Apparatus and method for determining the concentration of a component of a sample

This invention relates to improvements in instruments of the general type described in U.S. Pat. Nos. 4,963,814; 4,999,582; 4,999,632; 5,288,636; 5,366,609; and, 5,508,171. It is disclosed in the context of such instruments. However, it is believed to be useful in other applications as well. An instrument () has a well () for receiving a dry cell (), an opening () through which a first connector () is exposed to the well (), and a boss () adjacent the opening (). The boss () precludes the wrong terminal of the dry cell () from engaging the first connector () when the dry cell () is inserted into the well () in incorrect orientation. A second connector () includes a base (), a first leg () resiliently connected to and extending away from the base portion, a second leg () resiliently connected to and extending away from the first leg (), and a third leg () resiliently connected to and extending away from the second leg () and toward the first leg (). A display () for the instrument () has a lens () having a substantially transparent substrate with a polyurethane coating. The instrument housing has first and second portions. At least one locator pin () extends from one () of the housing portions and at least one complementary socket () extends from the other () of the housing portions for receiving the pin () to maintain the first () and second () portions in assembled orientation.