Powder Displacement

Displacement is defined as the occupation by a submerged body of a volume which would otherwise be occupied by a fluid.  If that definition sounds confusing, try my simplified one:  When you get into a tub filled with water, the water level increases.  You (the object) are a solid which is displacing the volume of water.

Displacement Factor for Enteral Powders and Modulars

Many enteral formulas are in liquid form straight from the manufacturer, in which case, this whole post is moot.  However, there are several which are distributed as a powder, and this powder must be reconstituted with water to be administered through a feeding tube.

Displacement is an important factor of powdered enteral formulas and modular products since it may be necessary to provide an exact volume of liquid to a patient.  When the powdered product (the solid object) is added to water, the level of water goes up.

Powdered enteral formula manufacturers will list on the packaging, the product guides, and their websites how much water is expected to be displaced per packet or per tablespoon of product.  For example, if one packet of formula is known to displace the water by 40 ml, and this is added to 250 ml of water, the final volume will be 290 ml.  Similarly, using the same packet of powder with 40 ml displacement, if one packet was added to 500 ml of water, the final volume would be 540 ml.  The displacement per packet is a constant.  Regardless of the volume of water the powder is added to, the level will always rise by the same amount.  The final volume will differ though.

Example Calculation

There may be times when the dietitian needs to determine how much water is needed per packet of powdered enteral formula based on some known variables provided by the formula manufacturer.  In this example, the manufacturer tells us only the number of calories in each packet and the displacement factor of the formula.  The third variable needed for the equation is one the dietitian decides upon, and this is the desired concentration expressed as the calories per ml.  Here is the question:

A powdered enteral formula provides 300 calories per packet and has a water displacement of 50 ml per packet.  Determine the amount of water needed to reconstitute this product to a 30 calorie per fluid ounce concentration.

The equation for calculating the amount of water needed is:

Water needed = (calories per packet / desired concentration in calories per ml) – displacement

The question provides everything needed to perform the calculation, except the desired concentration is provided in cals/fl oz when it should be cals/ml.  Important piece of information to commit to memory:

1 fluid ounce = 30 ml (29.57 to be exact, but rounding to 30 is sufficient)

The first step is to convert 30 calories per fluid ounce to calories per ml:

30 cals per ounce / 30 ml per ounce = 1.0 cals per ml

Now we can use the equation above.

Water needed = (300 calories per packet / 1.0 cals per ml desired concentration) – 50 ml displacement

Water needed = 300 ml per packet – 50 ml displacement

Water needed = 250 ml water

For each packet of this enteral formula, we will need 250 ml of water in order to reach a 1 cal/ml (30 cals/fl oz) concentration.