Envco – Environmental Equipment

The WQ201 pH sensor is a rugged reliable water pH measuring device. The sensor probe is mounted on 25 feet of marine grade cable, with lengths up to 500 feet available upon request.

The pH sensor output is 4-20 mA with a three wire configuration. The electronics are completely encapsulated in marine grade epoxy within a stainless steel housing. The sensor also uses a removable shield and pH element for easy maintenance.

Output: 4-20 mA
Range: 0-14 pH
Accuracy: 2% of full scale
Operating Voltage: 10-36 VDC
Current Draw: 5.5 mA plus sensor output
Warm Up Time: 3 seconds minimum
Operating Temperature: -5c to +55c
Pressure Rating: 0-100 psi
Size of Probe: 1 1/4" diameter x 10" long
Weight: 1 lb.

The pH value of a substance is directly related to the ratio of hydrogen ion (H+) and Hydroxyl ion (OH-) concentrations. If H+ is greater than OH-, the solution is acidic, i.e., the pH value falls in the 0 to 7 range. If the OH- is greater than the H+, the material is basic with a pH value in the 7 to 14 range. If equal amounts of H+ and OH- ions are present, the material is neutral with a pH value of 7.

The pH electrode can be thought of as a battery whose voltage changes as the pH of the solution in which it is inserted changes. It consists of two basic parts: 1) a Hydrogen ion-sensitive glass bulb, and 2) a reference electrode. The special glass of the sensitive bulb has the ability to pass H+ i.e., it is said to be H+ sensitive. This ability allows the H+ inside the bulb to be compared to the H+ outside of the bulb, and a voltage to be developed that is related to the difference. The bulb then is a half-cell that needs a reference voltage in order to function.

The voltage produced by the complete probe is a linear function of pH, generally about 60 mV per pH unit. For example, at 7.00 pH the probe produces zero volts while at 6.00 pH it produces +60 mV. If the voltage had been negative it would indicate that the solution had a value of 8.00 pH.

A buffer solution is a solution with a well-defined pH value and has the ability to resist changes in pH. These characteristics are well suited to the standardization of pH measuring systems. Buffers are available in a wide range of pH values and come either in pre-mixed liquid form or as convenient dry powder capsules. When selecting buffers for use with your particular system, a value should be chosen nearest to that of the sample being measured.