Gas detection system

In a gas detection system for detecting the content of inflammable, explosive and/or toxic or other gases an arrangement is proposed according to which a central electronic evaluation system is connected to a plurality of measuring heads, which are installed at the points of measurement, by a single common two-wire line and each measuring head is provided, at its place of installation, with its own intelligence (microprocessor) for processing at least partly the values supplied by the sensor, each measuring head being equipped with at least two sensors of different types or measuring ranges and with an additional temperature sensor and each sensor having an identification which is indicative of the type and measuring range of the sensor and which is picked up by the measuring head. The electronic evaluation system and the measuring heads communicate with each other by bidirectional digital communication, after the measuring heads have been addressed, and the measuring head is supplied with current via the two-wire line.

BACKGROUND OF THE INVENTION 
The invention relates to a gas detection system. 
Gas detection systems, where a plurality of sensors detecting the presence 
of gases and their respective quantities are connected to a common central 
evaluation system, have been known in many different, though complicated 
and complex, forms. 
For example, a system suited for monitoring larger garages or tunnels is 
designed in such a manner that gas sampling heads are arranged at 
different points and connected to a common evaluation center via 
mechanical hose connections the length of which may reach several hundred 
meters and even more. The gas mixture is supplied from different sampling 
points to the sensor in the evaluation center for analysis by switching 
over cyclically from one hose to the next, which action is usually 
effected by means of solenoid valves. This system uses of course only one 
sensor. Apart from the enormous mechanical input required not only for the 
initial installation, but also for the subsequent monitoring operations, 
for example with a view to preventing bending or breaking of certain 
hoses, such a measuring method is also particularly lengthy because 
important dead times result from the fact that the hoses must be scavenged 
every time before the gas sensor can be supplied with new gas to be 
measured. Such a system requires a great number of mechanically moving 
parts, particularly high performance of its pumps, frequent servicing 
operations, and all these circumstances are connected with the 
disadvantage that no redundancy of measurements is obtained, that any 
self-monitoring of the system is excluded and that the cycle times may be 
in the range of 30 minutes and above. 
One has therefore built up a gas detection system in such a manner that 
each of the points at which monitoring operations or measurements are to 
be effected is equipped with a measuring head provided with a sensor and 
that the measuring heads are connected to an electronic evaluation system 
via a 5-wire line, under certain circumstances only via a 3-wire line. In 
the case of a 5-wire line, two of the wires serve for firing the sensor, 
or for supplying it with current, two other wires serve for transmitting 
the measured value from the measuring head to the electronic evaluation 
system, and a fifth wire serves as connection to earth. In the case of a 
3-wire line, the earth wire performs simultaneously the function of one 
line of each of the two other pairs of lines. If another gas analysis is 
required at the same point or, as usually intended in the case of such 
systems, at a different point, identical parallel systems, comprising a 
measuring head and a separate connection cable, must be guided to the 
electronic evaluation system for detecting the second gas component or gas 
mixtures at different points, which means that the cost of the system and 
its installation is quite considerable. 
In addition, it is a problem of such known gas detection systems that it is 
not possible to combine different types of equipment because of the 
different supply voltages for the measuring heads, different sensitivities 
and connections. In addition, these systems require separate balancing of 
the electronic evaluation system up to the measuring head, after assembly 
and installation of each head, and lengthy calibration operations which 
will be described hereafter in more detail. 
Certain measuring heads require high, other measuring heads require lower 
supply currents, depending among other things on the sensor and the gases 
to be monitored, i.e. whether inflammable, toxic, explosive or other gases 
are to be detected, and in addition the very low analog signal voltages 
are responsible for the extremely high sensitivity to disturbing voltages 
which is normally a characteristic of such systems. Consequently, it has 
been general practice to install the individual connection lines between 
the individual measuring heads and the electronic evaluation system, 
whether they are of the 5-wire or of the 3-wire type, in the form of 
shielded cables. Normally, each measuring head is provided with a separate 
power pack feeding the sensor connected to it. Because of the many 
different requirements, measuring ranges, types of gases to be analyzed, 
and the like, one has heretofore never succeeded at arriving at a 
standardization in the meaning of a desirable simplification. 
It has been known in connection with certain chemical sensors, i.e. such 
with low current consumption, to supply the measuring head with a weak 
supply current and to feed back a measuring signal via the same line, in 
which case the supply current may, for example, be in the range of 
maximally 4 mA, while the measuring system may be in the range of between 
4 . . . 20 mA. 
However, in the case of these known systems, the calibration of the 
sensors, which has to be effected particularly frequently, is also very 
troublesome and has in any case to be effected every time the sensor is 
exchanged, and in addition the sensor must always be connected to the one 
measuring head to which it is assigned. 
The before-mentioned calibration may be carried out, for example, in such a 
manner that one service person is present at the central station, i.e. 
near the electronic evaluation system, while another service person, 
equipped with cylinders containing test gas of a given concentration, is 
present at the very place, i.e. the place of the respective sensor, to 
supply the latter with the desired test gas. The two persons at the 
central station and at the place of the sensor, respectively, communicate 
via walkie-talkie. Every time the respective sensor has been supplied with 
test gas in sufficient concentration, until an equilibrium has been 
achieved, this fact, and the type of test gas used, is then communicated 
to the service person waiting at the central station who then performs the 
calibration process on the setting potentiometers at the central station, 
provided no change of concentration has occurred in the meantime for some 
reason or other. This calibration method is lengthy and susceptible to 
trouble due to the fact that it has to be carried out by two people so 
that calibration errors are frequently encountered. Other problems 
encountered with the methods according to the prior art are connected with 
the fact that the shielded wires can be run only over a maximum length of 
1500 m, due to the high current of the measuring head and the 
susceptibility to trouble of the weak signals of the measuring head. In 
addition to the high cost of installation, it is also necessary in this 
connection to balance the bridge current of the measuring head relative to 
the length of the line, and of course also relative to zero and the 
sensitivity. 
Given the fact that the known gas detection systems do not comprise any 
"site electronics"--this term is being newly introduced by the present 
invention--one further requires, in addition to the balancing 
potentiometer for the bridge current in relation to the length of the 
line, one balancing potentiometer for zero point, one balancing 
potentiometer for sensitivity (measuring-head signals voltage), balancing 
potentiometers for the first and the second alarms, test jacks for bridge 
current and measuring-head signal (aging) and a separate possibility to 
balance the sensor or the head manually using an external measuring 
instrument, and all this without the possibility to combine different 
types of measuring components in the component carrier. 
Now, it is the object of the present invention to remedy all these 
inconveniencies and to improve the known gas detection systems in such a 
manner that it is now possible, without great input of work and cost, to 
arrange measuring heads at any desired point, which measuring heads are 
capable of determining simultaneously, i.e. in parallel, the concentration 
of a plurality of gases and of communicating fully with the electronic 
evaluation system arranged at the central station, or, to say it in other 
words, that it is now possible, in addition to the transmission of the 
supply voltage to the respective measuring head, to implement 
bidirectional signal transmission between the measuring head and the 
central electronic evaluation system. 
ADVANTAGES OF THE INVENTION 
The invention solves this problem with the aid of the characterizing 
features of the main claim and provides the advantage that by dividing the 
"site equipment" into an intelligent measuring head, which may for example 
be equipped directly with a microprocessor, and means for receiving at 
least two sensors in addition to the temperature sensor normally used, 
bidirectional digital data communication, i.e. digital signal transmission 
between the electronic central station and the measuring head is rendered 
possible in both directions so that any susceptibility to trouble is 
almost fully eliminated in this area. 
In addition, it is now possible to connect to a single 2-wire line 
basically any desired number of "site devices", each consisting of a 
measuring head comprising at least two gas sensors and a temperature 
sensor, with the possibility to select series and parallel connections as 
desired. 
If under exceptional circumstances, i.e. under explosive conditions, the 
line current may not exceed certain given values and the number of 
measuring heads, including their sensors, has to be limited, the lines 
leading to the central electronic evaluation system may be linked within 
the latter by the usual means of information technology. 
Another advantage of the invention is seen in the high measuring security, 
the required low maintenance input and the low-cost and simple 
installation, it being an additional particular advantage that the 
measuring heads may be equipped with any desired type of sensors, which 
means that when one or both or even more sensors of a measuring head have 
to be exchanged because of aging effects, or the like, the sensors newly 
installed in the standard or universal measuring head, in replacement for 
the old sensors, may either be identical to the latter or be designed for 
detecting different gases and/or for other measuring ranges. This is due 
to the fact that two different identification systems are incorporated 
into the gas detection system according to the invention, i.e. on the one 
hand an identification between the respective sensors connected to a 
measuring head, and on the other hand the possibility to address the 
sensors from the central electronic evaluation system as the central 
station, so that the existing two-wire line can be used without any 
problems for having the measuring data of plurality of "site devices" 
recorded by the central electronic evaluation system, with extremely short 
cycle times. 
In addition to the options provided for the equipment of the respective 
measuring head, the invention provides the possibility to perform dynamic 
temperature corrections, to monitor automatically the service life of the 
sensor by means of a special service signal, to perform automatic 
measuring compensation and range balancing, due to the sensor 
identification feature, and does away for the same reason with any 
balancing requirements when replacing existing sensors by identical or 
different sensors (exchange of sensors). The two-wire connection cable 
used may be installation or telephone cables, the possible transmission 
distances being in the range of up to 5000 m. In the case of the 
particular embodiment described herein, which is however not intended to 
restrict the invention, transmission is effected bidirectionally for 3 
measuring values (gas sensors and measured temperature value) and 
additional 8 status signals, at a transmission rate of 1200 baud, via a 
telephone cable serving as digital bus. The sensor identification is 
transmitted in this case by the "site electronics", for example a 
microprocessor arranged in the measuring head, and at the same time the 
service life of the sensor is monitored automatically. 
Another particular advantage of the present invention is further seen in 
the fact that the "site electronics" now permit automatic system 
calibration by means of a special calibration cap, without the need for a 
manual balancing operation. As the corresponding values are stored in the 
central electronic evaluation system, for example in an EPROM, the central 
electronic evaluation system is informed, by the identification of the 
sensor connected to the respective addressed measuring head, of the 
charactristics of the sensor, i.e. the gases it is capable of detecting 
and its measuring ranges. The identification may, for example, be detected 
physically by the intelligent measuring head, through a binary code, when 
the sensor is inserted in or mounted on the measuring head, and may then 
be held available for enquiry by the central electronic evaluation system. 
The features described by the subclaims permit advantageous improvements 
and developments of the gas detection system specified by the main claim. 
A particularly advantageous variant provides that the identification of 
each sensor takes the form of terminals arranged in the form of a binary 
code, for example by simply bridging soldered points according to the 
binary system, depending on the desired identification. This 
identification is given to the sensors already at the factory, before 
delivery, so that the sensors can be replaced later at desire as the 
intelligence of the measuring head will automatically inform the central 
electronic evaluation system of the type of sensor inserted, and it 
measuring range. It is then possible to provide a suitable display at the 
central station which automatically indicates the correct concentration 
and the component measured at any time. Compared with the known, lengthy 
measuring method using hose connections or multi-wire shielded cable lines 
for each measuring head, the invention therefore succeeds in achieving a 
really revolutionary simplification and measuring security over the prior 
art, as regards the detection of the presence of gases and their 
concentration.

DESCRIPTION OF THE EMBODIMENTS 
It is the basic idea of the present invention to provide local 
communication means, in the field of gas analysis, between the 
sensor/measuring head on the one hand and the central electronic 
evaluation system on the other hand, by giving the measuring head its own 
intelligence, for example in the form of a single-chip computer, a 
microprocessor, or the like, and by having the signals transmitted 
digitally in both directions, and by performing the data exchange between 
the different measuring heads and the central electronic evaluation system 
simultaneously and in serial succession, while the measuring heads are 
simultaneously supplied with the required supply voltage via a 
transmission line. 
In FIG. 1, one can see the central electronic evaluation system 10 and a 
common two-wire line 11 connecting the said system to the measuring heads 
12a, 12b . . . 12g. 
If very high currents are required by the measuring heads, as indicated in 
the drawing by 12j and 12k, it is also possible to provide a separate 
connection line whose digital signals are also supplied to the central 
electronic evaluation system 10, via suitable circuit means. 
Each of the measuring heads 12a to 12i comprises a separate microprocessor 
and may be equipped with a plurality, in the illustrated embodiment with 
two, of sensors having the most different functions for gas analysis 
purposes. In the particular embodiment illustrated in the drawing the 
sensors are designed to detect only toxic, explosive, or oxygen mixtures, 
as indicated at the measuring heads, although it would of course be 
possible also in the illustrated case, where each head is equipped with 
two sensors, to implement any desired combination, for example one sensor 
for toxic and one sensor for explosive gas, or two sensors for toxic gas 
mixtures of different components, or a combination of one oxygen sensor 
with one sensor for toxic or explosive gas mixtures, and in addition it 
would also be possible to employ a plurality of sensors for detecting one 
and the same gas, but for different measuring ranges. 
The abbreviations used in FIG. 1 for describing the functions of the 
individual sensors and/or measuring heads are TOX for the detection of 
toxic gases, UEG for the detection of explosive gases or gas mixtures, 02 
for the detection of oxygen or lack of oxygen, to give only a few examples 
of the possibilities that are opened up by the invention. 
All sensors illustrated in FIG. 1 are designated by reference numeral 13, 
regardless of their type or function (measuring range). The additional 
temperature sensors equipping advantageously each measuring head are not 
shown in the drawing for the sake of clarity. 
In the central electronic evaluation system, a main computing unit 
comprising extensive storage capacities is designated by reference numeral 
10a in the plug-in area. A two-wire line 11 connects this unit to the 
measuring heads 12a to 12g. The main computing unit contains data relating 
to each of the measuring heads 12a to 12g connected to it so that each of 
these measuring heads can be addressed by it by means of a predetermined 
code, in a desired predetermined order (there also exist different methods 
which permit certain priority measuring heads to be enquired by 
preference, which methods need not be described here in greater detail as 
they are known as such) which then gives rise to the bidirectional serial 
transmission of a protocol. Each measuring head forms an interface between 
the central electronic evaluation system and the sensors 13 connected to 
the latter. During the normal cyclical enquiry sequence (for the purposes 
of these explanations, the calibration steps are initially left out of 
regard) this interface is supplied by the respective measuring head with 
the sensor identification S1 or S2 for the connected gas sensors, and S3 
for the temperature sensor, and then with corresponding, standardized 
measuring values in a binary coded form, after the latter have been 
processed by the intelligence of the measuring head, on the analog side, 
to derive voltage values between 0 and 1 V, and have then be digitized, 
and supplemented by adjustments for peripheral conditions, such as 
temperature compensation, corrections for particular characteristics, 
linearization and other corrections, for example for humidity or air 
pressure influences. 
The measuring-head interface starts the process by addressing a 
corresponding storage area in the main computing circuit 10a of the 
central electronic evaluation system, using the supplied sensor 
identification, so that the measuring value transmitted by that sensor 
subsequently --or before--via the line in a coded binary, i.e. digital 
form can be classified and evaluated correctly. 
In fact, there are still other plug-in units in the area of the central 
electronic evaluation system, all of which are designated generally by 
reference numeral 14, and which are assigned to the individual measuring 
heads or sensors, for receiving from the main computing circuit 10a the 
measured values recorded for the respective sensor so that the plug-in 
unit assigned to the respective sensor is then capable of displaying the 
measured values so recorded in a suitable form, together with the other 
possible displays, such as alarm, diagnosis, trouble, calibration, and 
operating conditions, which are usually indicated by luminous diodes or 
other lamps. Consequently, the central electronic evaluation system, 
together with its main computing circuit, may be regarded as the 
administrator of the data received and transmits the latter via an 
internal bus to the plug-in units 14 where they can be redistributed 
according to their respective measuring points, if and to the extent this 
should be necessary or desirable. 
Given the numerous possibilities of this type of data collection and 
administration, there are of course many possibilities of taking into 
account other aspects, in addition to recording the environmental 
temperature dynamically at the measuring points, as for instance the 
remaining service life of the individual sensors, which can be derived 
without any problems from their general service life and the data stored 
in the main computing circuit. When a sensor is exhausted, it is then 
possible to have this information displayed on the corresponding plug-in 
unit. 
In addition, the indicated temperature may warn the operator, who is of 
course constantly watching the central electronic evaluation system with 
its plug-in units in the central station, of the development of any 
dangerous conditions in its system, and the location thereof, for example 
of flames or fire, as such data are of course also transmitted by the 
temperature sensor. 
It goes without saying that any functions that are not performed by the 
electronic system of the measuring head, especially by its microprocessor, 
because of its particular design or limitations, are then performed by the 
main computing circuit 10a as part of the processing of the data 
transmitted by the sensors, giving due regard to the data stored and any 
corrections that have to be made. This distribution of functions may be 
selected as desired, although it must be ensured that the main computing 
circuit 10a, with its storage capacities, must in any case be informed of 
the conditions which have given rise to the momentary measured value 
transmitted by the sensor. 
In any case, the environmental temperature of the measuring head will be 
displayed in the central station so that there is always the possibility 
to have the temperature sensitivity of the sensors compensated in the area 
of the central electronic evaluation system. 
In practice, two sensor housings are (initially) mounted on each measuring 
head (for gas analysis, it being of course also possible, depending on the 
design of the interface area, the storage capacitites and the set-up of 
the central electronic evaluation system, to connect additional sensors if 
this should prove necessary). For example--to give a few figures--10 
sensors of different types with, for example, 30 different measuring 
ranges may be provided. In case of failure, or if the operator so chooses, 
the sensors of any measuring head may be exchanged without any problems, 
according to their type and the measuring area, if the measuring head has 
to be exchanged for one adapted to different gas components or other 
measuring ranges. The electronic system of the microprocessor arranged in 
the measuring head, together with the digital transmission of the signals 
in both directions, allow any such exchange of sensors without any 
problems, due to the special identification of the sensors. 
FIG. 2 illustrates in this connection different sensor structures; the 
properties of the sensor are indicated below the latter by the 
corresponding abbreviations (02, TOX, UEG, IRA, the latter standing for 
infrared gas analysis). 
The sensor element 15 as such is connected to a simple electronic system 
16, and these two components together are arranged in a sensor housing 18 
with the associated base 19, as sensor unit 17. One then obtains a typical 
sensor component 20, in any case one with uniform bases 19 which can be 
mounted on the measuring heads 12 shown on top of the drawing, by 
inserting the base 19 into suitable plug-in contacts, as indicated in the 
drawing, whereby the electric connections are simultaneously established. 
As has been mentioned before, it is important that the identification of 
each sensor be arranged at the base or base portion 19 of the sensor in a 
manner that can be read by the measuring head. The sensor-related 
identification is provided on the respective sensor already in the course 
of its production so that the sensor can be exchanged later at the site at 
desire; the measuring head in its capacity as interface will in any case 
record the identification and inform the central electronic evaluation 
system automatically of the type of sensor present, and its measuring 
range. 
Advantageously, this coding relating to the type and measuring ranges is 
enquired and compared automatically during each enquiry cycle, in which 
case the corresponding processing routines for each sensor identification 
are stored in the storage (preferably a so-called EPROM) of the central 
electronic evaluation system so that the correct concentration and the 
component in question can be indicated automatically on corresponding, 
associated displays. It has been mentioned before that, according to a 
preferred embodiment of a sensor, the identification is realized by 
connecting electric contact points by means of bridges in the base area. 
This makes it possible for the microprocessor in the measuring head to 
pick up the necessary information, and for the same reason no balancing 
operation is required when a new sensor is mounted. 
Another advantage is seen in the fact that this basic design of the current 
supply in the area of the measuring head and of the digital transmission 
of the measured values and status signals makes it possible, without any 
problems, in any case for sensors requiring only low supply currents, to 
use enclosure type EEx ib (for the sensors EEx di) lines for the wiring 
system proper. In cases where excessively high currents prohibit the use 
of a single two-wire line 11 (FIG. 1) measuring heads 12h and 12i may be 
provided and connected to the respective plug-in units 14' via separate 
lines. 
In goes without saying that the system can be connected, via the internal 
bus line in the central electronic evaluation system 10 and corresponding 
interface circuits, to a higher-ranking computer capable of printing out 
and registering the measured values and status values, as well as other 
operating conditions, according to selected criteria. 
The basic concept described heretofore permits an additional embodiment of 
the invention to be implemented, namely the fully automatic calibration of 
the gas sensors in the area of the central station. In this case all risks 
of operating faults and errors are absolutely excluded because it is then 
only necessary for a single operator to handle the sensor directly at the 
site. 
FIG. 3 illustrates diagrammatically the basic principle underlying the 
calibration process. 
A calibration device 25 taking in practice the form of a calibration cap, 
is placed over each of the sensor housings which remain firmly connected 
to the measuring head. The calibration cap 26 is provided with current 
supply means of its own, for example a battery 27, an external gas 
connection 28, a cap-shaped housing--indicated at 29--of a design and 
shape permitting it to be placed over the gas-sensitive area of the sensor 
component in sealed relationship, and an electronic area 30 generating the 
calibration signals which are to be picked up by the measuring head. 
Gas sensors may require to be calibrated as to their zero point and 
sensitivity (steepness). For each calibration a test gas of a 
predetermined concentration has to be connected to the gas connection 28. 
The line 40 leading from the external gas connection 28 to the cap 29 is 
equipped with a flowmeter, preferably in the form of an NTC resistor, 
which determines the presence of the calibration gas flow and actuates a 
first switch 32, for example by means of an intermediate relay 31. When 
being fitted mechanically upon the sensor housing, the calibration cap may 
at the same time actuate a second switch 33, for example by means of a 
simple pin 34 which is pressed down by the mounting operation. It goes 
without saying that switching may also be effected by other means, for 
example manually by the operator after he has fitted the calibration cap. 
The calibration cap may further comprise guide means 35a, 35b serving to 
connect the sensors correctly to the measuring head, and it is of course 
also possible to have the calibration of both or more gas sensors at the 
measuring head and also at the temperature sensor carried out fully 
automatically. In FIG. 3, the two sensors used in this case are designated 
by S1 and S2, and the temperature sensor is designated by T. 
If both switches 32 and 33 are in the closed condition, a suitable 
transmitter 36 provided in the area of the calibration cap 25, preferably 
a luminous diode supplied with a suitable coded binary signal or simply 
with a predetermined frequency (1 kHz), generates a signal, for example 
this 1 kHz signal (for example in the infrared range) and supplies the 
latter to a receiver diode D2 in the measuring head assigned to the sensor 
S2. 
In the presence of the correct calibration gas flow, the calibration then 
generates the calibration signal (initially for a first sensor S2); the 
microprocessor of the measuring head 12 recognizes the calibration signal 
and transmits it to the central electronic evaluation system so that the 
latter commences a "calibration phase" in communication with this 
particular measuring head, while the central electronic evaluation system 
supplies an acknowledgement signal to the measuring head confirming that 
the signal has been recongized and that the system is ready for 
calibration. This acknowledgement signal is displayed in a suitable manner 
on the measuring head, by means of a luminous diode CR, which means that 
the "calibration" signal lights up to inform the operator that the total 
processor system is accepting and receiving the calibration values. 
In this connection, the following should be noted: Given the fact that the 
central electronic evaluation system must be capable of recognizing 
whether the gas present is a so-called zero gas intended for determining 
the zero point, or a test gas intended for determining the sensitivity, 
the central electronic evaluation system will interpret any gas 
concentrations of, for example, less than 10% recorded during the 
"calibration phase" as calibration or zero gas for determining the zero 
point, and any concentrations above 20%, for example, as test gas for 
determining the sensitivity. The test gas may then have a predetermined 
concentration of, for example, 50% of the final value of the measuring 
range. As a result of the values and data stored in the central electronic 
evaluation system, the latter is informed of the composition of the gas 
with which it is confronted and is in a position to carry out the 
corresponding calibration processes automatically. 
The calibration operation proceeds, on principle, as follows: As the 
measuring values are received during the calibration phase, the central 
electronic evaluation system monitors the incoming signal to detect the 
point where the concentration does no longer change, i.e. where the 
"concentration signal" assumes a zero state or any other stable state for 
a predetermined period of time. The central electronic evaluation system 
takes this condition as an indication that the sensor has assumed a 
balanced condition and is transmitting the test gas concentration, and 
corrects, if necessary, the zero point and steepness (sensitivity) data of 
this respective sensor stored in it, whereafter the calibration process is 
terminated, preferably not by the operator but automatically on 
instructions received from the central electronic evaluation system. Upon 
termination of the calibration process, the central electronic evaluation 
system removes the "calibration" signal (lamps of the diode CR), or causes 
the latter to light up intermittently so as to inform the operator that 
the calibration process for this particular sensor has been terminated and 
that the calibration cap can be removed and placed upon the next sensor of 
the same measuring head, for calibration thereof. The calibration cap 25 
is then fitted on the other sensor, the calibration signal is again 
generated by the calibration cap and supplied to a second receiver in the 
form of a photodiode D1. The frequency generated may now be 2 kHz, for 
example. In this case, the--different--calibration signal may even be 
supplied to the same receiver in the measuring head, and it would also be 
possible to give the calibration cap immediately a design comprising two 
adjacent partial caps by which the sensors on the measuring head can be 
supplied with the test gases in succession. 
In addition, the calibration cap may be provided with a circuit 37 
generating its own timing pulse. It should be noted expressly in this 
connection that the calibration may be performed by adjusting 
corresponding circuits in the area of the measuring head and/or by storing 
the new data resulting from the calibration in the measuring head. This is 
advantageous also insofar as the measuring head is anyway provided with 
electronic means 38, which are indicated in FIG. 3 by the amplifier symbol 
only and which are associated with each sensor so as to permit it, in its 
capacity as interface, to output standardized measuring signals of between 
0 and 1 V in a binary coded form. 
In summary, it can be noted that the gas detection system, including the 
measuring head and the central electronic evaluation system, detects 
automatically that the calibration process is running; it monitors the 
calibration process automatically and takes over the calibration values 
automatically, keeping the operator constantly informed, so that all the 
problems encountered according to the prior art in connection with the 
calibration of gas sensors, which always has to be performed, have been 
eliminated all at once. The operator charged with carrying out the 
calibration only has to fit the calibration cap over the sensor housing 
and to supply the system with the correct test gas. All other operations 
are carried out automatically, due to the capability of the system to 
communicate in both directions so that any errors are absolutely excluded. 
All features mentioned or shown in the above description, the following 
claims and the drawing may be essential to the invention either along or 
in any combination thereof.