A self-contained apparatus detects the absence of water in a
cut plant container and signals an audible alert. The alert sounds periodically until an attendant refills the water. The apparatus is a slender
rigid structure that can easily fit inside a narrow
vase of
cut flowers. The top portion that remains above the normal
water level holds the battery and electronic circuitry. The bottom portion, which rests against the container bottom, holds the water sensor. Various means of electronically distinguishing water from air are disclosed: thermal, vibrations,
viscosity, density, acoustic, electromagnetic (including
capacitance, microwaves, optical and beta rays), electrical contact, electrochemical, floats and pressure. The sensor has cavities in its structure to protect the sensor from handling,
water damage and
contamination. Some cavities are flooded having an opening above and below the normal container
water level; the water inside a flooded cavity matches the
water level in the container. Other cavities are dry to protect sensors and circuitry from
water damage. Circuitry to detect the absence of water includes bridge circuits for
variable resistance and variable
voltage sensors. In addition, both self-impedance and
mutual impedance circuits are disclosed.