Two-wire level measuring instrument

• US 4,851,831 A
• Filed: 05/29/1984
• Issued: 07/25/1989
• Est. Priority Date: 05/13/1981
• Status: Expired due to Fees

• Current Assignee: Drexelbrook Engineering Co. (Ametek Incorporated)
• Original Assignee: Drexelbrook Engineering Co. (Ametek Incorporated)
• Inventors: Stern, David M.
• Primary Examiner(s): NOT, DEFINED
• Assistant Examiner(s): Queen, Tyrone
• Application Number:
  US06/615,151
• Time in Patent Office: 1,883 Days
• Field of Search: 340/870.16, 340/870.39, 340/870.05, 340/870.11, 340/870.37, 340/618, 340/620, 73/304 C, 324/60 C, 324/61 R, 364/509, 364/850
• US Class Current: 340/870.16
• CPC Class Codes: G01F 23/266 measuring circuits therefor

Abstract

An improved system for generating a signal representative of the level of a material having a dielectric constant within a vessel is disclosed. The system features use of a first probe for generating a signal indicative of the dielectric constant of the material which is divided into a signal varying with the level of the material whereby a compensated level signal irrespective of the actual dielectric constant is measured. In a preferred embodiment, a single integrator is used to perform the division operation whereby inaccuracies due to component variation including long-term drift are eliminated. The instrument is disclosed as functioning in a two-wire instrument system having low power requirements, permitting use of conventional signal limits. Means for calibrating the instrument in a fail-safe manner such that the operator is assured of proper calibration are disclosed, as is means whereby bridge circuits used to measure in the capacitance measurements supplied to the division circuit are both supplied by a single oscillator means, again increasing reliability and accuracy of the system.

Claims

1. Apparatus for indicating a level of a dielectric material within a vessel at a first location, said apparatus comprising:

a first probe means for location within said vessel, said first probe means being adapted to provide a first analog signal representative of a dielectric constant of said material,

a second probe means for location within said vessel, said second probe means adapted to provide a second analog signal indicative of a net capacity of said material;

transmitter means located at said first location and electrically connected to said first and second probe means, said transmitter means comprising means for dividing said first analog signal provided by said first probe means into said second analog signal provided by said second probe means to produce a third analog signal indicative of said level of dielectric material within said vessel and means for transmitting said third analog signal;

a two-wire cable electrically connected to said transmitter means, said transmitter means transmitting said third analog signal over said two-wire cable;

power supply means at a second location remote from said first location, said power supply means electrically connected to said two-wire cable;

said power supply means transmitting power over said two-wire cable to said transmitter means; and

receiver means at said second location, said receiver means electrically connected to said two-wire cable, said receiver means comprising means for receiving said third analog signal indicative of said level over said two-wire cable from said transmitter means and indicator means for indicating said level;

wherein said dividing means comprises;

integrator means for generating a voltage signal;

voltage supply means for supplying an initial voltage to said integrator means;

means for supplying said first analog signal to said integrator means for a first period of time T dependent on said dielectric constant of said material to integrate said first analog signal;

means for supplying said second analog signal to said integrator means for a second period of time equal to said time T to integrate said second analog signal;

analog switching means connected to said integrator means for switching from said means for supplying said first analog signal to said means for supplying said second analog signal after said first analog signal has been supplied to said integrator means for said first period of time;

whereby a ratio of said voltage signal generated by said integrator means to said initial voltage signal is indicative of said level within said vessel.

2. The apparatus of claim 1 further comprising a common oscillator means for exciting said first and second probe means with an alternating current.

3. The apparatus of claim 2 wherein said first and second analog signals are comprised in individually calibratable bridge circuits.

4. The apparatus of claim 3 further comprising amplifier means for individual amplification of said first and second analog signals of said bridge circuits prior to division of said first and second analog signals to yield said third analog signal indicative of the level of said material.

5. Apparatus for indicating a level of a dielectric material within a vessel at a first location, said apparatus comprising:

a first probe means for location within said vessel, said first probe means being adapted to provide a first analog signal representative of a dielectric constant of said material;

a second probe means for location within said vessel, said second probe means adapted to provide a second analog signal indicative of a net capacity of said material;

transmitter means located at said first location and electrically connected to said first and second probe means, said transmitter means comprising means for dividing said first analog signal provided by said first probe means into said second analog signal provided by said second probe means to produce a third analog signal indicative of said level of dielectric material within said vessel and means for transmitting said third analog signal;

a two-wire cable electrically connected to said transmitter means, said transmitter means transmitting said third analog signal over said two-wire cable;

power supply means at a second location remote from said first location, said power supply means electrically connected to said two-wire cable, said power supply means transmitting power over said two-wire cable to said transmitter means; and

receiver means at said second location, said receiver means electrically connected to said two-wire cable, said receiver means comprising means for receiving said third analog signal indicative of said level over said two-wire cable from said transmitter means and indicator means for indicating said level;

wherein said dividing means comprises;

integrator means for generating a voltage signal;

voltage supply means for supplying an initial voltage to said integrator means;

means for supplying said first analog signal to said integrator means for a first period of time T dependent on said dielectric constant of said material;

means for supplying said second analog signal to said integrator means for a second period of time equal to said time T;

analog switching means connected to said integrator means for switching from said means for supplying said first analog signal to said means for supplying said second analog signal after said first analog signal has been supplied to said integrator means for said first period of time, said voltage signal generated by said integrator means defining a reference voltage signal after said first period of time, said voltage signal generated by said integrator means defining an output voltage signal after said second period of time;

means for outputting a signal representative of a ratio of difference between said output voltage signal and said reference voltage signal and differences between said reference voltage signal and said initial voltage;

wherein said ratio is indicative of said level.

6. Apparatus for generating a compensated analog level signal representative of a level of a material within a vessel at a first location remote from a second location at which indication of said level is desired, said apparatus adapted to receive power from said second location to perform a said measurement of said level at said first location over a pair of wires, said pair of wires transmitting said compensation analog level signal from said first to said second location for indication, said apparatus comprising:

first sensor means disposed within said vessel to generate a first analog signal proportional to a characteristic quality of said material;

second sensor means disposed within said vessel to generate a second analog signal by sensing variations in amounts of said material having said characteristic quality disposed in proximity to said second sensor means;

integrator circuit means for generating a voltage signal;

voltage supply means for supplying an initial voltage to said integrated circuit means;

means connecting said first sensor means and said integrator circuit means for supplying said first signal to said integrator circuit means for a first period of time T dependent on said characteristic quality of said material;

means connecting said second sensor means and said integrator circuit means for supplying said second signal for a second period of time equal to said time T;

analog switching means electrically connected with said integrator circuit means for switching from said means for supplying said first signal to said means for supplying said second signal after said first signal has been supplied to said integrator circuit means for said first period of time, said integrator circuit means voltage signal defining an output voltage signal after said second period; and

outputting means electrically connected to said integrator circuit means and said pair of wires, said outputting means outputting a signal proportional to a ratio of said output voltage to said initial voltage onto said pair of wires for transmission to said location;

wherein said ratio is indicative of said level.

7. The apparatus of claim 6 wherein said first and second sensor means are adapted to provide a signal indicative of capacitance between said second sensor means and a conductive member wherein said first sensor means outputs a signal indicative of a dielectric constant of said material and said second sensor means outputs a signal indicative of variations in capacitance between said second sensor means and said conductive member.

8. The apparatus of claim 7 wherein said capacitance is measured by impressing an alternating current across said first and second sensor means and a single oscillator means is used to supply said alternating current to said first and second sensor means.

9. The apparatus of claim 7 wherein said first signal output by said first sensor means and said second signal output by said second sensor means are individually amplified prior to division of said first signal into said second signal.