infinite impedance), the better.
4. Example of Calculation Using Impedance
Rx 
Error 
1kO 
Approximately 10% 
10O 
Approximately 1% 
