Microwave Devices – Why can’t I use a microwave oven that I can buy from an appliance store? Part 2

This blog is Part 2 on microwave devices. Please check the website for Part 1 of the blog where regulations and safety issues related to microwave devices are explained.

When I was shopping for a new kitchen microwave for my home, there seemed to be hundreds of models with different wattage, sizes, features, etc. These kitchen microwave ovens are useful for cooking at home, but better consistency and quality is necessary when working with patient tissue.


  1. Exposure Cycle: Most kitchen microwaves have a 30 second cycle. That means, if you set the exposure at half-power, the magnetron is on for 15 seconds, and off for 15 seconds. It does NOT mean that if you have a 1000 watt microwave at half power, it is now a 500 watt microwave. It means the power is on for half the cycle time. Let’s say you heat your coplin jar for 1 minute at half-power. That’s 15 sec on, 15 sec off, 15 sec on, 15 sec off, for a total of 30 seconds on. However, the solution in the coplin jar is cooling for the last 15 seconds of off time. So after 1 minute, the temperature may read 60 degrees C, but after 45 seconds it may have reached 70 degrees C and then cooled off 10 degrees during the last 15 seconds. Laboratory microwave devices are different, in that they have cycles of 1-2 seconds, with some having continuous power, allowing better cut off time, and thus better control of the final temperature.
  2. Hot and Cold Spots: Kitchen microwaves are designed to heat casseroles and other yummy meals. The magnetron releases beams of microwaves in a grid pattern, so that there are concentrations of beams in a low, wide pattern, similar to the size and shape of the casserole dish. What is the shape of a coplin jar? Tall and thin. If a corner of the food gets a little too cold or a little too hot, we can still eat it. But what about patient tissue that gets a little too hot, or not hot enough? That’s why we have to pour-off-pour-on the solutions in the coplin jar when heating in the kitchen microwave. The laboratory microwave has concentration of beams more evenly dispersed over the entire chamber (side to side, and top to bottom) so there are no hot and cold spots.
  3. Temperature probe: Most kitchen microwaves do not come with temperature probes. As a result, a histotech would have to rely on time to bring the solution up to a certain temperature (see Quality/Consistency #1). Most laboratory microwave devices come with a temperature probe, so you can program it to reach exactly, say, 60 degrees C, before cutting off.
  4. Agitation: With kitchen microwaves producing hot and cold spots, maybe the bottom half of the tall coplin jar gets too hot, or maybe the top half does. Either way, with a kitchen microwave, the histotech usually stops the microwave halfway through the cycle, pours the solution off and on the slides to mix it up, and then heats it for the remaining time, and again repeats the off and on pouring to mix the solution. With a laboratory microwave, there is a “bubbler” built into the temperature probe, so the solution is stirred the entire time it is being heated.
  5. Power and time settings: Kitchen microwave settings are not highly accurate. If the timer is off by 5 seconds, or the wattage is 50 W higher than the setting it is no big deal when heating food. We eat around the overcooked or undercooked areas. This kind of inaccuracy is not acceptable when staining slides or processing tissue. Laboratory microwave devices are designed with much more accuracy in the settings.

I hope I have given you enough information so that you can justify the additional cost of a laboratory grade microwave device.