pH measurement
→ pH (available only in version BP, if enabled in the Basic Settings)
→ selection between compensation 20°C, 25°C, 37°C and measured
temperature
→ reading min. value → reading max. value
Returns automatically back to measuring mode after 5 sec
→ reset of both the min. and max. value
→ return to measuring display
→ no function
Temperature compensation
The HDM97Pocket can perform pH measurement in four different ways of temperature compensation.
By pressing the key you can choose between these four compensation modes.
• pH at 20°C Display "PH 20.0 °C"
• pH at 25°C Display "PH 25.0 °C"
• pH at 37°C Display "PH 37.0 °C"
• pH compensated by simultaneous measured temperature Display "PH"
Background of pH measurement
For measuring pH, a combined electrode is used. Combination pH electrodes are combinations of one
reference electrode and one measuring electrode in a glass tube. The pH value in the dialysate is measured by
an unbreakable pH-electrode. The glass body is protected with a plastic coat and due to the jelly-electrolyte-
filling, it is maintenance-free. This electrode’s diaphragm must be stored in 3 mol/l KCl-solution. The
protective cap must be refilled every three to four weeks to prevent the electrode from drying out. Before use
the electrode must be checked for exterior damage and crushed glass. Crusts caused by leaking electrolyte
can be removed easily by rinsing with reverse osmosis-treated water.
The sample volume should be 100 ml of dialysate or 1 liter of untreated water. The sample should be poured
into a clean glass container with a tube or hose, coming into as little contact with air as possible. The pH-
value must be measured immediately in the same container. Make sure that the display stabilizes before the
value is read. In stirred solutions the response rate is faster; the value, however, must be measured at resting
fluid.
The pH-electrode must be dabbed only with a lint-free cloth and never rubbed dry. Rubbing destroys the
jelly layer on the glass surface which results in a longer response time for the electrode.
Pressure and fluid currents have considerable influence on the pH measurements. For high accuracy
pH measurement it is essential to take the pH measurement in a resting solution at environmental
pressure.
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