we have achieved very good results by using the following arrangement of light - emitting diodes ( led &# 39 ; s ) and photo detectors , when assessing hematocrit values . these values correlate very well with laboratory reference values . four led &# 39 ; s are arranged in a preferably — but not limited to — perpendicular fashion to each other around a receptacle , such as tubing , for the blood as apparent in fig1 . the light detectors are arranged in a fashion where they similarly are preferably perpendicular to each other according to fig2 , but at a distance longitudinally away from the encirclement by the led &# 39 ; s , as exhibited in fig3 . in a further embodiment , a second encirclement of light detectors is fitted . the arrangement is apparent in fig4 . the led and photo detector arrangement should for best understanding be viewed as groups of led &# 39 ; s and photo detectors : for instance , led 5 a , and photo detector 6 b is one group . another group can be led 5 b , and photo detector 6 a and 6 c . note that no led &# 39 ; s and photo detectors are aligned to achieve direct transmitted light . the invention does not make use of directly transmitted light , as often is the case in prior art . a sample of light detected from a group of one or several photo detectors can be taken at any one short instance in time . another sample can be taken from the same or another group as a second sample . preferably , a first sample is taken from a first group comprising led 5 a , and light detectors 6 b and 6 d , a second sample is taken from a second group comprising led 5 b , and light detectors 6 a and 6 c , a third sample is taken from a third group comprising led 5 c , and light detectors 6 b and 6 d , and finally a forth sample is taken from a fourth group comprising led 5 d , and light detectors 6 a and 6 c . a first result is derived from theses four sequentially acquired samples being signal processed . the process can include variations of amplification factors for the signals from the detectors , and also correlation factors between these signals , to further enhance the detection of the blood constituent to be measured . the results make a first result for blood constituents , such as hematocrit . in this process , the error occurring from variations in the cross section of the flow pattern in the vessel is reduced . furthermore averaging may reduce the effect the vessel wall has on the measurement . this is highly beneficial if the vessel is the extracorporal circuit of a dialysis system . one of the major advancements in the disclosed invention resides in the new possibility to measure hematocrit trough the walls of dialysis extracorporal circuit , namely the so - called transport tubing of the circuit . it is highly advantageous that no special cuvettes or dedicated arrangements to the disposable bloodlines are necessary . our process even makes it unnecessary to fit dedicated tubing to the extracorporeal circuit . this feature is considerably cost saving for the health care provider . fitting the hereby disclosed probe on the transport tubing also has the advantage that the probe is not interfering with the ordinary functions of the dialysis system . also , it furnishes the highly beneficial possibility to upgrade any already existing dialysis system with measurement of hematocrit , even if it is not prepared for such purpose . subsequently blood volume changes can be calculated and displayed . in one embodiment of the invention , two arrays of detectors are employed . downstream ( or upstream ) a blood flow in a vessel such as tubing , a second array of detectors is fitted . this is apparent in fig4 . the mathematical signal processing can further enhance the results by including this “ second order ” of detectors in the process . in another embodiment of the invention , a second arrangement of led &# 39 ; s and photo detectors , including a second array of detectors is fitted . this is exhibited in fig5 . in this embodiment , the led &# 39 ; s emits a different wavelength . this allows limited spectral analysis for further calculation of blood constituents , such as saturation of hemoglobin as known in the art . the results derived from this second array , can beneficially be incorporated in a signaling process with the values derived from the aforementioned first array . such process makes it possible not only to output all parameters from blood constituents , but also let the saturation value influence the input of signals from the first array to the signaling process . this is beneficial , as blood saturation may influence the first results of blood constituents from the first process from the first array . in the drawings and the above description a transparent blood transporting tubing is shown clamped between two essentially v - shaped profiles in the walls of which the led and sensors are arranged . in an alternative embodiment v - shaped groves in blocks may be used to clamp and shape the tubing so that its walls become essentially flat at leds and sensors . in a further embodiment the sensors may be arranged in small holes with even smaller openings serving as collimators towards the tubing . it is not today clear why the invented measuring method and device are so superior in relation to the prior art , one theory could be the offset between sensors and light source . only light that has been dispersed from the volume of the blood in the path of the light and into the sense sector of the sensor and from this into the sensor will be registered . in other words only light that has been dispersed at least twice will reach the sensor . by arranging source and sensor perpendicularly blood cells in a major part of the tube cross section will have the opportunity to contribute so that the signals from the sensors become a function of the hematocrite value .