to achieve the objectives , the invention consists of an on - line method and equipment for detecting , determining the evolution and quantifying a microbial biomass and other substances that absorb light along the spectrum during the development of biotechnological processes . the method is based on the measurement of the light intensity absorbed by a culture medium at a specific wavelength , in other words , the measurement of the optical density . as an innovation , according to the invention , said method consists of the following : a first light beam of variable intensity and pre - set frequency range is made to pass through a first test - tube through which the substance to be controlled is running . a second light beam of fixed intensity and the aforementioned pre - set spectrum is made to pass through a second test - tube containing a reference sample of the substance to be controlled . the intensity of the first and second beams is continuously compared after passing through the respective test - tubes . the intensity of the first beam is continuously varied so that the intensities of the first and second beams will be identical in the aforementioned comparison . the corresponding electrical signal is processed , which determines the aforementioned continuous variation for the continuous real - time calculation of the biomass concentration or parameter of interest in the first test - tube . the calculation of the biomass concentration or parameter of interest can be made based on a calibration pattern which is a model correlating the values of the aforementioned electrical signal with the biomass concentration or parameter of interest , the concentration of all of the other products of interest being estimated based on this model by way of a suitable observer . the aforementioned model can be obtained by means of a suitable calibration test affording the possibility of comparing the measurement provided by the sensor for different samples of which the concentrations are known . means of pumping and circulating the substance to be controlled from a bioreactor to a first test - tube . a second test - tube in which a reference sample of the culture medium present in the bioreactor is placed . means of emitting light of a variable intensity and pre - set range of frequencies that are applied to the first test - tube . means of emitting light of a constant intensity and the same frequency range mentioned in the immediately preceding paragraph hereinabove , which are applied to the second test - tube . means of sensing the light which passes through the first test - tube . means of sensing the light which passes through the second test - tube . means of comparing the two signals corresponding to the light intensities sensed by the above - mentioned sensing means . means for adjusting the output signal , such that by means of the modification of the power applied to the light - emitting means , the difference between the two output signals of the sensing means will be nil or , equivalently , that the output signal of the aforementioned means of comparison will be nil . means for varying the power applied to the means of emitting light of variable intensity as a function of the output signal of the means of adjustment . means for sensing and remote transmission of the instant value of the signal which is being applied to the means of emitting variable intensity light . and means for processing the output signals of the aforementioned sensing and transmission means , and for calculating ( in terms of the output signal and of a calibration pattern ) the parameter of interest of the aforementioned substance . according to a preferred embodiment of the invention , the light - emitting means consist of led diodes which emit at wavelengths within the light spectrum and which provide for absorbance values sensed by the sensing means within the 0 . 1 to 40 optical density range . the equipment of the invention may incorporate means for eliminating any possible cases of interference caused by gas bubbles or other particles . said means of eliminating any possible cases of interference may consist of an air valve located downstream from the first test - tube , or may also consist of the use of a filtering algorithm integrated into the system processing means . the aforementioned pumping and circulating means of the system shall preferably consist of a hydraulic pump with the suitable pipage . according to the aforementioned preferred embodiment , the above - mentioned test - tubes are of a sturdy material with a low absorption index for light at the wavelengths emitted by the light - emitting means . additionally , in the example embodiment of the invention , the aforementioned test - tubes are located in one same compartment , such that the temperature inside the same will be the same for the purpose of preventing any drift effects due to temperature changes . the means for varying the power may consist of a voltage - intensity converter . the aforementioned means of processing and calculation may have some functional blocks that include a reading data block followed by a filtering block and an estimating block which calculates the concentration of substances of interest based on a mathematical model , this estimating block being connected to a recalibration block and to a results display block . in addition thereto , an optional control block can be incorporated , affording the possibility of obtaining the instant value of the flow of substrate to the bioreactor , thus optimizing the production of biomass or substance of interest . the equipment can be recalibrated on - line by means of the modification of the internal calibration model ( estimating block ) based on the comparison of the result provided by the instrument and the measurement obtained based on an occasional analysis of a sample taken from the bioreactor . in the following , in order to facilitate a better comprehension of this description and comprising an integral part thereof , some figures are provided , in which , for non - limiting purposes of illustration , the object of the invention is shown . fig1 . illustrates the elements comprising the equipment which employs the method of the present invention according to one embodiment thereof . fig2 . provides a functional block diagram of the processing and calculation means included in fig1 above . fig3 . provides the real - time measurement graphics of the concentration of the t73 strain of the saccharomicees cerevisae yeast in different culture situations , the graphic lettered ( a ) corresponding to the concentration of biomass in a culture with glucose and that lettered ( b ) to the culture made with ethanol . fig4 . samples the graphics of the evolution of the concentration of biomass ( a ), of ethanol ( b ) and of glucose ( c ) obtained from an asymptotic observer ( sensor software ) complemented with a kalman filter for the case of glucose feeding . a description is provided in following of one embodiment of the invention , making reference to the numbering used in the figures . thus , the equipment shown in fig1 avails of some means for pumping and circulating the substance ( 1 ) to be controlled from a bioreactor or similar ( 2 ) to a first test - tube ( 3 ), said means being comprised of a hydraulic pump ( 9 ) and the necessary pipage . there is also a second test - tube ( 4 ) in which there is a static control sample of the substance ( 1 ) to be controlled . some means of emitting light of variable intensity ( 5 ) and of a pre - set frequency range act on the test - tube ( 3 ), while some means of emitting light of a constant intensity ( 6 ) within the same aforementioned frequency range act on test - tube 4 . behind the light - emitting means ( 5 , 6 ) and the test - tubes ( 3 , 4 ) are means for sensing light ( 7 , 8 ), the signals of which are transmitted to some means of comparison ( 10 ), the output of which is transmitted to some means of adjustment ( 11 ) which may consist of a pi controller . the regulator output ( 11 ) is transmitted to a voltage - intensity converter ( 12 ) which modifies the intensity of the light - emitting means ( 5 ) such that said variation in the intensity determines the difference between the two output signals of the sensing means ( 7 , 8 ) being nil . fig1 also shows some means for sensing and transmitting ( 13 ) the instant value of the signal which is being applied to the means ( 5 ), which may consist of a current sensor . lastly , fig1 includes means of processing and calculation ( 14 ) to which the signal picked up by means 13 is transmitted . these means of processing and calculation ( 14 ) act as a function of the signal transmitted thereto and of a pattern for the calibration of the parameter of interest of the substance ( 1 ), such that the signal applied to the light - emitting means ( 5 ) is converted into an indication of the instant variation of biomass in test - tube 3 . in this example , the light - emitting means ( 5 , 6 ) consist of led diodes which emit at wavelengths within the visible or infrared spectrum and which allows the possibility of absorbance values sensed by means 7 and 8 between 0 . 1 and 40 in optic density . these means ( 7 , 8 ) consist of silicon photodiodes . in the present example , it has been anticipated to avail of means for eliminating any possible interference caused by gas bubbles or other particles by means of a filtering algorithm integrated into the means of processing and calculation ( 14 ). the test - tubes ( 3 , 4 ) are of a sturdy material having a low light absorption index at the wavelengths emitted by means 5 and 6 . additionally , test - tubes ( 3 , 4 ) are located in one some compartment , such that the temperature in these test - tubes will the be same such that there will be no drift effects due to temperature changes . the functional blocks of the means of processing and calculation ( 14 ) are shown in fig2 , showing a reading data block ( 15 ), the mission of which is that of reading the signal transmitted to means 14 at a pre - set frequency . this block ( 15 ) is connected to a filtering block ( 16 ), the mission of which is that of eliminating and offsetting , by means of a suitable algorithm , the variations caused by the bubbles present in the medium . in turn , block 16 is connected to an estimating block ( 17 ) which calculates the concentration of the substances of interest based on a dynamic mathematical model of the behavior of the microorganisms in the bioreactor ( 2 ). this block ( 17 ) is connected to a recalibration block ( 18 ), the mission of which is that of recalculating the parameters of the model used according to available additional information , as well as the values of the concentrations sporadically obtained by analytical means within a quality control policy ( samples every eight or twelve hours ). additionally , block 17 is connected to a results display block ( 19 ). the diagram in fig2 could optionally be expanded by means of a control block , allowing the possibility of instantaneously obtaining the value of the flow of substrate to the bioreactor ( 2 ), such as to make it possible to optimize , according to a previously - set criterion , the production of biomass or protein . the graphics shown in fig3 and 4 show actual cases of utilization of the system of the invention , measuring , in real time , the concentration of saccharomicees cerevisae in a continuous experiment . in fig3 , the y axes show the absorbance of the medium ( do600 ), whilst the x axes show the time in hours . in fig4 , the coordinate axes show the concentration of compounds in gr / l in the example corresponding to said graphics , the concentration of the t73 strain of the sacchromicees cerevisae yeast ( sc ) is made in real time . an amber - colored led has been used as the light - emitting element , said led emitting a 600 nm wavelength . as the sensing element , a silicon photodiode has been used . apart from this , the electronic device has been built using operational amplifiers . as a computing element , a personal computer with a data acquisition card has been used . algorithms for the filtering of the signal in order to eliminate the effect of the bubbles and to estimate the yeast concentration based on the measurement of the corresponding sensor have also been programmed . the results are those shown in fig3 and 4 . the graphics show different situations , corresponding , in the case of fig3 , to the concentration of biomass in the culture according to the calibration pattern . the graphic ( a ) in fig3 corresponds to the concentration when the culture is fed with glucose , while graphic ( b ) is related to a situation similar to the preceding one , but feeding the culture with ethanol . apart from the above , as a continuation of the application , a sensor software has been designed for the concentrations of glucose and ethanol in the t73 culture for case ( a ) ( glucose - fed ) based on an asymptotic observer complemented with a kalman filter , the results for which are shown in fig4 . the method corresponding to the equipment in fig1 is inferred from the description which has been provided of said equipment , additionally perfectly fitting the method description provided in the previous “ description of the invention ” section , as a result of which , it is not considered necessary to repeat said description of the method in this section .