Two-wire compensated level measuring instrument

Method of dividing the value of a first signal V.sub.L by the value of a second signal V.sub.R comprising the steps of:

• storing an initial condition V.sub.i in an integrator circuit means;
• applying said second signal V.sub.R to said integrator circuit means;
• measuring a first time period T equal to the time taken for said initial condition V.sub.R to said integrator circuit means to reach a predetermined second value;
• applying said first signal V.sub.L to said integrator circuit means for a second time period equal to said first time period T;
• and
• providing an indication of a content of said integrator circuit means after said second time period has passed, whereby the ratio of the difference between the contents of the integrator circuit means E.sub.o (T) and the predetermined second value to the difference between the predetermined second value and the predetermined initial condition V.sub.i, is the ratio of the first signal to the second signal V.sub.i /V.sub.R.

Apparatus for generating a signal representative of the level of materials in a vessel and transmitting said signal over two electrical conductors to remotely-located means for responding to said signal, wherein said remotely-located means also supplies power over said two conductors to said apparatus for generating the signal, comprising means for dividing a first signal indicative of the dielectric constant of the material the level of which is to be measured into a second signal varying with the level of said material, whereby the ratio of said first and second signals is a third signal representative of the level of said material irrespective of its dielectric constant, wherein said means for dividing comprises:

• means for supplying an initial condition to integrator means;
• means for supplying said first signal to said integrator means, thus modifying said initial condition at a rate dependent on the magnitude of said first signal;
• means for measuring the time T taken for said initial condition modified in said integrator to be varied to reach a predetermined value;
• means for supplying said second signal to said integrator means for modifying its output;
• means for controlling the time said second signal is supplied to said integrator such that said time is equal to the time T taken for said predetermined value to be reached;
• and
• means for outputting a signal indicative of the ratio of the change in the output of said integrator while said second signal was supplied to said integrator to the change in the output of said integrator while said first signal was supplied to said integrator, said ratio being equal to the quotient of said first and second signals.