Infiltration indicator and alarm

Claim 1

A flow restrictor, for use in an infusion system comprising a source of liquid infusate pressurized to a substantially constant known driving pressure driving a flow rate of liquid infusate through sid restrictor, comprising a small-bore flow passage, defined within a length of capillary tubing and having a length and a diameter, the ratio of said length to said diameter being at least 10, and said length being at least 3 millimeters, said flow restrictor being further capable of indicating pressure and flow of said liquid infusate and further comprising: a tubular duct sealably surrounding said capillary tubing and extending upstream and downstream thereof; an upstream hole and a downstream hole in the wall of said duct at locations respectively just upstream and downstream of said capillary tubing; thin-wall elastomeric tubing surrounding said duct and sealed thereto at distances upstream and downstream of said holes to provide sealed upstream and downstream annuli inflatable by pressurized infusate flowing outward through said holes; a transparent cylindrical jacket surrounding said elastomeric tubing and radially spaced therefrom at a small distance such that said elastomeric tubing touches said jacket when said annuli are inflated, the thickness and elastic modulus of said elastomeric tubing providing said downstream annulus being sufficiently small that said downstream annulus may be inflated to touch said jacket when the pressure of said infusate, flowing through said downstream hole, is only 35 to 110 Torr and, the thickness and elastic modulus of said elastomeric tubing providing said upstream annulus being sufficiently large that said upstream annulus may be inflated to touch said jacket only when the pressure of said infusate, flowing through said upstream hole, is at least 200 to 500 Torr.