In the electromechanical thermal or lagged-type demand meter, the pointer is made to move according to the temperature rise produced in elements of the meter by the passage of currents. Unlike the integrating demand meter, the lagged meter responds to load changes in accordance with the laws of heating and cooling, as does electrical equipment in general.

Because of the time lag, momentary overloading, instead of being averaged out, will have a minor effect on the lagged meter unless the overloading is held long enough or is severe enough to have some effect on the temperature of equipment.

The demand interval for the lagged meter is defined as the time required for the temperature sensing elements to achieve 90% of full response when a steady load is applied. Like the integrating meter, the lagged meter is generally designed to register kilowatt demand.

The lagged type is essentially a kind of wattmeter designed to respond more slowly than an ordinary wattmeter. An important difference in these methods of metering demand is in the demand interval. In the integrating meter, one demand interval follows another with regularity, giving rise to the term block interval.

The thermal or lagged meter measures average load with an inherent time interval and a response curve which is based on the heating effect of the load rather than on counting disk revolutions during a mechanically timed interval.

Electronic thermal demand emulation is the logarithmic average of the power used, with a more recent load being weighted more heavily than a less recent load, (approximated exponentially). The meter will record 90% of a change in load in 15 minutes, 99% in 30 minutes, and 99.9% in 45 minutes.

Because thermal demand emulation is the logarithmic average, the demand is not set to zero on a demand reset. On a demand reset, present demand becomes the new maximum demand.

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