Designing Heat Meters Using PIC16F9XX Microcontrollers

TB1092 Designing Heat Meters using PIC16F9XX Microcontrollers Water meters output in three forms: cubic feet, gallons Author: Rodger Richey and Ezana Haile or cubic meters. We need to calculate the mass flow Microchip Technology Inc. rate in terms of kilograms per second for use in our heat transfer equation. The following list shows the conversion factors from each of the three water meter INTRODUCTION outputs into kilograms. The electronic heat meter would Heat in many parts of the world is supplied to rooms then need to record this measurement over a particular using hot water and radiators. A boiler in a basement or time period. The specific heat and density of water adjoining building heats water and pumps it throughout change as a function of the temperature of the water the building. Radiators in each room use convection and therefore depends on the output temperature of and radiation to distribute heat. By changing the flow of the boiler. For the examples below, lets assume that water through the radiator, the amount of heat also var- the boiler outputs hot water at a temperature of 40°C. ies proportionally. This technical brief will show the Table 1 shows both the density and specific heat of basic details to building an electronic heat meter. water over temperature. At 40°C, the density of water 3 . Table 1 shows the density of wateris.991 grams/cm versus temperature. HEAT METER FUNDAMENTALS 3 3 3 1 ft of water = 28,316.8467117cm x 0.991 gram/cm Water is heated by a boiler of some type. It is then x 1 kg/1000 grams = 28.061995 kg pumped to the rooms in a building and then returns to 3 3 the boiler to start the process over again. The idea x 0.991 gram/cm 1 gallon of water = 3,785.4118 cm behind a heat meter is to measure the amount of x 1kg/1000 grams = 3.751343 kg energy dissipated in the building by the hot water 3 3 3 1 m of water = 1,000,000 cm x 0.991 gram/cm radiators. To measure this energy consumption, the x 1kg/1000 grams = 991 kg heat meter must measure three parameters: * Incoming temperature of water The most common residential water meters are designed to measure mass flow rates of 0-20 gallons Outgoing temperature of water per minute. This would equate to 1.25043 kg/s (20 gpm Mass flow rate of water * 3.751343 kg/gal * 1m/60s). As an example, lets We can obtain the temperature of the incoming and calculate the energy usage when the mass flow rate is outgoing water using RTDs, thermocouples or similar 5 gallons per minute with an incoming temperature of devices. The mass flow rate can come from standard 40°C and an outgoing temperature of 34°C. residential water meters. Once we know these Calculate the mass flow rate: parameters, we can calculate the amount of energy dissipated in the building using the following 5 gpm × 3.751343 kg/gal × 1m/60s = 0.31261 kg/s equation: Use specific heat of water 4.179