Multiple-function fluid measuring and transmitting apparatus

A measuring and controlling system is provided which has a control function as well as a measurement function to perform the measurement and control of fluid amounts such as the flow rate, pressure, liquid level, weight and so on by itself in the same field. A pressure receiving unit is directly mounted on a pipe or a container, in which a fluid under measurement exists, in order to detect the temperature, differential pressure and static pressure of the fluid under measurement independently of each other. Means for storing the characteristic of fluids under measurement is provided such that the mass flow rate, weight and liquid level can be calculated. By providing the transmitter with the measurement and control functions, the measurement and control of fluid amounts such as flow rate, pressure, liquid level, weight and so on can be performed in a closed form, thereby making it possible to simplify a correction procedure for the static pressure and temperature, improve the responsibility, and enhance the control performance of the whole plant. Also, a measuring and controlling system can be realized which reduces a wiring amount for connecting with an upper level control unit.

BACKGROUND OF THE INVENTION 
The present invention relates a novel multiple-function fluid measuring and 
transmitting apparatus and a measuring, controlling and transmitting 
system for fluids in a plant which have the function of a differential 
pressure transmitter which is widely used as measuring equipment for 
detecting a flow rate, pressure or liquid level in a chemical plant and so 
on as well as a control function such as a loop controller. 
An example of a conventional differential transmitter is shown in FIG. 14. 
The pressure is delivered to be measured by differential transmitter 100 
through high pressure pipe 158 and three valves 157 provided at the 
opposite ends of orifice 560 installed at a part of pipe 550 through which 
water to be measured flows. 
Conventional differential transmitters are made so large and heavy for 
protecting a sensor disposed therein that they cannot be directly 
installed on a pipe 550 through which a fluid under measurement flows. 
Also, due to their low reliability, they are removably mounted on a 
special post 167 separate from a pipe or a container, so as to facilitate 
a regular maintenance. For this reason, such a differential pressure 
transmitter requires not only a pressure introducing pipe durable to a 
high pressure and a valve but also gas and drain releasing means 157, 
thereby causing a cost to be increased. 
Since the detection of the flow rate requires a sensing of a temperature T 
and a static pressure Ps of a fluid under measurement, a temperature 
transmitter and a pressure transmitter are disposed on an identical pipe 
line to measure these two parameters and transmit them to an upper level 
control unit which in turn calculates the flow rate from the measured 
parameters. In other words, two or more transmitters respectively for 
sensing a pressure, differential pressure and temperature are used to 
calculate the mass flow rate from measured parameters as shown in U.S. 
Pat. No. 4,562,744. 
Recently commercialized intelligent differential pressure transmitters are 
also used in accordance with this convention, so that even if they are 
provided with a composite sensor constituted of differential pressure, 
static pressure and temperature sensors formed on a single substrate, the 
temperature of a fluid under measurement cannot be correctly measured 
because the fluid is cooled by a pressure introducing pipe, whereby the 
temperature sensor is presently used for merely correcting the 
characteristic of the differential pressure transmitter. 
For detecting the flow rate, therefore, a temperature transmitter and a 
pressure transmitter both are disposed on an identical pipe line, as 
mentioned above, for sensing a temperature T and a static pressure Ps of a 
fluid under measurement and transmit them to an upper level control unit 
which performs a calculation to measure a flow rate. For this reason, this 
method is problematic in that the measurement and control of a rapidly 
fluctuating mass flow rate is particularly difficult, and an additional 
cost is needed as there is required two or more transmitters. 
Further, the above-mentioned conventional differential pressure transmitter 
is so large and heavy that it cannot be directly installed on a pipe 
through which a fluid under measurement flows and therefore is separately 
mounted on a special post apart from a pipe or a container so as to 
facilitate removal of the differential pressure transmitter since the 
reliability is so low that a regular inspection is necessary. This 
installation requires not only a pressure introducing pipe durable to a 
high pressure and a valve, but also gas and drain releasing means, thereby 
resulting in increasing a cost. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a multiple-function 
fluid measuring apparatus which is capable of singly measuring a 
differential pressure, a pressure, a temperature, a mass flow rate of a 
fluid under measurement as well as a liquid level and a mass of a liquid 
in a container, in other words, to provide a multiple-function fluid 
measuring apparatus provided with three functions conventionally performed 
by differential pressure, pressure and temperature transmitters. 
It is another object of the present invention to provide a 
multiple-function fluid measuring apparatus of a type directly mountable 
on a pipe or a container which is economical and so reliable that no 
maintenance is necessary. 
It is a further object of the present invention to provide, in addition to 
the above multiple-function fluid measuring apparatus having three 
measuring functions, a multiple-function fluid measuring apparatus which 
is capable of controlling an actuator on the basis of a calculated 
deviation between a measured value and a target value. 
It is a further object of the present invention to employ, in a fluid 
plant, the multiple-function fluid measuring apparatus having a control 
function to highly accurately control the temperature, pressure, mass flow 
rate, liquid level and mass. 
It is a further object of the present invention to provide an economic 
control managing system for a fluid plant with a good responsibility by 
using the above-mentioned multiple-function fluid measuring apparatus 
having a control function. 
To achieve the above object, a pressure receiving unit is adapted to be 
directly mountable on a conduit or a container, whereby a composite sensor 
provided therein detects a temperature as well as differential pressure 
and pressure of a fluid under measurement. Further, the mass flow rate of 
the fluid under measurement in a conduit and the liquid level and mass of 
a fluid in a container are calculated from a characteristic map of the 
fluid which has previously been prepared in a memory serving as storage 
means. 
Further, to achieve the other objects, parts which have been employed in a 
conventional transmitter are reduced in number, size and weight by using 
the semiconductor IC technology to simultaneously enhance the economical 
efficiency and reliability. Also achieved are the remote setting by the 
use of digital communications, elimination of the necessity of maintenance 
by diagnosis and update operations, and improvement of the reliability. 
To achieve the other objects, a transmitter is provided with a 
bi-directional communication circuit for receiving and transmitting 
measured values of fluid amounts, such that a deviation between a target 
value set and stored by instructions from an upper level control unit or a 
portable communicator and a currently measured value is calculated by a 
calculating and storage means, and a control amount is directly 
transmitted to an actuator in accordance with a predetermined algorithm, 
thereby controlling fluid amounts such as pressure, mass, liquid level, 
flow rate and temperature. 
Also, measured values, measuring conditions and control states of the fluid 
amounts to be measured and controlled, and the result of self-diagnosis of 
the apparatus are remotely monitored via digital communications by the 
upper level control unit or the portable communicator. Since the fluid 
amount measuring and controlling apparatus is provided with a means for 
displaying these data and results, the states of the apparatus and the 
system can be monitored even in the field. 
To achieve the other objects, until the calculating and storage means in 
the fluid amount measuring apparatus receive a preferential instruction 
from the upper level control unit or the portable communicator for 
changing the setting of target values, control amounts are directly 
transmitted to related actuators and resulting fluid amounts are measured 
again in accordance with a previously stored algorithm for performing a 
control of the fluid amounts corresponding to a previously set target 
value in the field. By thus eliminating the necessity of communications 
with the upper level control unit and correspondingly reducing a time 
required for the communications, an in-field closed control system with a 
fast response is provided. 
A pressure receiving unit is directly mounted on a conduit or a container 
so as to allow the multiple-function fluid measuring apparatus to directly 
contact a fluid under measurement, thereby making it possible to detect 
the temperature as well as static pressure and differential pressure of 
the fluid under measurement from signals generated by static pressure, 
differential pressure and temperature sensors formed inside the pressure 
receiving unit. 
The bi-directional communication circuit provided in the multiple-function 
fluid measuring apparatus allows the apparatus to mutually perform digital 
communications with the upper level control unit or the portable 
communicator. Thus, deviations between target values which have been set 
and stored by instructions from the upper level control unit or the 
portable communicator and currently measured values are derived by the 
calculating and storage means, and control amounts calculated on the basis 
of the deviations are directly transmitted to actuators in accordance with 
a predetermined algorithm to control fluid amounts such as pressure, flow 
rate and temperature. 
Likewise, measured values, measuring conditions and control states of the 
fluid amounts to be measured and controlled, and the result of 
self-diagnosis of the apparatus are remotely monitored via digital 
communications by the upper level control unit or the portable 
communicator, and the fluid amount measuring apparatus is provided with a 
means for displaying these data and results, whereby the states of the 
apparatus and the system can be monitored even in the field, thus 
achieving the improvement in reliability of the apparatus and the system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of apparatus for implementing the present invention will 
hereinafter be described with reference to FIGS. 1 to 13. 
Generally, a flow rate is detected according to the following equation (1) 
by a differential pressure .DELTA.P developed when a liquid pressurized by 
a pressurizing pump passes through an orifice arranged in a pipe line: 
##EQU1## 
where Q represents a volume flow; C a flowout coefficient; .beta. a ratio 
of the diameter of a restriction hole to the inner diameter of a conduit; 
.epsilon. a gas expansion coefficient; .rho..sub.1 a correction 
coefficient of the density (upstream side) (which is proportional to the 
temperature and pressure in the case of a gas); and Fr the dimension of 
the restriction hole. 
The output from a differential pressure sensor is influenced by a static 
pressure Ps and a temperature T, if these parameters largely fluctuate. 
Also, when a flow rate is to be measured, a static pressure Ps added to a 
pipe line and a change in ambient temperature act as external disturbances 
which must be removed to the utmost. Specifically, for correcting the 
characteristic of a transmitter, an intelligent differential pressure 
transmitter is employed which comprises a composite sensor having a 
differential pressure sensor, a static pressure sensor and a temperature 
sensor formed on a single substrate. 
FIG. 1 illustrates an example of a fluid measurement and control system for 
a chemical plant or the like according to the present invention; FIG. 2 
illustrates a modified example of the system shown in FIG. 1; and FIG. 3 
is a flow chart illustrating an algorithm of a measurement and control 
method of the present invention. A fluid measuring and transmitting 
apparatus 300 of the present invention is directly installed on a pipe 
line or a container for measuring and controlling physical amounts of a 
fluid under measurement, as will be later described. 
Description will now be made for the case where a flow rate is measured and 
controlled. Referring to FIG. 1, a differential pressure .DELTA.P 
developed on both sides of a differential pressure generating element 560 
such as an orifice arranged in the middle of a pipe line 550, a static 
pressure Ps and a temperature T are measured, and transmitted to the fluid 
measuring and transmitting apparatus 300 for calculating a flow rate in 
the pipe line and to a control unit 500 as electric signals. 
Simultaneously, a deviation value .DELTA.Q is calculated, and a control 
amount .DELTA.v is sent to a valve actuator 200 which in turn controls a 
valve in accordance with the sent control amount to control the flow rate 
of a fluid in the pipe line 550. This procedure is executed in the 
following manner as shown in FIG. 3: 
(1) In response to an interrupt instruction from the total control unit 500 
or a portable communicator 400, 
(2) the fluid measuring and transmitting apparatus performs a 
self-diagnosis and transmits a report on the presence or absence of 
abnormality and; 
(3) measured or calculated fluid amounts such as a flow rate, static 
pressure Ps, temperature T and so on are transmitted to the total control 
unit and the portable communicator. 
(4) If the total control unit or the like does not instruct any change in 
measurement conditions such as a target control amount of the fluid, a 
control condition (such as PID control, fuzzy control parameters and so 
on), the fluid measuring and transmitting apparatus; 
(5) calculates a deviation value .DELTA.Q; 
(6) and sends a control amount .DELTA.v to the valve actuator as an 
electric signal, and the valve actuator opens or closes the valve on the 
basis of the control amount transmitted thereto to control the flow rate 
of the fluid within the pipe. 
(7) The fluid measuring and transmitting apparatus again measures a 
differential pressure .DELTA.P, static pressure Ps and temperature T; 
(8) calculates a flow rate Q; and 
(9) calculates the deviation value .DELTA.Q to control the flow rate to a 
target value. 
Generally, minor loops in the field are performing the flow rate control 
from (5) to (10), however, when an interrupt is issued from an upper level 
apparatus, this interrupt is preferentially processed to respond to the 
instruction, diagnosis and monitoring from (1) to (4). 
The portable communicator 400 is provided with a keyboard for input and a 
display unit for output so as to allow the operator to monitor the state 
of the system in the field and instruct a change in control amount and so 
on. 
A conventional system has been provided with a temperature transmitter, in 
addition to differential pressure and pressure transmitters, on an 
identical pipe line in order to correct the influence of compression of a 
liquid due to a static pressure and expansion of the liquid due to a 
temperature rise. However, according to the fluid amount measuring 
apparatus of the present invention, since a differential pressure and 
static pressure of a fluid under measurement are detected by a single 
detector, the flow rate detecting accuracy and responsibility are 
improved. Also, a closed loop control is performed in the field so as to 
share part of functions which have conventionally been performed by an 
upper level control unit, thereby making it possible to simplify the 
system and efficiently operate the whole plant. 
FIG. 2 illustrates a system which has a temperature sensor 150 arranged 
separately from the fluid amount measuring apparatus 100, wherein the 
output from the temperature sensor 150 is supplied to the fluid amount 
measuring apparatus 100 and the measurement and control are performed 
according to a processing procedure shown in FIG. 3. 
The structure of fluid amount measuring apparatus 300 according to the 
present invention is shown, by way of example, in FIGS. 4-9. Referring 
first to FIG. 4, a pressure receiving unit 1000 is installed together with 
a differential pressure generating element 560 in part of a conduit 550 
through which a fluid under measurement flows, where an air tight 
structure is assured by a flange 108. Seal diaphragms 103a, 103b, which 
isolate a sensor disposed in the apparatus from the external environment 
for protection, directly contact with the fluid under measurement and 
receive a pressure thereof. The pressure of the fluid under measurement is 
received by the seal diaphragms 103a, 103b located on both upstream and 
downstream sides of the differential pressure generating element 560 which 
add the respectively received pressures to each side of a composite sensor 
16 through a silicon oil 107 which serves as a pressure transmitting 
medium. The composite sensor 16 has a differential pressure sensor, a 
static pressure sensor and a temperature sensor formed on a single silicon 
chip. These sensors respectively convert a pressure, differential pressure 
and temperature to electric signals which in turn are sent from air tight 
terminals 109. A signal processing unit 106 amplifies and converts these 
signals into digital amounts, and thereafter performs a correction and 
calculation which have been previously programmed by a microprocessor, and 
transmits the values of measured flow rate, static pressure and 
temperature to an upper level control unit through an output cable 125 as 
well as displays these values on a display means 25. Since the silicon oil 
directly contacts with the fluid under measurement through the seal 
diaphragms, the temperature of the silicon oil is substantially equal to 
that of the fluid under measurement, thereby removing the necessity of 
separately providing a temperature sensor. Also in the present embodiment, 
the signal processing unit 106 is directly mounted on the pressure 
receiving unit 1000 such that outputs of the composite sensor 16 are 
connected to the signal processing unit 106 through terminals 109, whereby 
noises are not easily mixed in the signals and accordingly accurate 
measurements are enabled. 
However, if the signal processing unit 106 is directly mounted on the 
pressure receiving unit 1000, a thermal insulating structure must be 
provided between the signal processing unit and the pressure receiving 
unit if a fluid under measurement is at high temperatures. 
FIG. 5 illustrates another embodiment of the present invention, where the 
same reference numerals as those in FIG. 4 designate the same parts. The 
structure shown in FIG. 5 differs from that of FIG. 4 in that the signal 
processing unit 106 is located separately from the pressure receiving unit 
1000 and connectors 110, 120 of the signal processing unit 106 and the 
pressure receiving unit 1000 are connected by an extension line 115. This 
structure can prevent electronic parts with a relatively low heat 
resistivity from malfunctioning even if the temperature of a fluid under 
measurement in a pipe or a container exceeds 100.degree. C. It should be 
noted, however, that variations in accuracy occurring during the 
manufacturing process of the apparatus are stored in a memory as the 
results of characteristic measurement of the pressure receiving unit, 
which is performed upon shipping from the factory, as a characteristic map 
of each pressure receiving unit, so that this memory is located on the 
pressure receiving unit side. The memory is located near the connector 110 
where external air produces a cooling effect in order to prevent 
malfunctions due to a temperature rise. If the circuit portion up to an 
A/D convertor is disposed in part of the pressure receiving unit, the 
extension line 115 passes only digital signals and accordingly the 
influence of external noise is reduced, whereby the extension line may be 
made longer. 
FIG. 6 illustrates a further embodiment of the present invention. 
Conventionally, the flow rate measurement by using the differential 
pressure generating element 560 such as an orifice is widely employed in 
industrial plants and so on because the shape and size of the orifice have 
been globally standardized and maintained on the basis of the results of 
experiments made in real flows. The value of differential pressure differs 
in dependence on the pressure introducing positions on the upstream and 
downstream sides of the orifice. Further, as illustrated, it is also 
necessary to form tap holes 1001a, 1001b through part of the conduit to 
define this size. When the holes are positioned respectively at an equal 
distance of 1 inch (25.4 mm) from the orifice as illustrated, this is a 
method called "flange tap". There are also a vena contracta tap method, a 
corner tap method and so on. In accordance with these methods, actually 
measured values of the flow rate coefficient .alpha. shown in the 
foregoing equation (1) are prepared as standard values by the JIS, ASME 
and DIN standards. The present embodiment provides the pressure receiving 
unit with a mounting structure corresponding to a variety of tap methods 
as mentioned above. Incidentally, it is preferable that the seal 
diaphragms are located as closely as possible to the conduit in order to 
measure the temperature of a fluid under measurement in a required 
accuracy. Practically, the temperature of a fluid under measurement may be 
measured within .+-.1.degree. C. For practically designing this structure 
so as to prevent the temperature from falling more than 1.degree. C., the 
distance between a seal diaphragm and a conduit may be approximately 10 mm 
if the apparatus is made of a metal, although it differs depending on the 
kind of a fluid under measurement and an external temperature. 
FIG. 7 illustrates a further embodiment of the present invention. In this 
embodiment, seal diaphragms, which have been disposed on the transmitter 
side in a conventional intelligent transmitter, are disposed on the 
pressure receiving unit side, and the pressure is introduced to a 
composite sensor 16 through capillary tubes 111 which are filled with 
silicon oil. A temperature sensor 30 is also disposed on the silicon oil 
filled side of one of the seal diaphragms for detecting a temperature of a 
fluid under measurement and generating a signal indicative of the detected 
temperature. The output from the sensor 30 is transmitted to a signal 
processing unit 106 through a shield line 113. In summary, the present 
invention may be implemented by a conventional intelligent transmitter 100 
with simple modifications. The temperature sensor 301 only detects a 
temperature from a pressure receiving unit 1000 which is disposed near the 
seal diaphragm in contact with the fluid and generates a temperature 
signal, while a differential pressure and static pressure of the fluid 
under measurement are measured by conventional intelligent transmitters 
and a flow rate of the fluid flowing through the conduit is calculated in 
accordance with the processing procedure shown in FIG. 3. 
Incidentally, the shield line 113 is employed in order to prevent noise 
from influencing the signal from the temperature sensor 30 which generally 
presents a small value. Although the present embodiment is more 
complicated than those of FIGS. 4, 5 and 6, it is compact in comparison 
with a conventional system. It is possible to form a closed measurement 
and control loop in the field by adding simple modifications to a 
conventional intelligent transmitter. 
FIG. 8 illustrates a further embodiment of the present invention. This 
embodiment is an example which employs a Venturi tube 560 as a 
differential pressure generating element. The structure shown in FIG. 8 
differs from that of FIG. 6 in that the differential pressure generating 
element 560 itself is integrated with a pressure receiving unit 1000 of a 
fluid amount measuring and controlling apparatus. However, there is no 
essential difference in action. Although the size of the apparatus is 
slightly larger, this structure is appropriate if a pressure drop given to 
a fluid under measurement must be reduced. 
FIG. 9 illustrates an embodiment which employs the present invention for 
measuring and controlling a liquid level. As illustrated, a pressure 
receiving unit 1000 is mounted on a lower portion of a container 570 for 
storing a fluid L, while a flange provided with a seal diaphragm 103a is 
mounted on an upper portion where the liquid does not reach. Silicon oil 
107 is introduced to a composite sensor 16 by a capillary tube 111. The 
pressure receiving unit 1000 is provided with the composite sensor 16, 
isolated by and located in the vicinity of a seal diaphragm 103b, which 
can directly measure a pressure, differential pressure .DELTA.P, and 
temperature T of the fluid L under measurement. If the differential 
pressure can be detected from the output of a differential pressure 
sensor, .DELTA.P is calculated; if the outer diameter of the container is 
known, the volume of the container is calculated; and if the specific 
gravity .rho. of the fluid L is known, its weight is calculated, whereby a 
liquid level h can be finally calculated from the following equation (3). 
Also, a correction calculation can be performed for expansion and 
contraction of the fluid by using outputs of a static pressure sensor and 
a temperature sensor comprised in the composite sensor. The inner diameter 
d of the container, the specific gravity .rho. of the fluid, and an 
expansion and contraction ratio of the liquid have been previously stored 
in a memory. 
EQU .DELTA.P=.rho.gh (3) 
where .rho. represents the specific gravity of the fluid; g the 
gravitational acceleration; and h a liquid level. 
The foregoing measurement and calculation results are transmitted to an 
upper level control unit through a transmission line 125a. Also, a 
deviation value between a control target value and a measured value of the 
liquid level is calculated, and a control amount is transmitted as an 
electric signal to valve actuators 500, 501 through a transmission line 
125b to control the liquid level. 
As described above, the measurement and control apparatus of this system 
controls a plurality of actuators in relation with each other depending on 
a control object and transmits the situation to an upper level control 
unit. Since the apparatus of the present embodiment performs functions 
such as measurement and calculation for each detection terminal, the upper 
level control unit can perform upper level controls such as monitoring of 
a plurality of systems, harmonic control and so on which should be 
originally performed by the upper level control unit. 
The fluid amount measuring apparatus of the present invention achieves the 
simplification of the structure and reduction of parts in size and weight. 
The pressure receiving unit is disposed on part of the conduit 550 so as 
to directly contact with a fluid under measurement, thereby making it 
possible to measure a temperature, in addition to pressure and 
differential pressure, of the fluid under measurement by the composite 
sensor disposed nearby. For sensing a level and mass of a fluid in a 
conduit or a container or a flow rate of a fluid flowing through a 
conduit, the volume and density of that fluid are necessary. These 
characteristic values, since they depend on temperature, have been 
previously measured and stored in a memory which serves as a storage means 
of the apparatus of the present embodiment. Therefore, the mass flow rate 
of a fluid under measurement can be calculated by a microprocessor with 
signals from the above-mentioned sensors and the characteristics of the 
fluid which have previously been stored in the memory. Unlike a 
conventional differential pressure transmitter shown in FIG. 14, the fluid 
amount measuring apparatus of the present invention does not require a 
protection mechanism because a high pressure durable three valve is not 
used and accordingly an excessive load is not charged on the pressure 
receiving unit, whereby the pressure receiving unit omitting a sensor 
diaphragm 120 is extremely simple. 
FIG. 10 illustrates a block diagram of the flow rate measuring apparatus of 
the present invention. Outputs of the differential pressure, pressure, 
temperature and density sensors included in the composite sensor 16 are 
selectively fetched by a multiplexer 17, amplified by a programmable gain 
amplifier 18, and converted to digital signals by an A/D convertor 19. A 
memory 20 comprises map data MS 201-204 which have previously stored the 
characteristics of the differential pressure, static pressure, temperature 
and density sensors in the form of maps, and a map MF 205 which has stored 
the characteristic of fluids under measurement such as density. A 
microcomputer 21 calculates the temperature, static pressure and 
differential pressure by referring to the map data MS 201-204 and also the 
mass flow rate and weight of a fluid under measurement by using the map 
MF, as will be later shown in FIG. 11. These measured values are displayed 
on a display unit 25 as well as transmitted through a communication 
circuit in an I/O circuit 23 in response to a request from an upper level 
control unit. When used in an analog control system, the signals generated 
by the sensors are again converted to analog signals by a D/A convertor 22 
and outputted to an analog control unit through a voltage-to-current 
convertor within the I/O circuit 23. The apparatus of the present 
invention further compares the measured value derived as described above 
with a target value given from the upper level control unit, and sends a 
deviation value therebetween to a valve actuator to control the flow rate 
or liquid level of the fluid under measurement. The setting of a target 
value may be carried out even in the field by a keyboard 24 and a portable 
communicator 29 provided for the apparatus. As described above, the 
apparatus of the present invention can control fluid amounts such as flow 
rate, pressure, liquid level and so on in a closed loop form in the field, 
in addition to a measurement function, thereby realizing not only the 
improvement of the characteristics such as responsibility but also a 
measurement and control system which is highly economical, for example, in 
a wiring cost. 
FIG. 11 schematically illustrates how to correct outputs of the respective 
sensors. Although not shown, a correction map MS represents a differential 
pressure sensor output Ed, a static pressure sensor output Es and a 
temperature sensor output Et in a three-dimensional form. The 
microprocessor 21 executes the processing in accordance with a processing 
procedure shown in FIG. 12. Specifically, at a node 1, after removing a 
cross talk of the static pressure sensor, a correct static pressure P is 
derived from the outputs of the static pressure sensor and the temperature 
sensor by using a correction map 202 and outputted. Next, the differential 
pressure sensor output Ed is corrected with respect to the temperature by 
using a correction map 203, and further corrected by using the static 
pressure sensor output P to derive a correct differential pressure sensor 
output .DELTA.P. Stated another way, since the characteristics of the 
static pressure and differential pressure sensors are changed due to the 
ambient temperature, a signal generated by the temperature sensor is used 
to precisely execute a correction calculation. Further, these measured 
values thus derived and the map MF 205 which has stored the 
characteristics of fluids under measurement such as density p are used to 
calculate the mass flow rate, weight and liquid level of the fluid under 
measurement. 
Next, the structure of the composite sensor will be described in detail 
with reference to FIGS. 13A, B and C. FIG. 13A is a plan view of the 
composite sensor; FIG. 13B a cross-sectional view taken along the line 
A--A in the plan view; and FIG. 13C an example of wire connection when the 
static pressure sensor and the differential pressure sensor are connected 
to each other. In FIG. 13A, reference numerals 1-4 designate gage 
resistors for differential pressure detection formed on a semiconductor 
substrate 10 which is made of a silicon monocrystal by doping impurities 
into the semiconductor substrate 10 by ion implantation or thermal 
diffusion. These gage resistors 1-4 are formed in a region of a diaphragm 
9 processed by alkali etching, dry etching or the like. Reference numerals 
5-8 designate gage resistors for static pressure detection. Stresses are 
developed in the gage resistors 1-4 due to a differential pressure load. 
The gage resistance of 1-4 are clanged. A gage resistor 30 is a 
temperature gage which is located in a fixed portion. The gage resistor 30 
is also oriented in the &lt;100&gt; direction in which it is insensitive to 
stress. Reference numerals 13a-13f designate electrode pads. Incidentally, 
while a humidity sensor, later referred to, is added in the structure of 
FIG. 13, it is not necessary during a normal liquid measurement. After 
wire connection has been made as shown in FIG. 13C (when a liquid is to be 
measured, resistors 31, 40 and 41 in the drawing is not necessary, and a 
temperature is measured from a terminal 13g), a fixed voltage is applied 
between the electrode pads 13a and 13b to derive a differential pressure 
output between the electrode pads 13c and 13d and a static pressure output 
between the electrode pads 13e and 13f. Further, FIG. 13C illustrates a 
structure in which a sensor 40 for humidity detection is connected, in 
which case the terminal 13g is connected with a dummy resistor 31 while an 
output 13h of the humidity sensor is connected with a dummy resistor 41. 
Since these dummy resistors need not be located on the sensor substrate, 
they are provided separately in the present embodiment. 
Reference numeral 11 in FIG. 13B designates a fixing base made of 
boro-silicated glass for the semiconductor substrate 10. When a 
differential pressure is loaded on this composite sensor, which results in 
increasing the resistance values of the gage resistors 1 and 3 and 
decreasing the gage resistors 2 and 4, the values of which are equal to 
the gage resistors 1,3 and 2,4, and the direction of which are opposite to 
same. Therefore, by forming a bridge circuit as shown in FIG. 13C, a 
static pressure sensor output can be derived which is free of fluctuations 
due to a differential pressure. 
If a humid gas is to be measured by a differential pressure type sensor, a 
moisture correction is required in addition to the foregoing corrections 
for static pressure and temperature. 
The composite sensor shown in FIG. 13A includes a humidity sensor 40 of a 
type having a thin film resistor made of polymer or ceramics, the 
resistance value of which changes in accordance with humidity. As shown in 
FIG. 13C, the humidity sensor 40 is connected to resistor bridge circuits 
constituting the static pressure sensor and the differential pressure 
sensor together with the dummy resistor 41, where humidity is detected 
from a terminal 13d as an electric signal. In this implementation, the 
seal diaphragms are removed from the structure shown in FIGS. 4 or 5, 
whereby the composite sensor is directly exposed to a humid gas. The 
temperature, differential pressure and static pressure sensors are 
therefore coated with films to prevent the characteristics thereof from 
being deteriorated by humidity. When the composite sensor provided with 
the humidity sensor is substituted for the composite sensor shown in FIG. 
10, the processing unit 21 can correct for a moisture included in a gas, 
thereby making it possible to provide a flow rate measuring apparatus 
suitable for a humid gas. Stated another way, since a moisture correction 
based on a signal from the humidity sensor can be provided for a volume 
flow rate calculated from a differential pressure signal, in addition to 
corrections made by signals from the temperature and static pressure 
sensors, the provision of a separate humidity meter, hitherto required, is 
not needed, thereby making it possible to measure the flow rate of a humid 
gas with a simple structure. A specific method of correction calculation 
is described in "Measurement Handbook", Section 16.3, "Restriction Flow 
Meter", pp 830-837, and so on. 
To calculate the flow rate, a correction for density is necessary in 
addition to that for humidity. Generally, a separate means is used to 
previously measure and store the density of a fluid, and then the 
processing unit 21 performs a correction calculation by using the stored 
density. If a fluid under measurement is a liquid, the density is a 
function of temperature alone, whereas, the density of a gas also depends 
on the pressure, so that it is preferable if a density sensor is also 
formed on the composite sensor. A conventionally used density sensor is so 
large that a composite sensor including a density sensor has not been 
realized on a single chip. However, by applying a sensor technique 
described in Applied Physics Letter, pp 1653-1655, published on May 16, 
1988, a composite sensor having a density sensor can be constructed as 
illustrated in FIG. 13D. 
This composite sensor forms a density sensor having a surface elastic wave 
element on part of a silicon substrate constituting the sensor, which 
makes use of the fact that the frequency of a surface elastic wave 
propagating between electrodes varies depending on the density of fluid. 
Since the density sensor according to this method is influenced by static 
pressure and temperature to result in a change in frequency, the density 
sensor need be corrected by output signals from the static pressure and 
temperature sensors formed on the same silicon substrate. This correction 
is made in accordance with the correction method for the differential 
pressure characteristics described above in connection with FIGS. 10 and 
11. This composite sensor is realized by the recent progress of the fine 
processing technique which can compose the respective small sensors on a 
silicon and integrate, if necessary, a signal processing circuit on the 
same silicon, thereby making it possible to provide a small sensor which 
is excellent in mass productivity. 
As described above, since the apparatus according to the present invention 
is provided with a control function in addition to a measurement function, 
the measurement and control of fluid amounts such as flow rate, pressure, 
liquid level, weight and so on can be carried out in a closed form in the 
field, thereby simplifying correction procedures for static pressure and 
temperature, reducing a processing time required for the measurement and 
control, and improving the characteristics such as the responsibility. 
According to the present invention, burdens to an upper level control unit 
are alleviated to reduce a processing time required by the system for the 
measurement and control operation, which leads to improving the control 
performance in view of the whole plant. 
According to the present invention, since the measurement and control can 
be performed in a closed form in the field, an amount of wire for 
connecting with an upper level control unit is reduced, thereby realizing 
a highly economical measuring and controlling system. 
According to the present invention, the temperature, differential pressure 
and static pressure of a fluid under measurement can be detected 
independently of each other and corrected for density and temperature on 
the basis of previously stored characteristic data of the fluid under 
measurement to calculate and measure the mass flow rate, weight and liquid 
level. 
According to the present invention, since the temperature, differential 
pressure and static pressure can be independently detected with a high 
accuracy, the functions of a temperature transmitter and differential 
pressure and pressure transmitters can be achieved by a single apparatus. 
Furthermore, according to the present invention, the fluid amount measuring 
apparatus does not require a protection mechanism because a high pressure 
durable three valve is not used and accordingly an excessive load is not 
charged on the pressure receiving unit, whereby the pressure-receiving 
unit omitting a sensor diaphragm becomes extremely simple. Therefore, a 
highly economical measuring system can be realized.