Liquid metering unit responsive to the weight of the metered liquid

In a continuous blending process in which an additive liquid is fed at a controlled rate into a continuously flowing stream of a second liquid, which comprises feeding the liquid additive from a supply alternately into two fixed reservoirs mounted in parallel and discharging the liquid additive from said reservoirs at a controlled rate through a flow control valve into said stream of second liquid, the one reservoir being filled while the other is being discharged, the improvement which comprises:

• (a) generating an electrical signal having a value proportional to the rate of mass discharge from the reservoir being emptied;
• (b) using that signal for comparison with a demand signal representative of the desired mass flow rate of the liquid additive;
• (c) allowing adjustment of the setting of the flow control valve in response to any difference between said first signal and said demand signal, thereby to increase or decrease the flow rate of the additive from the reservoir being emptied, as may be necessary to achieve equalization of said first signal and said demand signal;
• (d) automatically switching the discharge from one reservoir to the other when the signal from the reservoir being emptied reaches a predetermined value;
• (e) immediately prior to switching over from one reservoir to the other, memorizing the said value of said electrical signal as it exists immediately before the change over, generating a substitute signal equal to that memorized value, and substituting that substitute signal, during the period of change over, in place of the actual signal for comparison with said demand signal, thereby to maintain the setting of the flow control valve unchanged during the period of change over;
• (f) cyclically repeating operations (a) to (e) with respect to each reservoir in turn as it is being emptied.

In an apparatus for the continuous control of liquid flow at low flow rates, comprising two fixed reservoirs connected in parallel between an inlet line and an outlet line, means for alternately connecting each of said reservoirs in turn to the inlet line while the other is connected to the outlet line, so that the one reservoir can be filled with said liquid while the other is being emptied, a flow control valve in said outlet line, means associated with each reservoir for sensing the amount of liquid in the reservoir, means for generating a signal proportional to the amount of liquid in each reservoir, means for generating a flow signal proportional to the rate of discharge of said liquid from each reservoir during the discharge phase of the operating cycle, and means for automatically adjusting the setting of the flow control valve depending on the deviation of said flow signal from a desired value, the improvement comprising:

• said sensing means being sensitive to the mass of the liquid contained in the reservoir, said signal generating means generating a signal which is proportional to the mass of liquid in each reservoir and said flow signal generating means generating a signal which is proportional to the rate of mass discharge of said liquid from each reservoir during the discharge phase of the operating cycle, and further including means for memorizing said flow signal as existing immediately prior to the change over from one reservoir to the other, means for generating a substitute signal equal to the memorized signal and means for feeding that substitute signal to said comparing means for the period of the change over, thereby to maintain the flow control valve at a constant setting during the change over.

In a process for the control of liquid flow at a desired low flow rate, which comprises:

• alternately filling and emptying two fixed reservoirs, the one reservoir being filled from a supply of the said liquid while the other is being emptied through a flow control valve to an outlet pipe, the flow of liquid to and from said reservoirs being automatically switched from one reservoir to the other as the liquid level in the reservoir being emptied sinks to a predetermined level, continuously monitoring the rate of discharge of the liquid from the reservoir being emptied;
• generating an electrical signal having a value proportional to the rate of discharge from the reservoir being emptied;
• utilizing the electrical signal for comparison with a demand signal representative of a desired flow rate;
• adjusting of the setting of the flow control valve in response to any difference between said first signal and said demand signal, thereby to increase or decrease the flow rate from the reservoir being emptied, as may be necessary to achieve equalization of said first signal and said demand signal;
• continuously monitoring said signal, and when said signal reaches a predetermined value, automatically switching over the discharge from one reservoir to the other;
• the improvement which comprises:
• memorizing the value of said signal as it exists immediately before said change over, generating a substitute signal equal in value to the memorized value, and substituting that substitute signal in place of the genuine signal for comparison with said demand signal during the period of change over.