What is a Liquid Earth (LEVP) for a pH sensor?
The liquid earth (LEVP) connection is a separate wire lead immersed in the electrolyte within the reference half-cell of the pH probe. It gets the naming as liquid earth or "solution ground" as it provided a direct connection from the pH transmitter to the solution being measured thus can serve as an earth ground. The ground connection is achieved through the conductive KCl electrolyte and the porous reference liquid junction (diaphragm).
Advanced Diagnostics
The liquid earth allows for additional diagnostics measurements to be performed by the pH transmitter. These measurements give some insight into the overall health of the pH sensor. For example, a common diagnostics measurement would be the measured resistance between the liquid earth to the Ag/AgCl wire within the measurement half-cell. This measurement must account for the pH membrane glass. A normal pH sensor will exhibit approximately 200 to 600 megohm. If the glass membrane becomes cracked this resistance can drop to near 0 ohms (short circuit). If the resistance increases then a non-conductive coating such a grease or oil may have fouled the glass membrane or diaphragm.
The liquid earth is also used for diagnostics checks of the reference half-cell. The resistance measurement from liquid earth to the reference element is through the electrolyte. It is generally a value between 250 to 5000 ohms. Increasing resistance is often an indication of contamination of the electrolyte or poisoning of the AgCl element.
Products such as the H220X pH transmitter offer diagnostics warning messages when these resistance limits are exceeded. Skilled users, familiar with these measurements, can monitor the diagnostics values over time to perform preventative maintenance on the pH sensor.
Reducing Noise
Liquid processes moving through non-conductive piping or vessels may develop a static voltage potential. This is especially common with glass-lined tanks and plastic piping. If the process is not grounded (earthing) then the static voltage may find the pH sensor and use the reference element as a ground path. This static voltage can create noise on the pH measurement circuit creating measurement error, difficulty in pH calibration, and, eventually harm the transmitter. The pH sensor liquid earth can help in this application by functioning as a separate grounding path outside of the measurement circuit. The LEVP wire can be tied off to chassis ground within the transmitter. Stray static voltage can then be dissipated without affecting the measurement. The end result is a more stable measurement.
LEVP pH Sensor Products
pH sensors with the LEVP liquid earth option include the EasyFerm Plus and EasyFerm Bio. These sensors require additional contact points thus the VP electrical connector is used. Cabling options include the VP6 and VP8 open end cable products.
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