Selection Guide
The following data is compiled to assist you in selection of the proper instrument and sensing probe for capacitance applications. Please make a choice below.
The following definitions will help to ensure proper use of the data contained in this section.
NOTE: All instruments do not have 100% differential, i.e. 314B has only a 50% differential adjustment. Ensure that the differential required is within the capabilities of the instrument selected.
This is:
NOTE: Sheath capacitance is the same pF/ft. as gain of probe. Cooling extensions are 4 pF for each 4” of extension. Standard coaxial cable capacitance is 13.5 pF/ft. Low noise coaxial cable is 21.5 pF/ft. Triaxial cable capacitance is 17 pF/ft.
NOTE: The pF/ft. minimum and maximum span of the instrument selected must be met. Ensure that enough capacitance change is generated to meet the minimum span requirements and that it does not exceed the maximum span of the instrument.
Before you begin, please view the appropriate dielectric constant graph.
These graphs illustrate capacitance in picofarads per foot (pF/ft.), in various vessel diameters, generated by standard probes offered by Robertshaw. In the graphs, the sensing probe is calculated as being mounted in the center of a cylindrical vessel of the diameter shown. The diameters shown in these graphs are: 4in., 14 in., and 60 in. The probe capacitance values can be approximated for other vessel diameters by interpolation from the values obtained from these graphs/diameters.
To calculate picofarad values for any product level, the following values must be known:
The procedure for utilizing the dielectric constant graphs for making calculations for a given installation is as follows:
Example: Assumed data
For calculation of probes near an “infinite” flat wall or for probes mounted in a large cylindrical vessel where the probe is near the wall of the vessel, calculate the value from the appropriate probe graph using the graph vessel diameter as twice the distance the probe is from the flat wall or vessel wall. Take this value and multiply by .77 (77%). This is not exact, but is close enough for all probe picofarad calculations.
CAUTION - conductive coatings on the probe will cause an error in measurement. ALWAYS use anti-coating transmitters for continuous measurement and shortstop probes/instruments for point level measurement where products will leave a conductive coating (see “special features” in selection guide matrix). For continuous measurement with anti-coating transmitters, always use a probe or probe mounting that provides the highest pF/ft. gain but does NOT EXCEED the span of the instrument. Gain can be increased by using our high gain 741A probe by mounting the probe close to the wall of the tank or vessel.
NOTE: Only Models 318A and 5100 with shortstop option can be used with 727A/732A probes.
CAUTION: concentric shields can only be used in free flowing, non-viscous liquids. Concentric shields are always the same length as the probe.
Bare, non-insulated probe calculations are simple and straightforward mathematical functions. Graphs (curves) for bare probes are unnecessary, since the “curve” is a straight line. The table below shows the picofarads per foot for various probes in differing vessel diameters. All values given are for that given probe in that given vessel with AIR surrounding the sensing probe. To determine the pF/ft. for a probe in a given vessel size, multiply the listed value below by the dielectric value of the product being measured. This resultant value is for that probe and vessel with that product per foot of covered probe. For total capacitance values, remember to add the probe terminal capacitance value (in the definitions section) to the above obtained figure.
NOTE: When using the above tables and graphs for determining the span in picofarads (change in capacitance for a given distance due to the rising product in a vessel) the dielectric constant (K) of the product must be reduced by 1 to compensate for the displaced air.