From a pharaoh's water clock to the 4-20 milliamp signal running factories
In 1663 Christopher Wren showed the Royal Society a design for a weather clock, with sensors steering pens across paper driven by clockwork. Meteorologists ignored the idea for about two centuries. Once they adopted it, it barely changed: later pneumatic chart recorders did much the same job, with pressurised bellows nudging the pen.
Measurement began with time and weight. Balances and simple position pointers are ancient, and one of the oldest known water clocks came from the tomb of the Egyptian pharaoh Amenhotep I, buried around 1500 BCE. By 270 BCE such clocks already had the rudiments of automatic control. The suffix meter, as in thermometer or speedometer, comes from the Greek metron, meaning that by which something is measured.
Industrial plants long relied on people walking the site, reading gauges and turning valves by hand. From the early 1930s pneumatic transmitters and automatic three-term controllers took over, sending air-pressure signals usually between 3 and 15 psi. Controllers later moved into a central room, mounted on a wall called a control board, where operators paced back and forth watching indicators instead of trudging around the plant. Pneumatics kept a following in corrosive and explosive atmospheres.
Transistors, commercialised by the mid-1950s, let wires replace air tubes. Early electronic loops ran 20 to 100 milliamps at up to 90 volts, but every maker invented its own signal until 4 to 20 milliamps became the norm, formalised in the 1970s as the standard ANSI/ISA S50. Electronic instruments proved more dependable and more accurate than their mechanical predecessors, cutting maintenance costs. Computer-based distributed control systems then swapped racks of dedicated controllers for software, adding alarm handling and automatic event logs and making paper chart recorders unnecessary.
Plenty of instrumentation hides in homes. A mechanical thermostat senses heat with a bimetallic strip, shows the reading with a needle on its free end, and tips a mercury switch to fire the furnace. A gas oven's flame supervision device cuts the gas if its sensor cools, signalling that the flame has gone out. Even a toilet cistern counts, its float acting as a water-level sensor that shuts the valve.
Source: Instrumentation
