Abstract: A method and apparatus for measuring the relative density of a gas. The apparatus includes a chamber to which a gas in question, for example natural gas, is supplied through an inlet and leaves through an outlet. The speed of sound SoS at ambient temperature is measured using any suitable method such as electronic control and a calculating device and an ultra-sound emitter and an ultra-sound receiver. The ambient temperature Ta is observed by a temperature sensor, and a thermal conductivity sensor measures the thermal conductivity of the gas at two different temperatures above the ambient temperature. One value ThCH of the thermal conductivity is measured at 70° C. above ambient and the other value ThCL of the thermal conductivity is measured at 50° C. above ambient. The control calculates the relative density RD of the gas according to the formula RD=g·ThCH+h·ThCL+i·SoS+j·Ta+k·.Ta2+l where (g, h, i, j, k and l) are constants.

Abstract: A method and apparatus for synthesizing an approximation to a sine wave comprising generating a number of pulse width modulated signals, each having a predetermine duty cycle and being the same frequency as each other, from a clock signal. The generated pulse width modulated signals are then combined to produce an approximation to a sine wave having the same frequency as the pulse width modulated signals. The clock signal is provided by an oscillator arranged to produce clock signals over a continuous range of frequencies so that approximations to a sine wave can be produced over a continuous range of frequencies.

Abstract: An energy meter which measures a volume of gas supplied, which measures a calorific value of the gas supplied, and which calculates an energy value corresponding to the measured volume of gas supplied and the calorific value. The structure to measure the volume of the gas supplied and to calculate the energy value may be provided in a single unit. The structure to measure the calorific value of the gas supplied preferably includes a structure to measure the speed of sound in the gas and further preferably measures a first thermal conductivity of the gas at a first temperature and measures a second thermal conductivity of the gas at a second temperature which differs from the first temperature, and is arranged to produce the calorific value of the gas corresponding to the measured speed of sound in the first and second thermal conductivities.

Abstract: A method and apparatus for determining the speed of sound in a gas. An acoustic transmitter is arranged to apply an acoustic signal to the interior of a resonator and an acoustic receiver is arranged to detect the amplitude of the acoustic signal in the interior of the resonator. A control means determines the speed of sound of gas within the resonator from the frequency of a detected non-radial resonant mode.

Abstract: A method and apparatus for measuring the calorific value of a gas. The apparatus includes a chamber to which a gas in question, for example natural gas, is supplied through an inlet and leaves through an outlet. The speed of sound SoS at ambient temperature is measured using any suitable method such as electronic control and a calculating device and an ultra-sound emitter and an ultra-sound receiver. The ambient temperatures Ta, is observed by a temperature sensor, and a thermal conductivity sensor measures the thermal conductivity of the gas at two different temperatures above the ambient temperature. One value ThCH, of the thermal conductivity is measured at 70° C. above ambient and the other value ThCL of the thermal conductivity is measured at 50° C. above ambient. The control calculates the calorific value CV of the gas according to the formula: CV=a·ThCH+b·ThCL+C·SoS+d·Ta+e·Ta2 +f, where a, b, c, d, e and f are constants.

Abstract: Determininig a frequency at which a resonator resonates comprising: driving an acoustic transmitter for applying an acoustic signal to the interior of a resonator by scanning through a first range of frequencies in substantially equidistant steps of a first size; detecting a signal from an acoustic receiver arranged to detect the amplitude of an acoustic signal within the resonator produced by driving the acoustic transmitter over the first range of frequencies; determining a frequency at which a maximum occurs in the detected signal for the first range of frequencies; driving the acoustic transmitter by scanning through a second range of frequencies containing the determined frequency from the first range of frequencies, the second range being smaller than the first range, and scanning through the second range of frequencies in substantially equidistant steps of a second size which is smaller than the steps of the first size; detecting a signal from the acoustic receiver produced by driving the acoustic tran

Abstract: A meter to measure the volumetric flow rate of a liquid, for example water, is formed with a pipe forming a liquid flow path containing a low power electrical heater. A first temperature sensor measures the temperature T of the liquid adjacent to an upstream end of the heater and a second temperature sensor measures the temperature T.sub.2 of the liquid adjacent to a downstream end of the heater. The heater adds heat power of a value P to the liquid. A control arrangement comprising a computer means calculates the flow rate Q using the expression ##EQU1## where ST.sub.1 and DT.sub.1 are respectively the specific heat capacity of the liquid and the density of the liquid at the temperature T.sub.1 measured by the first sensor.

Abstract: A heat meter for measuring the amount of heat consumed in a region. A heat exchanger is supplied with heated fluid through an inlet pipe which carries temperature sensors for sending signals to a control. The control also receives signals from a temperature sensor in an outlet pipe carrying fluid away from the region. A low power electric heater is mounted between the sensors to add a known amount of heat to the fluid. The control calculates the heat power consumed in the region based on the temperature difference across the regions and the temperature difference across the region and the temperature difference across the heater.