Analytical instruments are used to measure physical and chemical properties such as mass, pH, conductivity, density etc. So that such instruments have to be maintained periodically. Some of the common maintenence process are listed below:
Calibration:
Sensor and analyzer / transmitter system should be frequently calibrated. For two particular purposes, a sensor loses calibration: changes in slope or changes in offset. Slope changes are generally caused by the measuring electrode aging. Offset modifications are often caused by the reference point being blocked and contaminated.
The offset is only corrected by a single point calibration. The offset and the slope are corrected by a two-point calibration. Calibration frequency is dictated by process rigors such as temperature, pressure, abrasives, harsh chemicals, etc. It also relates to your precision requirement.
Temperature Calibration:
Dolphin sensors include an element for measuring temperature accuracy. This temperature measurement is used by Foxboro analyzers and transmitters to automatically compensate the pH readings.
Measurements of oxidation do not require compensation of temperature. The precision of the temperature measurement should be verified and adjusted if needed for optimum pH measurement precision. This is particularly crucial when using a lengthy cable length with2-wire RTD element sensors. Cable length automatically compensates for mistakes by sensors with 3-wire RTD components.
Electrode Inspection:
Fouling on the measuring electrode (the build-up of a film) and the reference location can result in erratic production. Inspect as required the electrodes. Recommended for fresh facilities once a week. Clean the electrode as outlined in the following sections if fouling is apparent.
Cleaning a Glass Electrode:
Choose the most mild solvent, soap, or chemical, but remove the contamination. For glass electrodes, Caustic is a dangerous option. The glass may be attacked by stronger levels. Dilute HCl is often a nice option. The HCl concentration should be as small as possible and the contamination should still be removed. Consider a maximum of 4% or 1 N.
Storing a Sensor:
Sensor’s shelf life relies on the storage conditions. While IPS does not indicate a shelf life, an estimate of 6 to 12 months is sensible. Sensors can last well over a year on the shelf under the best conditions.The key to adequate storage is to keep the measuring electrode hydrated at ordinary room temperature as well as the reference point.
Store your PH10 or ORP10 sensor in a solution of 1 M potassium chloride or a buffer solution of pH 4 or pH 7. Sensors in distilled or de-ionized water should not be stored.
In a protective cap containing liquid potassium chloride salt solution, new sensor assemblies are delivered with the measuring and reference intersection sealed. The cap should stay in position until your sensor is prepared to be installed in the process.
To store a sensor, the protective cap can be reused by replenishing the solution and attaching it to the sensor.