BS EN 61307-2011 pdf download.Industrial microwave heating installations – Test methods for the determination of power output.
4.4 Effective microwave power and efficiency
Typically, actual microwave workloads are not well suited for calorimetric measurements. Liquid substitutes are then used In calorimetric measurements, and give the effective microwave power (see Clause 7 and Clause 8).
5 Calorimetric power measurements
5.1 General
Only the principles are outlined in this standard. The applied measurement instrumentation and use shall conform to known engineering techniques. Water is the directly or indirectly power-absorbing substance.
5.2 Direct water power measurements
It is important that any directly power-absorbing water has a microwave absorption capability and load geometry which provides a good and essentially temperature independent impedance matching over the actually used temperature interval. A sodium chloride solution with specific conductivity between 200 pS/cm and 600 pS/cm shall be used for the direct absorption at frequencies below 900 MHz.
The power meter typically consists of a waveguide section, equipped with a microwave transparent tube through which the water can flow. The water shall be thoroughly mixed. The recommended water flow rate is about 1 1/mm for each kilowatt but not less than 0,5 11mm. The difference between the outlet and inlet temperature shall be at least 10 K.
The inlet temperature of the water shall not exceed 35 °C, and the outlet temperature shall not exceed 60 °C. However, for microwave power levels less than 3 kW, these temperatures should be on both sides of the ambient temperature, to reduce heat loss errors.
Under operating conditions, the voltage standing wave ratio (VSWR) as measured by a network analyser with a matched waveguide transition or an equivalent measurement device replacing the microwave generator assembly and within the water temperature interval specified above, shall not exceed 1 .25.
If a circulator is used, its isolation shall be greater than 20 dB and the impedance matching of the circulator with dissipative termination is to comply with this subclause.
The water flow shall be monitored, for Instance by means of flow interlock switches, to avoid the formation of steam which may lead to eruption.
Q is the water flow rate, in kg/mm; the factor 4187 Is its specific heat in J/(kg• K) and 60 is a factor resulting from units applied;
△T is the temperature difference in K between the water outlet and inlet temperature.
NOTE If the microwave generator assembly contains a circulator with a dissipative termination protecting the microwave generator, this may be used as power meter by short-circuiting the load port. IL is then to be noted that twice the Insertion kiss appáes for this measurement, but not In the evaluation for determination of the available microwave power output
5.3 Dummy load power measurements
The dummy load is a matched low-reactance resistor, cooled by natural air convection, by forced air or by water. It is generally connected to the microwave generator or generator assembly by a 50 W coaxial feeder, or by a TE1O waveguide. At low power levels, natural air convection is applied and at higher power levels up to about 2 kW, forced air cooling can be applied.
NOTE Applicable dummy loads in two-port desiyn ate commercially available, providing a calibrated insertion loss at levels of -30 dB to -60 dB, suitable for the use of a commercially available power meter at its output port.
It Is necessary to use high-accuracy components and instruments, to ensure that the inaccuracy of power output measurement is less than 5 %.
6 Determination of microwave workload power
This test is applicable only if the normal load is well specified with regard to specific heat and temperature rise in the process. Furthermore, it shall be possible to accurately measure the average temperature rise after processing. If the set-up is suspected to provide an inaccuracy of more than 5 % of the final result, the method described in Clause 5 or Clause 7 is instead used.
NOTE Typically, accurate tests according to this clause can be made only in continuous processing of pumpable workloads. These loads are representative only if their microwave properties are similar to those of the normal load
The input temperature T1 (°C) of the microwave workload is measured. During steady-state processing, a suitable length of processed microwave workload exiting the microwave heating equipment during a predetermined time t (s) is quickly taken out as sample and thermal insulation is provided. Temperature equilibration is then accomplished by either forced convection (stirring or kneading of the sample) or by internal heat conduction, after which the output temperature T01 and the mass m of the sample are measured. Its specific heat c
been pre-determined.
The microwave workload power P is then calculated from the following equation:BS EN 61307-2011 pdf download.