Tank filling monitor based on the attenuation of a light signal transmitted by an isosceles prism with total internal reflection as the prism is progressively immersed in a liquid of differing refractive index from the atmosphere. Optical fibres of appropriate length may be used to convey the incident light from the source to the prism and from the prism to a light detector.

Apparatus is described for monitoring the blood level in a blood oxygenator by means of light emitting elements projecting through a transparent wall of the oxygenator and light responsive elements receiving light reflected from the blood within the oxygenator. The apparatus detects drops in blood levels and generates suitable alarms in response thereto, including circuits for regulating the blood flow rate through the oxygenator, and circuits for detecting malfunctions of the inventive apparatus.

A control circuit for maintaining any medium between two predetermined limits as for example the water level in a container. High and low limit probes in the medium are connected through carefully chosen resistances to a neon lamp. Although the low limit probe does not conduct enough current to ignite the neon lamp, when both probes become activated by the medium sufficient current does flow to ignite the lamp. If the level drops below the high limit probe the current still flowing due to the lower probe remaining active is enough to maintain the lamp ignited. When the level drops below the lower limit probe the current stops or drops low enough so that the lamp is extinguished. Suitable means are included in the control circuit to monitor the state of the neon lamp and control the power to a pump, a motor, or other device so as to maintain the level between the high and low limit probes.

A postage meter attachment or conversion apparatus is disclosed that will effectively convert a manually lever settable postage meter into an electrically controlled postage meter. The conversion apparatus is easily assembled to and disassembled from a standard lever settable postage meter. Each lever of the meter is settable simultaneously. The motion of the drive mechanism of the conversion apparatus conforms to the arcuate motion of the meter setting levers, thus eliminating the need for precise alignment between the levers and the drive mechanism. The conversion apparatus can be used to make the mechanical meter compatible in an electrical environment containing equipment such as a computer, a digital scale, and other postage system processing devices.

A device for transmitting predetermined volumes of liquid from one container to another container consists of a pipe leading from the first container to the second container and immersed into the liquid in the first container, means sealing the first container, a pressure transmitting tube introduced into the first container for producing pressure means pressing the liquid through the pipe and means measuring a predetermined liquid volume. The invention is particularly characterized in that there is a substantially constant super pressure in the first container and there are two signalling devices spaced from each other and located in the path of the flowing liquid. When the liquid front passes them they transmit a signal to a calculator which determines from these signals the volume of liquid passing through the pipe per time unit. Closure means for the liquid are connected to the calculator. They receive a closing signal formed by the incoming signals after the passing of time required for the transmission of desired liquid volume set at the calculator.