OFTEN a recipe, even an American recipe, sneaks in a line about peeling two pounds of potatoes or mincing one ounce of ginger. Trying to judge whether the item in question is lighter or heavier than a 5-pound bag of sugar, or than your last child at birth, doesn't get you very far.

A kitchen scale can be an essential part of your support system. Recipes for canning and preserving tend to be given in pounds, and French and other foreign cookbooks use weight exclusively as measurement. Dieters need scales to mete out their portions of lean fish and meat. Bakers may require fine measurement of pastry ingredients. And those of us who never could learn what a pound of potatoes looks like use them to weigh vegetables.

There are two basic types of home kitchen scales on the market. One is the balance-beam scale, which operates on a system of counterbalancing weights. It looks like a smaller version of the scale in a doctor's office. The other is the spring scale. In the spring scale the weight of the ingredients pulls down on a spring, which in turn moves the indicator.

For lasting accuracy, the balance scale is preferable. Springs tend to wear out and get mushy with heavy long-term use. In some cases they can be repaired, but most often not. Readings can also be influenced in some spring scales by the position of the ingredients. Think of your bathroom scale: You know you weigh two pounds less if you stand on your left foot, right?

Balance beam scales have two disadvantages: They are expensive and, because of the way the indicator fits into little grooves, they can indicate either ounces or grams but not both. Spring scales generally have one row of numbers for ounces and pounds, another for grams and kilograms. Some cooks also find that their extra accuracy is not worth the work involved in jiggering the weight back and forth across the beam to find the measurement.

The balance beam scale most frequently encountered is made by Terraillon. You can buy it calibrated in either kilos or pounds, to weigh amounts as light as 1/16th ounce and as heavy as 22 pounds. It is fitted with a flat tray, not with a bowl. At around $45, its range is sufficient to cover most kitchen needs, and it should stay accurate for decades. Most balance scales can also be repaired or adjusted, if necessary.

Spring scales are available in greater variation, from tiny and cheap to sizeable and expensive scales that will weigh a side of beef or two. Terraillon makes several varieties. One interesting variation is the "zeroing" scale. The indicator on a zeroing scale can easily be moved back to zero, meaning that you can weigh your flour, move the indicator back to zero and weigh your sugar in the same bowl. They are great for things like plum puddings that call for lots of ingredients. One company, Soehnle, makes a zeroing scale that will measure up to 11 pounds and a smaller version that weighs up to 4 pounds. Krups also makes a sleek-looking zeroing scale that measures up to about 5 pounds. All three of these indicate pounds and kilos at the same time. The Krups is about $30, the larger Soehnle about $32.

A wall scale is necessary for cooks with limited counter space. The one most usually seen on the market is by Eva, and comes with a bowl that folds up, making the whole scale about the size of a small wall clock. This is a spring scale that measures in pounds and kilos, up to 6 1/2 pounds or 3 kilos. It's expensive, about $40, but experienced cooks swear by it.

In buying a kitchen scale there are several questions to keep in mind. The first, of course, is what you will be weighing and in what quantities. Many of the smaller scales will measure up to 5 pounds or so but have such small containers that you couldn't get five pounds of potatoes or five pounds of flour in them.

The second question is how much accuracy you need. If you need to measure things to the quarter-ounce, a balance scale is probably your best bet. On the other hand, it's convenient for most cooks to have both pounds and kilos on the same scale, a feature not available in most balance scales.

Look at the dial, too. Some dials are difficult to read because of the distorting qualities of their plastic-coated protection or because the indicators aren't long enough.

If you need a scale to measure huge quantities or for some other special use, try a restaurant equipment house such as E. B. Adams (1612 U St. NW) or The Scale People Inc., at 11017 Veirs Mill Rd. in Wheaton.