Sir
The recent article by Dr D. F. Evans (Br J Surg 1987; 74: 157-61) is an interesting exposition of the uses and potential pitfalls of this technique. However, as this review article could influence readers' choice of pH monitoring equipment, it is essential that the following points are clarified.
When describing the characteristics of the miniature glass electrode M1506 (Microelectrodes Inc.) the author states that it is 'particularly useful for paediatric measurements although its response time is slow and its cost relatively high'. We have recently completed a study comparing the electrode operating characteristics of the M 1506 with those of the GK2801C combined glass electrode (Radiometer, Denmark) and the antimony electrode 001 1 (Synectics, Sweden) under different conditions'. As can be seen in Table I , the response of the M1506 is extremely fast. If, when measuring response time, both sensing and reference electrodes are transferred between solutions, their combined response will be obtained. Studies conducted in this way with a slow reference electrode could account for the slow response time quoted by Dr Evans.
Two points arise from the 98 per cent response time values shown in Table 1 of Dr Evans' review. Firstly, in our experience the selection of a 98 per cent level to establish response time is too high to be useful. In this region of the voltage/time graph, very small voltage changes are taking place over a relatively long time span and the choice of a 98 per cent response time is therefore highly susceptible to error. Secondly, Dr Evans mentions neither the pH range nor the direction over which response was measured and both can markedly affect the result. While we realize that this Table is only intended as a guide, the information provided could be misleading. In our own study using a 1&90per cent response time', a clear difference emerged between the two glass electrodes, with antimony being slower than either (Table I ) . As with glass, plastic electrodes show response time variation dependent on electrode geometry (J. M. Rawlings and M. L. Lucas, unpublished observations). It must be emphasized that when comparing response times of different electrodes they must be tested under identical conditions which should be clearly stated. If these criteria are observed, it would beseen that the response timeofplasticelectrodesissimilarand not superior to that of glass', in contradiction to the information given in Dr Evans' table.
Referring again to Table I in Dr Evans' paper, it is difficult to