Controlled temperature blood warming apparatus

In an in-line warmer for heating parenteral fluid and the like supplied from a fluid container (2) through a flexible supply conduit (4) of the type having a box-like fluid warmer enclosure (5) including a closable cover (11), a substantially planar heating plate (19) positioned within the enclosure, one surface of the plate including a sinuously-shaped groove (23) configured to maintain a length of the supply conduit in heat transfer relationship with the plate for heating the fluid within the conduit, electric heating means (34) for heating the plate, and a semiconductor switch (TR1) operatively connected between said heating means and a source of alternating voltage (52) for controlling current flow to the heating means when said switch is activated, the improvement in combination therewith comprising means for maintaining the temperature of the plate at a predetermined temperature to prevent overheating of the parenteral fluid or the like comprising:

• a first temperature sensor (37) attached in heat transfer relationship to said plate for providing a first electrical signal representative of the actual temperature of the plate;
• a voltage controlled oscillator (101) responsive to said first electrical signal producing an oscillating output signal having a frequency of oscillation which varies with the temperature of the plate as sensed by said first sensor, said circuit being so designed that said oscillating output signal ceases whenever the temperature of said plate as sensed by said first sensor exceeds said predetermined temperature;
• a second temperature sensor (38) attached in heat transfer relationship, to said plate for producing a second electrical signal representative of the actual temperature of the plate;
• comparison means (A3) responsive to said second electrical signal for producing an output signal when the temperature of the plate as sensed by said second sensor exceeds a predetermined maximum permissible plate temperature value representative of the overheating temperature of the fluid;
• and
• switch control means (105) for activating said semiconductor switch to cause current flow to the heater means in proportion to the frequency of the oscillating output signal to heat the fluid only in response to the presence of said oscillating output signal and for deactivating the semiconductor switch to prevent current flow to the heater means in response to either the absence of the oscillating signal or to the presence of said comparison means output signal, or both,
• whereby component failure associated with said oscillator (101) or switch control means (105) will cause the oscillating output signal to cease, thereby de-energizing the heater means to prevent overheating of the fluid.