Robertshaw Model 738A Capacitance Level Probe

These probes are used with Robertshaw RF and microprocessor based ON/OFF or continuous level controls for measuring applications involving liquids or granular materials.

When the probe is used with an ON/OFF (fixed level point) type, it may be installed in the vessel in either a vertical or horizontal position. If, however, the probe is to be used on a continuous level measuring application or on an application with a differential (such as Pump On/Pump Off), then it must be installed into the vessel in a vertical position.

Bent probes are available to allow the active length of the probe to be different from the installation direction. See "Bending Specifications."

When the process is a conductive liquid (such as water or sulfuric acid) and the vessel is non-metallic or non-metallic lined, a probe with a ground wire may be employed to provide a ground path to the liquid (second plate).

The liquid must be clean and free of foreign materials and be no more viscous than water.

The Robertshaw Level Measuring instruments have been designed so that they attach directly to the 1/2" NPT fitting on the top of the probe. Typically, these are either the complete transmitter housing or a conduit outlet box with a coaxial cable to the instrument. The 6-32 threaded fitting in the end of the probe rod accepts either a contact pin or a machine screw as required.

This special purpose, alumina insulated probe was designed for use with Robertshaw's capacitance level and bulk measuring instruments and is offered for use in special, corrosive material measurement applications involving high temperature and pressure conditions.

The capacitance sensing probe is made up primarily of a pressure tight Lava seal, a 3/4" entrance gland and a stainless steel coil spring conductor surrounded by a ceramic insulator of alumina having a normal outside diameter of 3/8" with an inside diameter of 1/4".

High purity alumina is used to give the probe high compressive strength, improved resistance to chemical attack and in general allows for installation in many high temperature operations where standard, Teflon covered probes could not withstand the conditions.

The alumina probe is intended for use in high temperature-high pressure applications where the product being measured is either conducting or nonconducting liquids. It can also be used in granular materials however, providing they are not excessively abrasive or have high densities which would subject the probe to extreme side forces.

The alumina probe should not be subjected to high side loads, high amplitude-low frequency vibrations or high thermal shock.

The high compressive strength of the cast alumina makes this a suitable non-porous insulator capable of withstanding ery high pressures in the compressive mode. The crushed lava seal (magnesium silicate) provides a tight seal against leakage under pressure conditions and yet is resilient enough to withstand high temperature conditions; therefore, this probe can used reliably in many applications.