Using kitchen thermometers for meat is the best way to tell if meat items are done cooking or not. Time is ok for approximations but due to different calibrations of ovens the kitchen thermometer should have the final say in whether your pork roast has cooked enough. The usual parts of a kitchen thermometer are the probe that gets inserted into the meat and a read out of the temperature of some type.
These thermometers are made with a probe that contains a coil of two different metals that have different temperatures at which they expand. The coil operates a dial type read out and is the average of the two metals. These thermometers are oven safe and made to remain in the food during cooking time. The are of the probe that can accurately measure temperature is around 2 inches from the tip. a bimetallic-coil thermometer is good for thick meats such as roasts.
Thermocouple thermometers have two thin wires at the tip of a probe that the temperature is measure and read on a digital read out. Thermocouple thermometers are used in laboratories and are very fast at a read out of a temperature. They can display a temperature in from 2 to 5 seconds. Because the reading of the temperature is in the tip of the probe thermocouple thermometers can be used with foods off all thickness especially thin foods. Thermocouple thermometers are used for a quick reading and are not intended to remain in the food while cooking.
A more recent technology called platinum resistive thermal device is quickly becoming a replacement for thermocouple probe measurement devices in food service. The Fluke FoodPro Plus is a new offering well worth looking into.
Thermistor type food thermometers are compact and product a reading very quickly. They are designed with a ceramic semiconductor that measures the temperature. The semiconductor is in the tip of the probe secured with a temperature sensitive epoxy. The placement of the measuring device in the tip of the probe allows measurement of all thickness of foods. The temperature will be read in approximately 10 seconds. Thermistor thermometers are not made to remain in food. The readout on the thermistor thermometer can be digital or dial.
Any of these types of meat thermometers can do the job. You need to know where the temperature measuring device is located in the probe of the thermometer as the center of thicker cuts of meats will be cooler than closer to the skin. You want to take your reading from the coolest point of the meat. If it is a boneless piece the place to take the temperature would be the center while a bone in item the temperature would be close, but not touching the bone. For instance a Turkey would be the thigh near the bone. Finally if you are not using an oven safe thermometer that you will leave in through out the cooking time then push the thermometer in on the top of the meat rather than on the side. Since you will be taking the thermometer out a hole on the side of the meat will leak those lovely juices that give flavor and moisture while a hole on top will not leak much.
There are different models even among these types of meat thermometers. I have used most of them and find the simplest of design to be the best, that is if you cannot afford the couple of hundred for a thermocouple thermometer.