An air flow transducer for controlling heated humidifiers used in neonatal artificial ventilation, suitable for in-line application in monopatient breathing circuits, is described here. The sensor is built with two nominally identical bipolar junction transistors, with different packages, as hot elements operated at a regulated constant voltage. The operation principle is based on the differential convective heat power exchanged with the fluid stream due to the different thermal conductivities of the transistors' packages. The underlying theory is described in mathematical terms and the theoretical model is validated with experimental data in the mass flow rate range from 4 to 215 mg s(-1). The nonlinear behavior allows sensitivities from -5 mV/(mg s(-1)) at flow rates in the range of 4-130 mg s(-1) to -2 mV/(mg s(-1)) at higher flow rates up to 215 mg s(-1). The linear range extends from 40 to 130 mg s(-1), with constant sensitivity equal to -5 mV/(mg s(-1)). The differential nature of the output allows to obtain repeatabilities in the order of 2% for fluid temperatures between 20 and 25 degrees C and of about 6% if the fluid temperature lies in the range of 15-35 degrees C. The relatively long time constant, in the order of 20+/-5 s, makes the sensor suitable for average flow rate measurements. Using the sensor's output as a control variable of a heated humidifier for artificial ventilation, the relative humidity of gases varies by only 20% in the flow rate range of the sensor (from 95% to 75%), whereas the same parameter shows a variation of about 40% (from 100% to 60%) with the same humidifier without flow control.
[1]
H. Turndorf,et al.
Humidity and the anesthetized patient.
,
1979
.
[2]
L. Machattie.
The transistor as an anemometer
,
1979
.
[3]
Gerard C. M. Meijer.
Thermal sensors based on transistors
,
1986
.
[4]
B. Escudier,et al.
Heat and moisture exchanger vs heated humidifier during long-term mechanical ventilation. A prospective randomized study.
,
1991,
Chest.
[5]
Anton F. P. van Putten,et al.
Silicon thermal anemometry : developments and applications
,
1996
.
[6]
D. Galler,et al.
Relationship between the humidity and temperature of inspired gas and the function of the airway mucosa.
,
1996,
Critical care medicine.
[7]
R. Branson,et al.
HUMIDIFICATION FOR PATIENTS WITH ARTIFICIAL AIRWAYS
,
1999
.
[8]
A. Gustafsson,et al.
A new device for 100 per cent humidification of inspired air
,
2000,
Critical care.
[9]
T Mashimo,et al.
Performance of heated humidifiers with a heated wire according to ventilatory settings.
,
2001,
Journal of aerosol medicine : the official journal of the International Society for Aerosols in Medicine.
[10]
M. Davies,et al.
Inspired gas temperature in ventilated neonates
,
2004,
Pediatric pulmonology.
[11]
J. Guttmann,et al.
Moisturizing and mechanical characteristics of a new counter-flow type heated humidifier.
,
2007,
British journal of anaesthesia.