pH METER

Typical Number in Hospital: 5

Cost Bands: 3

References: 9

The pH (hydrogen ion concentration or acidity) of samples of body fluids, including blood, may be measured as part of the chemical analysis of such samples. The pH meter has two electrodes, one for reference and one for measurement. The measurement electrode employs a membrane of special glass containing a solution of known pH. The approach of a hydrogen ion to the outside of the membrane causes a positive charge to pass into the ionic solution inside the electrode. The voltage developed across the electrode changes by approximately 60 mV/pH unit. Since the whole range of physiological pH values is only 0.6 pH units, the meter must be capable of accurately measuring changes of 0.1 mV. The reference electrode is usually a calomel cell in a potassium chloride salt bridge connecting with a porous plug at the tip of the electrode.

The voltage produced by a pH electrode varies with the temperature of the specimen, and so it is usual to maintain the test solution at 37[d]C, although the meter may also have a temperature correction control. The meter is normally calibrated by the use of two solutions of known pH. The internal impedance of the pH electrode is very high (10-100 M[O]) and so the meter must have a suitably high input impedance.

For blood pH estimation electrodes are used which enable measurement to be made with very small blood samples, or may even operate inside the body. pH meters may be used in the clinical chemistry laboratory and in intensive care areas.

IUXTR

PHANTOM r=3

Performance evaluation of radiotherapy apparatus may be conducted using a model, known as a phantom, which has properties similar to human tissue. Water demonstrates similar absorbing properties to normal tissue, and a water filled phantom may be constructed in which a radiation sensor (e.g. a small ionization chamber) may be moved to map the level of radiation at each point in the phantom. In this way the distribution of radiotherapy dose can be calculated for real treatment situations.

Phantoms are also used as teaching aids, to simulate real conditions with X-ray or ultrasonic machines. Such phantoms are also useful when setting up or calibrating imaging equipment. In nuclear medicine, phantoms are used for making absolute activity measurements.