Most landfill gas flow measurement devices create differential pressure (DP). The DP reading is then used to calculate flow. Flow readings are highly dependent on the accuracy of the DP reading. Keep in mind that DP can be affected by barometric pressure, temperature, and system dynamics.
When DP gets below 0.5” WC, accuracy is sacrificed. The best practice is to maintain a lower limit of 0.5” WC, which makes taking a reading easier and gives more confidence in your measurement. The upper limit of 5.0” WC is based on the energy required to power the gas collection system. As DP gets higher, the gas system blower works harder to maintain a constant back pressure. On average, the cost of electricity in the United States is about $0.10 per kWh, so just one extra horsepower used to run the blower can cost an extra $700* over one year. Putting more back pressure on the system than required results in more electrical power used and money spent to operate the blower.
Question: “What differential pressure range should I use for the best flow measurements?”
Answer: A good practice is to have DP between 0.5” - 5.0” WC. "When should I change orifice plates?" Answer: When the DP is outside the recommended range.
In essence, use the orifice plate that will provide a measureable reading of at least 0.5” WC but not more than 5.0” WC − this will generate optimal flow measurements. DP should be large enough so it is easy to measure, but not so large that you consume too much available vacuum, and waste energy.
*1 hp @ $0.108 kWh running 24 hours = $1.93/day $58.76/month $705.49/year
Note: kWh vary throughout the country between $0.07 and $0.33, with the Commercial average being $0.108