CA2157582A1 - Dual sensor air-in-line detector - Google Patents

Abstract

A device is provided for the detection of gas, e.g. air in tubing (14) carrying a liquid. The device includes a transmitter (21) for transmitting a light beam to the liquid contained in the tubing (14), and first and second light receivers (20a, 20b) for receiving reflected and transmitted light emitted from the transmitter (21) and which is incident upon the liquid in the tubing. The light receivers are located within a body member (12) around the liquid conducting tubing and each produces a signal according to the condition of the liquid in the tubing. Both receivers produce such a given signal when gas is present in the liquid but each receiver also produces such a signal under a different liquid condition. A processing means (33) is provided which is responsive to the outputs from the two receivers and indicates that gas is present in the tubing only when both provide said given signal.

Description

wo 95/21374 2 1 ~ 7 5 8 2 Pcr/usss/0l424 DUAL SENSOR AIR-IN-I,I~E DETECTOR

Field of the Invention:

S This invention relates to devices for tletecting gas, e.g. air or air bubbles,in fluid con~lucting tubing and in p~rticular in fluid con~llJctin~ tubing forlTlin~ part of a fluid flow system utili7~ for the intravenous supply of fluid to a meAiC~l p~ti~l~t 10 Discussion of Bac~ d:

Typically ll~l~a~cl~t walled p.v.c. tubing is employed in systems as last-mentioned bec~nse it is hygienic and cheap it being common practice to change and discard the length of tubing in use frequently. A known 15 arrangement for use in clini~l analysis and capable of ~etecting air in tubing utili7~s a device as illusl,~ted in transverse section in Figure l of the acco~ yi"g drawings.

R~fç~rin~ to Figure l, the known device inc~ es a body member 1 having 20 a passage 2 p~ing thel~tlllough in which may be ~ccornmodated a length of tran~ar~nt walled p.v.c. tubing 3. Passage 2 is open at the top (as viewed) in order that the tubing 3 may readily be slotted into position and of course removed after use. Fxt~n-lin~ into the body l from its base and right-hand side (as viewed) l~"peclively are two circular-cylindric~l 25 passages 4 and 5 which are orthogonal to each other and exit via a~l Lul~,s 6, 7 respectively into tubing p~cs~ge 2. ~ oc~t~ in circular cylinclric~l passage 4 is an infra-red receiver 8 (a phototransistor) which receives infra-red energy tr~n~mitte~ by an infra-red tr~ lr~ 9 (an LED).

30 In operation the output level of receiver 8 depends upon the nature of the W O 95/21374 21 5 7~ 82 PCTrUS95101424 i ~ . .. . .
fluid p~ing through the tubing 3 past receiver 8 and tr~n~mitt~-r 9.
Difr~r~ fluids will result in different output levels with a si~nifir~nt change if a gas, e.g. air, is ~r~selll. For example, in a test a voltmeter 10 co~ P~lecl to the output of a suit~ble ~ietector circuit 11 was found to S in~ t~ 0.1 volts when the fluid p~sins~ through tubing 3 was ~i~till~A
water; 0.2 volts when the fluid was semi-skimm~l milk; 1.4 volts when the fluid was a 20% intralipid solution and 4.2 volts when air passed through.

10 While not a ~ y function of a clinir~l analyzer, a device as shown in Figure 1 will therefore operate as a detector of air p~in~ through tubing 3. Ho~ t;l, as is l~r~sel-~ed, p.v.c. tubing typically used in a clinir~l analyzer is small bore thick-walled tubing with an outside ~i~meter of 2.5mm and an inside ~ meter of O.9mm. In the intravenous supply of 15 fluids to a patient however, the standard p.v.c. tubing used is of relatively large bore and thin walled having an outside ~ et~ of 4mm and an inside rli~mpter of 3.1mm.

EA~1111e11lS have been carried out in con~e~;lion with the det~oction of air-20 in-line with a device generally as ill~:ilrated in Figure 1, but adapted ionally to accept the relatively large bore, thin walled tubing l.~tili7Yl for the intravenous supply of fluid to a patient but the results achieved were not s~ti~f~ctory~ Particularly bearing in mind the critical importance of ~et~.cting air-in-line in such applications, the changes that 25 took place in the output of the receiver c~ ,s~onding to receiver 8 in Figure 1 were inmffici.o.ntly m~rk~l for the device to be regarded as useful in this connection However, further e~ ..P.nt~tion led to the intro~uctio~ of an optical spacer between the ll~ c. and the tubing, and ~e tubing and the receiver and with this, markedly il~roved results 30 were achieved.

wo 95/21374 2 1 5 7 5 8 2 PCT/US95/01424 Such an optical spacer is known from EP-A-0481656, which discloses a deYice for ~let~cting the presence of air in liquid con~lucting, tr~n~lu~ent or ttar,s~alenl tubing, the device co~ ing a passage for ~ccol.. odating of said tubing, a l~ er for tr~n~mitting radiation (in this case, light) 5 towards said passage; a receiver for receiving radiation from said passage which has passed through the tubing, the receiver being operable to produce an output signal (i) when air is plcsenl in the tubing and/or (ii) when the dilution ratio of the liquid in the tubing is below a first predet~lnlil.ed threshold; and proces~ing means. The ~loces~ g means 10 proc~es said output signal to provide an in-lic~tion that air is ~l-,se-ll inthe tubing. Hence, the device tends to make a false detection that air is present in in~t~nces define~ in (ii) above.

Claim 1 relates to an improvement of the device of EP-A-0481656, 15 wherein the improvement comprises means for eli...i..A~ false ~let.~ctionc made by the receiver, that gas is present in the tubing, and comrrising means (iii) a second receiver for receiving from said p~S~ge r~ tion which has passed through said tubing, the second receiver being operable to produce an output signal when gas is ~reselll in the tubing, the second 20 receiver m~king false detections under lirrt;lc;nt liquid conditions from the first receiver, and (iv) proces~in~ means opelati~ely connPct~ to both receivers to receive said output signals to provide an intlic~tion that gas is present in the tubing only when the output signals from both receivers are Se,.l~.
Preferably the tl~s~ lfr and receivers are respectively a light energy tr~..~...;l~er and light energy receivers and are all o~e,~livt; in the infra-red spectrum, and preferably the tr~n~mitter is a LED (light emitting diode) and the receivers are phototr~n~i~tors.

WO95/21374 21 5 75 8 ~ PCT/US95/01424 Preferably, the device includes an optical spacer defining said passage and occupying space between said passage and said tr~ncmi~ter and between said p~cs~Ee and said receivers, the optical spacer comrri~ing a cylindrical C1~P~I~P~I~ having a ~iP-lectric co":i~nL g1eater than that of air, said tubing S passage e~rtPnrling along a lon~itl~lin~l axis of the cylintlTic~l eltoment for ~coInmodating said tubing in intim~te contact with said cylin~ iC~l n ÇlF .~F.I~t r~ bly,the optical spacer is in the form of a collar ~u11o.~ linE the 10 tubing, the L~ and the receivers being housed in the body.
P~,f~ly the tr~ and the receivers are located in passages ekl~ndillg through the body and opening towards the tubing acco."lllodating passage.

1~ Normally the ~.~....Il~,r and receiver locating passages open lo~ r~s the tubing accommodaLillg passage via respective apc1~u,es. The a~11u,~s may be in fixed walls, integral wi~ the body, which otherwise close the passages or in plugs ins~l~d in the passages otherwise to close the same.

20 The a~JG~ Lur1s may be of dirr~ sizes chosen to provide o~Li",ulll effect in any given device. Commonly, the a~e1lu,~, through which the lln.-~ ,r co.. ~ ir-~tes will be of smaller cross-sectional area than the a~1Lu~Gs through which the receivers comm-lni~te. In one embo~imt~-nt wherein the ~c.lLIrcs are of circular cross-section, the diameter of the 25 apc1lu-e through which the tr~ncmitter communicates is at least approxim~t~ly half the diameter of the ~C1lu1Gs through which the receivers communic~t~.

P~ bly the lr~..s..~;tl.,r and receivers are spaced around the tubing 30 accomlllodating passage, prl_fGr~ly with their principle optical axis in the WO 95/21374 2 1 5 7 ~ 8 2 PCT/US9~/01424 same transverse plane. Preferably again the receivers are arranged with their prinriple optic axes orthogonal one to the other. Where the axes lie in the same transverse plane it may be found that s~ticf~r-tory results are obtained with the optic axes at some relative angle other than 90 but it 5 is believed that o~Li--,u-l- results are obtai,lcd when the axes are orthogonal one to the other. Again, spacing the LIA~ and receivers along the length of the tubing accommodating passage may be found to give s~ticf~rtory results but arranging the l~ ;ller and receivers such that their l";,lr;ple optic axes are spaced around ~e tubing accommodating 10 passage is believed to provide optimum results. The tubing used in the illLIa~,.,.lOus supply of fluids to a patient is of course of circular cross-section and therefore for this application the tubing accommodating passage is normally of circular cross-section.

15 Typically the outer ~ meter of the optical spacer, when pl~ scnl, will be bet~ twice and three times the outside rli~metp-r of said tubing. In a ell~d embo~ f-l-~ the outer rli~met~r of the spacer is 2.5 times the outer ~ te~ of the tubing. r~ c~c.~bly the l~An~"~ and receivers are discrete devices and pfc~bly an LED (light emitting diode) and 20 phototli~nc;~ respectively. Norrnally such co,ll~onenls are generally circular-cylindrical in overall outline and accor~ingly, the tr~ncmit~er and receiver p~Cc~ges are normally circularly cylintlriç~l. In order to simplify the insertion and removal of the tubing, ~rcfe~ably the tubing aCco~ o~ting passage has a linearly e~ct~ ing slot through which the 25 tubing may be slotted.

The m~teri~l chosen for the optical spacer, when present, should have a ~lie~ectric coll~lalll which is an optically reasonable match to the ma~erial of said tubing. Preferably the material is acrylic.

WO 9St21374 2 1 5 7 ~ ~ 2 ` PCTrUS95/01424 In a preferred embo~limrnt the ll~n~n~ fr projects a light beam to the fluid con~ucfing tube and ligh~ receivers in the form of a pair of sensors disposed perpen~lir~ rly to each other o~te in opposite modes such that if an air bubble is ~r se.,l in the line the ~letector will be able to reliably 5 distinguish this cit~-~tion from the case where there is no air bubble. One sensor is disposed 90 from the optical axis of the Ll~n~ln;~ while the other sensor is disposed 180, ie., along the tr~ optical axis.
Light beams projected from the L~ er are incident on the tubing and are thus reflected or Li~ ...;ll~ d~en~ g upon the ch~r~cterictics of the 10 fluid (eg., the opacity of the Ruid and/or dilution ratio of the fluid) and also upon the presence or absence of air or air bubbles in the line. The two ~~ lic~ -ly disposed se.lsols provide oulpuls in~lir~tive of the amount of light received. A processor determines the presence or absence of air in the line based on the combination of the o~l~uLs of the two 15 se-~o,~. When the oul~u~ of both SenSGl:j are high, the processor d~,t~,~...i.-r~ that air is lJr~s~lll in the line, v~l,e.~as if one of the two sel~ols' outputs is low, the pl~,`ces~ g device deLel.l-i--es that no air is plese.lt in the line. With this arrange...enl, both sensor outputs are used in order to del~l,.,ine the presence or absence of air or air bubbles in the 20 fluid con~ucting tubing, thus providing a reliable det~ction ~aLus regardless of the initial calibration of the sensols and IlA.~ ler.
-Brief Description of the Drawings 25 Figure 1 is a cross-sectional view of a conventional device for ~et~cting air in tubing;

Figure 2 is a transverse cross-sectional view of the device ~ close~ in EP-A-0481656 for ~etecting the presence of air in tubing forming part of 30 a fluid flow system for the intravenous supply of fluid to a m~ic~l ~ 21~7~82 -WO 95t21374 PCT/US95/01424 p~ti~ nt Figure 3 shows an arr~ng4...~ of an embodiment according to the ~l. sent invention wherein a single IIA..~ el and a pair of sensors which are S provided perpen~lir -l~rly to each other are shown;

.
F~gure 4 illu~ les the output of sensor 20a shown in Figure 3;

Figure 5 shows the output of sensor 20b shown in Figure 3;
Figure 6 illu~Llales a graph of the values shown in Table 1 for a Ubest calibrationn case;

Figure 7 is a graph of the values of Table 1 for the worst case lS calibration; and Figure 8 is a circuit diagram illustrating the relationship between the lln~ and se.-~o~ ~ of Figure 3.

20 Description of the Plerelç~d Embodiments RP-f~rrinE-now to the drawings, wherein like reference numerals ~esign~te i~ntic~l or co~ ,yonding parts throughout the several views, and more partir -l~rly to Figure 2 thereof, a device as ~licçlose~ in EP-A-0481656 inrludes a body member 12 having a tubing accorl.ll,odating passage 13 passing the.Glllrough in which may be accGIIllllodated a length of Llanspalelll large bore thin-walled p.v.c. tubing 14. Tubing 14 has an internal di~mçter of 3.0mm and an outside ~ me~ter of 4.1mm. Passage 13 y~ y has a ~ meter equal to lOmm. ~-oc~tçd within passage 13 30 is an optical spacer 15 of material chosen to be a good optical match with ; ~ ~ r ~ ~ ~

the material of the tubing 14. In this case the material of the optical spacer 15 is acrylic. The optical spacer 15 sull~ullds the tubing 14 save for a gap 16 which is of width sufficient for the tubing 14 to pass through.
Gap 16 is ~lign~d with a slot 17, of similar width, e~ten~ling longit~-din~lly S through the top (as viewed) of the passage 13. The slot 17 and gap 16 enable the tubing 14 to be readily slotted into position and removed after use. F~cterl~ling into the body 12 from its base and right-hand side (as viewed) respectively are two circular-cylindrical passages 18 and i9 in which are located respectively an infra-red receiver 20 in the form of a 10 phototr~n~i~tor and an infra-red tr~n~mitter 21 in the form of an LED.
Circular-cylinrln~l passages 18, 19 are orthogonal to each other and exit via a~l~ures 22, 23, respectively, into tubing acco,lllllodating passage 13.
The oper~ing~ of a~elLul~s æ, 23 in passage 13 are covered by the outer s~ re of optical spacer 15. In this particular example the receiver and 15 ~ f a~llur~,s 22, 23 are not of the same ~ mtott r. The fli~meter of ~ .r a~,lul~, 23 is one-half that of receiver a~cllul~ æ.

With an arrangement as descrihed above and ~Itili7ing for infra-red tln.lc..-;llef 21 a type TSTS 7202 LED and for infra-red receiver 20 a type 20 BPW 77B and with receiver and tr~t~ er a~ures of rli~m~ottor 3.0mm and 1.5mm respectively, a test corresponding to that described earlier in r~f~ ce to Figure 1 provided an indication of 0.3 volts when the fluid passage through tubing 14 was .listi~ water; 1.2 volts when the fluid was semi-skimmPA milk; 1.2 volts when the fluid was 20% intralipid 25 solution and 4.0 volts when air passed through. It will be recalled that with an arrangement generally as illustrated in Figure 1 (but adapted dimensionally to accept relatively large bore, thin-walled tubing such as tubing 14 in Figure 2, in contrast) provided a change in output in the p1esG.,ce of air which was insufficiently marked to be useful in the terms 30 of "air-in-line" detection.

WO 95/21374 2 1 ~ ~ 5 ~ 2 PCT/US95/01424 An embodiment of the ylese~ll invention will now be described with reference to Figure 3 of the accompanying drawings. In this embodiment an additional sensor 20b is provided along the longih-~lin~l axis of C~ cr 21 at a 90 angle to sensor 20a. As shown in Figure 3, S inr~ d LED l~ er 21 of type TS 7302, for example, is provided for projecting a beam of light to fluid con(lucting tubing 14. The photoll~ tors 20a, 20b of type BPW 77, for e~mrle, also o~ale in the il~ ;l spectrum wherein sensor 20a is provided at a right angle to the direction of inr;~en~e of the light beam from the L~ ...;tl~, while 10 sensor 20b is provided along a path parallel to the direction of light beam inr;denfe, ie., at a 180 angle. A tra..:j~ar~nl optical spacer 15 which may be of acrylic .~ cl.~l, for example, is also provided within a tubing accol...uodating passage 13 in the body member 12 for holding the large-bore thin-walled tubing 14.
The ILAII!I~ and first and second energy receivers are loc~t~A in respective passages which extend through the body member and open into the tubing acco-l----ol~ting passage. A~LUrGS 22, 23, 24 are shown which connect the tubing acco...---odating passage with each of the 20 r~sye~cLi~fe passages provided for the ll~n~...;llPi and receivers. The a~llules may be of dirr~ L sizes for each of the receivers and ,r or, alternatively, may be the same size for two of these elPmPntc and of a dirr~,r~ t size for the third cle~..e.lL. Also, the e~ ~y~lLulG may be of smaller cross-sectional area than the 25 ay~lulG through which light is received by the first and second receivers.
In a yl~rell~d embodiment, the ayel Ll11GS are of circular cross-section with the ~ meter ofthe a~lL.~ through which the tr~ el Lli~ ...itC light being aypl~o~ Ately 1.5 times g~akr than the (li~meter of the apertures through which the first and second receivers con~ icate with the tubing 30 aCconlmo~ting passage. However, the invention is not limited to such a WO 95/21374 21 5 7 ~ 8 2 PCT/US95/01424 ~
~, i;,; .
circular cross-sectional shape for the ~lur~s. A suitable ~ m~t~r for the optical spacer is between 2 and 3 times larger than the outer ~ mPter of the large-bore thin-walled tubing. A preferred outer ~ m~-t~r of the tubing ~ccommo-l~ting passage is ~ t~ly 2.5 times greater than the 5 outer di~meter of the tubing.

With the arr~n~...e.-~ of the tr~n~mitt~-r and sensors shown in Figure 3, the voltage oul~uls provided by se-n~ors 20a and 20b are as shown in Figures 4 and 5. As ~lisc~lssed above, using the output of sensor 20a 10 alone will provide s~ti~f~ctory results provided that the calibration of the sm;ller and sensor is within a predetermined range. However, as shown in the worst case calibration illustrated in Figure 7, the output of sensor 20a at a dilution ration of 9/1 for a 20% intralipid solution may be equal to the calibrated output of sensor 20a, ie., the air setting value.
15 Since this air setting value may be the same as the output when an air bubble is ~r,senl in the line, the device shown in Figure 2 of the dl~wings (which only uses sensor 20) will be unable to distinguish between the ~r~ sence of an air bubble in the line and the case where no air bubble is ~ç~sent. With the device illustrated in Figure 3, on the other hand, even 20 in the worst case calibration situation, the detector will be able to distinguish between an air bubble and the absence of an air bubble at any dilution ratio due to the use of the second sensor 20b which operates in an opposite mode from that of sensor 20a. In other words, at low dilution ratios such as a~lok;m~tely 10/1, for example, the output of sensor 20b 25 will be low, when the output of sensor 20a is high. Similarly, when the dilution ratio is high, the output or sensor 20b will be high while the output of sensor 20a will be low.

It is in~olLallt to note that for either sensor when an air bubble is ~l~,sellt 30 in the line the output of each of the sensors will be high, ie., will be equal to the air setting or calibration value. Thus for the situation when an air bubble is ~l-,senL in the line and the dilution ratio is such that the output of sensor 20a is subst~nti~lly equal to the output of sensor 20a for the case when an air bubble is yrcsellt~ the output of sensor 20b will also be high 5 (3 volts) and equal to the air setting value. In this m~nner, the detector device accor~ling to the ~reSe~lt invention will be able to reliably detect when an air bubble is in fact ~r~sellt in the fluid con~cting tubing. On the other hand, if no air bubble or bubbles are ylesen~ the output of sensor 20a may be high but the output of sensor 20b will be low, thereby 10 inrli~ting that no air bubbles are present.

The results of the experim~nt~l data are shown in the following table.

Table 1 Worst case Best case Dilution Sensor20a Sensor20b Sensor 20a Sensor20b Intra 100% 2.04 0.01 1.27 0.01 2:1 2.61 0.02 1.87 0.02 4:1 3.87 0.14 2.44 0.04 8:1 4.15 0.01 2.70 0.1 10:1 3.16 0.24 2.57 0.15 16:1 3.99 0.07 2.53 0.12 20:1 3.55 0.18 2.20 0.22 30:1 2.34 0.52 1.73 0.41 40:1 2.34 0.48 l.SS 0.57 50:1 1.95 0.62 1.30 0.82 60:1 1.81 0.74 1.3i 0.65 70:1 1.55 0.97 1.13 0.84 80:1 1.30 1.30 1.08 0.91 90:1 1.10 1.60 0.87 ~ 1.25 100: 1 0.99 2.00 0.70 1 .90 500:1 0.68 4.74 0.46 4.71 1000:1 0.58 4.76 0.48 4.77 The results of the prece~in~ Table 1 are illustrated graphically in figures 6 and 7 of the accol~,pallying drawings. Figure 6 shows the best calibration results, ie., the calibration of output sensors such that the 10 ou~put of sensor 20a does not intersect the output of sensor 20a for an air set~in~ ie., 4.0 volts. However, as shown in figure 7, for worst case calibration the output of sensor 20a at relatively low dilution ratios may be the same as the output of sensor 20a for the air setting case, and also the cæe when an air bubble is ~l~se,ll in the fluid line. HoweveL, using 15 the two sensors together, the situation when no air bubble is present can be eæily deterrnined since the output of sensor 20b will be low at the point where the output of sensor 20a is close to the air setting value. If, on the other hand, an air bubble is p~sel-t in the line, both sensors 20a and 20b will inr~ te high outputs which will never be the case when air 20 is not ~resenl in the line. The results can be summ~rized in the following table.

Table 2 FLUID SENSOR20a SENSOR 20b Air High High Water Low High Dense Fluid Low or High Low 21~82 WO 95/21374 ~ PCT/US95tO1424 Figure 8 illustrates the circuit diagram arrangement of the tr~nsmitt~r and . SenSGl~ of Figure 3. As shown, the tr~n~mitttq.r 21 is connected in serie with a resistor R, equal to 100 ohms in a preferred embodiment, and is conn~cteA. between a 5 volt source and ground. Also, it should be noted 5 that the angle between the light beams received by sensor 20a and those received by sensor 20b is equal to 90 since the sensors are at right angles to each other, and thus the angle illustrated in Figure 8 is not to be considered the actual angle between sensors 20a and 20b. Output l~ni~lals 32, 31 of sensols 20a, 20b rGs~ccli~ely~ are connect~A to ground 10 through 200 Kn variable resistors R2, R3, respectively. The outputs of sensors 20a, 20b are input to a processing device 33 which delel,l,ines the presence or absence of air in the tubing 14 and oul~uls ~yl~liate signals to a display device (now shown). Proces.sing device 33 outputs a first signal when the outputs of sensors 20a and 20b are both high, a second 15 signal when the output of sensor 20a is low and sensor 20b is high (inAir~ting water or a highly low diluted solution), and a third signal ~I,e~ er the output of sensor 20b is low (indic.~ting a dense fluid at low dilution). The proc~ssing device 33 may incll~de a microprocessor o~ali.lg under program control for gcne~ati.lg the ~pr~liate output 20 signals colrespollding to the oul~llls received from the sensors 20a, 20b thereby inr~ir.~ting the ~l~sence or ~l~se~ce of air in the line.
Alte~ ely, the proces~;ng device 33 may inrluAe discrete logic CilCuilS
for receiving the outputs of the light receivers and generating the requisite outputs.

Claims (17)

WE CLAIM

1. In a device for detecting and indicating the presence of gas in liquid conducting tubing, the device comprising a member defining a passage for accommodating tubing; a transmitter for transmitting radiation towards said passage; and a receiver for receiving from said passage radiation which has passed through said tubing, the receiver being operable to produce an output signal when gas is present in the tubing, the improvement comprising means for eliminating false detections made by said receiver that. gas is present in the tubing length and comprising:-(i) a second receiver for receiving from said passage, radiation which has passed through said tubing, the second receiver being operable to produce an output signal when gas is present in the tubing, the second receiver making false detections under different liquid conditions from the first receiver; and (ii) processing means operatively connected to both receivers to receive said output signals to provide an indication that gas is present in the tubing only when the output signals from both receivers are present.

2. The device of Claim l, including an optical spacer defining said passage and occupying space between said passage and said transmitter and between said passage and said receivers the optical spacer comprising a cylindrical element having a dielectric constant greater than that of air, said tubing passage extending along a longitudinal axis of the cylindrical element for accommodating said tubing in intimate contact with said cylindrical element.

3. The device according to Claim l, wherein the principle optical axis of said transmitter and said first and second receivers intersect the axis of said passage and are in a common transverse plane.

4. The device according to Claim 1, wherein said first and second receivers are mounted in said device substantially perpendicular to each other.

5. The device according to Claim 4, wherein said transmitter is mounted perpendicularly to said first receiver and along the optical axis of said second receiver so as to be facing said second receiver.

6. The device according to Claim l, wherein said transmitter and said first and second receivers are operative in the infrared spectrum.

7. The device according to Claim 1, wherein said transmitter comprises an LED and said first and second receivers comprise phototransistors.

8. An apparatus for supplying a medical fluid to a patient, comprising tubing having a portion thereof mounted in the passage of a device according to Claim 1, wherein each receiver is operable to produce an output signal when air is present in said portion of the tubing.

9. A device for detecting and indicating the presence of gas in liquid conducting tubing, the device comprising a member defining a passage for accommodating tubing;
a transmitter for transmitting radiation towards said passage;
a first receiver for receiving from said passage radiation which has passed through said tubing, the first receiver being operable to produce an output signal when gas is present in the tubing and making false detections that gas is present in the tubing under first liquid conditions.

a second receiver for receiving from said passage radiation which has passed through said tubing, the second receiver being operable to produce an output signal when gas is present in the tubing and making false detections under different liquid conditions from the first liquid conditions;
processing means operatively connected to both receivers to receive said output signals to provide an indication that gas is present in the tubing only when the output signals from both receivers are present and an optical spacer defining said passage and occupying space between said passage and said transmitter, and between said passage and said receivers, the optical spacer comprising a cylindrical element having a dielectric constant greater than that of air, said tubing passage extending along a longitudinal axis of the cylindrical element for accommodating said tubing in intimate contact with said cylindrical element.

10. The device accoding to Claim 9, wherein the principle optical axis of said transmitter and said first and second receivers intersect the axis of said passage and are in a common transverse plane.

11. The device according to Claim 9, wherein said first and second receivers are mounted in said device substantially perpendicular to each other.

12. The device according to Claim 11, wherein said transmitter is mounted perpendicular to said first receiver and along the optical axis of said second receiver so as to be facing said second receiver.

13. The device according to Claim 9, wherein said transmitter and said first and second receivers are operative in the infrared spectrum.

14. The device according to Claim 9, wherein said transmitter comprises and LED and said first and second receivers comprise phototransistors.

15. An apparatus for supplying a medical fluid to a patient comprising tubing having a portion thereof mounted in the passage of a device according to Claim 9, wherein each receiver is operable to produce an output signal when air is present in said portion of the tubing.

16. A device for detecting and indicating the presence of air in liquid conducting tubing, the device comprising:
a member defining a passage for accommodating tubing;
a transmitter for defining light towards said passage;
a receiver for receiving from said passage light which has passed through said tubing, the receiver being operable to produce an output signal when (a) air is present in the tubing and/or (b) when the dilution ratio of the liquid in the tubing is below a first predetermined threshold;
a second receiver for receiving from said passage light which has passed through said tubing, the second receiver being operable to produce an output signal when (c) air is present in the tubing and/or (d) when the tion ratio of the liquid in the tubing is above a second predetermined threshold; and processing means operatively connected to both receivers to receive said output signals to provide an indication that air is present in the tubing only when the output signals from both receivers are present.

17. The device of Claim 16 including an optical spacer defining said passage and occupying space between said passage and said transmitter, and between said passage and said receivers, the optical spacer comprising a cylindrical element having a dielectric constant greater than that of air, said tubing passage extending along a longitudinal axis of the cylindrical element for accommodating said tubing in intimate contact with said cylindrical element.