The pH measurement circuit has inherently a high internal resistance (up to 5x109 ohms in some cases) due to the fact that the potential voltage must pass through the membrane glass which adds resistance. Therefore the electrical connection to a pH meter or transmitter is prone to all kinds of electro-magnetic interference. For this reason specialized cable and connectors become important for the success of the pH measurement.
Measurement Electrode Connection Cable
The measurement electrode is always connected with a low-noise screened coaxial cable directly to the pH transmitter. This cable has very high insulation resistance, always higher than the input impedance of the measuring instrument (normally 1012 Ω). The market offers coaxial cables having an insulation resistance of 1014 Ω to 1017 Ω per meter of cable length.
Another important specification is the cable capacitance. Capacitance should be as low as possible to avoid increasing the time constant of the signal transmission. Experience has shown that the capacitance of a pH connection cable should not be higher than 150 pF/m. For example, a 200 pF/m cable capacitance on a 50 m long cable could add approximately 50 seconds time delay to the response time of the pH sensor. Coaxial cables used for pH measurements normally have a capacitance of between 64 pF/m to 102 pF/m.
The temperature rating of the pH connection cable should also be considered. For normal measuring applications below 70°C, standard pH connection cables have a temperature rating of -30°C to +80°C. Special high temperature cables are available having a maximum temperature rating of 130°C.
Reference Electrode Connection Cable
The connection of a separate reference electrode to the measuring instrument is not as challenging provided that the cable is shielded from any electro-magnetic interference. Normally a standard single core, screened cable suffices. Temperature ratings of the cable must be observed.
Combination Electrode Connection Cable
What has been said about the cable requirements above must also be observed for the connection of a combination electrode. It is highly advisable to use a double screened coaxial cable (triax cable). To save costs it is normal practice to connect the inner screen of the coaxial cable to the reference electrode part of the combination electrode.
Cable Preparation and Cable Routing
The insulation of the internal conductor of a pH connection cable is not only screened with copper wire mesh but also with an additional black semiconductive layer. This layer suppresses voltages which might be created when moving the coaxial cable. This phenomenon is known as triboelectric noise. It is also critical that this black layer must be carefully removed when preparing the cable ends for connection to the electrical connector and/or to the measurement instrument. If the black layer is not fully removed, it will cause a short circuit between the internal conductor and the copper screen. When stripping the coaxial cable, tools and hands of the technician should be dry and clean to prevent contamination. After stripping, the cable ends should be cleaned with alcohol, using a cloth or a brush, both of which should be absolutely clean. Testing has shown that touching the stripped cable ends with wet or oily fingers will reduce the insulation resistance down to, or below, 107 ohms, which will result in a short circuit of the high resistance measuring chain – a pH measurement now becomes impossible.
When routing the pH connection cable from the sensor to the transmitter, care must be taken not to route the cable parallel to power cables. Parallel power cables in the vicinity of the pH connection cables can lead to electro-magnetic interference (induction), which must be avoided at all cost. The outer screen of the triax cable (combination electrode) should always be earthed (grounded) on one side only. In principle, every pH connection cable should be as short as possible, but should under no circumstance be longer than 50 meters.
pH connection cables should not be buried straight into the ground. If this has to be done these cables must be installed in a metal or plastic conduit.
Plug or Cable?
Sensor manufacturers supply their pH probes in two configurations: either with an integrally installed cable (normally 1 to 3 m in length), or with a plug-style electrical connector. Both configurations have their advantages and disadvantages.
If the cable is integral to the pH sensor, the customer can be assured that the cable connection is water tight and measuring faults attributed normally to the electrode/ cable connection (short circuit, moisture ingress) can be ruled out. On the other hand however, when the pH electrode assembly has to be replaced (remember: a pH electrode is a consumable item with a certain life expectancy), the cable also has to be re-purchased.
A plug connection eases the electrode removal or replacement during the maintenance period and is more economical in the long run (saving of cable cost). However, care must be taken to always mate the connection socket firmly to the cable plug, otherwise moisture, the biggest enemy to the electrode/cable connection, might penetrate the socket/plug coupling. Once moisture has entered this joint, a reliable pH measurement is no longer possible.