Method and apparatus for automatic measurement of evapotranspiration

• US 5,311,769 A
• Filed: 03/04/1992
• Issued: 05/17/1994
• Est. Priority Date: 03/04/1992
• Status: Expired due to Term

• Current Assignee: Henry T. Hetzel
• Original Assignee: Henry T. Hetzel
• Inventors: Hetzel, Henry T.
• Primary Examiner(s): RAEVIS, ROBERT R
• Application Number:
  US07/845,611
• Time in Patent Office: 804 Days
• Field of Search: 73/866.4, 73/198, 73/223, 73/224, 73/61.77, 137/391, 137/392, 137/395, 137/396, 392/386, 392/394, 392/400, 392/402, 392/403, 392/325, 392/327, 392/322, 261/154, 261/DIG. 65
• US Class Current: 73/61.77
• CPC Class Codes: G01N 33/497 of gaseous biological material, e.g. breath

Abstract

An automatic atmometer includes an evaporating head that is coupled to a metering vial through a three-way valve controlled by fluid sensors disposed proximate inlet and open ends of the metering vial. The three-way valve is ordinarily in a quiescient state that connects the evaporating head to the metering vial. As water evaporates from the evaporating head water is drawn by suction out of the metering vial. When a sensor proximate the inlet of the metering vial indicates that the vial is empty the three-way valve is activated. This refills the metering vial from a fluid reservoir above the metering vial. As soon as one or more sensors proximate the open end of the metering vial indicate that the vial is full the three-way valve is returned to its quiescient condition. An output signal activates a counter or other logging device to record the number of times the metering vial has been emptied and refilled. Fluid presence at various portions of the metering vial is sensed by capacitive structures each producing a varying amount of capacitance according to the presence or absence of the fluid. The amount of an RF signal coupled through each sensor is measured and compared to a reference voltage. The reference voltage is itself produced from a similar capacitive structure so that the measured voltage and the reference voltage have the same temperature coefficient.

Claims

1. An atmometer comprising:

an evaporator, having a fluid inlet, that evaporates a fluid whose evaporation is to be measured;

a fluid supply that supplies fluid to be evaporated;

a flow metering reservoir having a service orifice that receives, retains and subsequently supplies retained fluid, through the service orifice;

a valve, hydraulically coupled to the fluid inlet of the evaporator, to the fluid supply and to the service orifice of the flow metering reservoir, that while in a first state (a) hydraulically couples the fluid inlet of the evaporator to the service orifice of the flow metering reservoir and (b) blocks fluid flow from the supply outlet of the fluid supply, and also that while in a second state (c) blocks fluid flow between the valve and the fluid inlet of the evaporator and also (d) hydraulically couples the supply outlet of the fluid supply to the service orifice of the flow metering reservoir;

a fluid sensing circuit, disposed proximate the flow metering reservoir, that produces a first signal indicative of when the flow metering reservoir contains an amount of fluid less than or equal to a first selected amount and a second signal indicative of when the flow metering reservoir contains an amount of fluid greater than or equal to a second selected amount that is greater than the first selected amount; and

a valve control circuit, coupled to the valve and responsive to the first and second signals, that causes the valve to be in the second state upon the presence of the first signal, to then remain in the second state until the presence of the second signal, to return to the first state upon the presence of the second signal, and to subsequently remain in the first state in the absence of both the first and second signals.

2. An atmometer as in claim 1 further comprising a recording signal output, coupled to the valve control circuit, that produces a cycle in a recording signal for each time the valve control circuit causes the valve to be in the second state.

3. An atmometer as in claim 1 further comprising a switch, coupled to the valve, for momentarily forcing the valve to be in the second state.

4. An atmometer as in claim 1 further comprising a timer, coupled to the valve control circuit, that forces the valve to return to the first state if the second state lasts longer than a preselected length of time.

5. An atmometer as in claim 1 wherein the fluid sensing circuit includes first, second and third fluid sensors and further wherein (a) the second and third fluid sensors are disposed a first distance apart along a generally cylindrical portion of the flow metering reservoir distally removed from the service orifice, and (b) the second signal is derived from the logical conjunction of the second and third fluid sensors.

6. An atmometer as in claim 5 wherein the flow metering reservoir includes an expansion chamber.

7. An atmometer as in claim 6 wherein the interior volume of the expansion chamber is greater than the interior volume of the generally cylindrical portion of the flow metering reservoir that lies between the second and third fluid sensors.

8. An atmometer as recited in claim 1 wherein the flow metering reservoir further includes a vent open to the atmosphere.

9. An atmometer as in claim 8 wherein the fluid sensing circuit includes first, second and third fluid sensors and further wherein (a) the second and third fluid sensors are disposed proximate the vent and a first distance apart along a generally cylindrical portion of the flow metering reservoir, and (b) the second signal is derived from the logical conjunction of the second and third fluid sensors.

10. An atmometer as recited in claim 1 wherein the fluid sensing circuit comprises a radio frequency oscillator, a detector of radio frequency voltage, and a capacitive path therebetween whose dielectric constant varies according to the presence or absence of fluid proximate the capacitive path.

11. An atmometer comprising:

an evaporator, having a fluid inlet, that evaporates a fluid whose evaporation is to be measured;

a fluid supply that supplies fluid to be evaporated;

a flow metering reservoir having a service orifice that receives, retains and subsequently supplies retained fluid, through the service orifice;

a valve, hydraulically coupled on one side to the fluid supply and on an opposite side via a tee connection to the service orifice of the flow metering reservoir and to the fluid inlet of the evaporator, that while in a first state blocks fluid flow from the fluid supply, and also that while in a second state hydraulically couples the fluid supply to the service orifice of the flow metering reservoir;

a fluid sensing circuit, disposed proximate the flow metering reservoir, that produces a first signal indicative of when the flow metering reservoir contains an amount of fluid less than or equal to a first selected amount and a second signal indicative of when the flow metering reservoir contains an amount of fluid greater than or equal to a second selected amount that is greater than the first selected amount;

a valve control circuit, coupled to the valve and responsive to the first and second signals, for causing the valve to be in the second state upon the presence of the first signal, to then remain in the second state until the presence of the second signal, to return to the first state upon the presence of the second signal, and to subsequently remain in the first state in the absence of both the first and second signals; and

a timer, coupled to the valve control circuit, that forces the valve to return to the first state if the second state lasts longer than a preselected length of time.

12. An atmometer comprising:

an evaporator, having a fluid inlet, that evaporates a fluid whose evaporation is to be measured;

a fluid supply that supplies fluid to be evaporated;

a flow metering reservoir having a service orifice that receives, retains and subsequently supplies retained fluid, through the service orifice;

a valve, hydraulically coupled on one side to the fluid supply and on an opposite side via a tee connection to the service orifice of the flow metering reservoir and to the fluid inlet of the evaporator, that while in a first state blocks fluid flow from the fluid supply, and also that while in a second state hydraulically couples the fluid supply to the service orifice of the flow metering reservoir;

a fluid sensing circuit, disposed proximate the flow metering reservoir, that produces a first signal indicative of when the flow metering reservoir contains an amount of fluid less than or equal to a first selected amount and a second signal indicative of when the flow metering reservoir contains an amount of fluid greater than or equal to a second selected amount that is greater than the first selected amount;

a valve control circuit, coupled to the valve and responsive to the first and second signals, for causing the valve to be in the second state upon the presence of the first signal, to then remain in the second state until the presence of the second signal, to return to the first state upon the presence of the second signal, and to subsequently remain in the first state in the absence of both the first and second signals; and

wherein the fluid sensing circuit includes first, second and third fluid sensors and further wherein (a) the second and third fluid sensors are disposed a first distance apart along a generally cylindrical portion of the flow metering reservoir distally removed from the service orifice, and (b) the second signal is derived from the logical conjunction of the second and third fluid sensors.

13. An atmometer as in claim 12 wherein flow metering reservoir includes an expansion chamber.

14. An atmometer as in claim 13 wherein the interior volume of the expansion chamber is greater than the interior volume of the generally cylindrical portion of the flow metering reservoir that lies between the second and third fluid sensors.

15. An atmometer comprising:

an evaporator, having a fluid inlet, that evaporates a fluid whose evaporation is to be measured;

a fluid supply that supplies fluid to be evaporated;

a flow metering reservoir having a service orifice that receives, retains and subsequently supplies retained fluid, through the service orifice;

a valve, hydraulically coupled on one side to the fluid supply and on an opposite side via a tee connection to the service orifice of the flow metering reservoir and to the fluid inlet of the evaporator, that while in a first state blocks fluid flow from the fluid supply, and also that while in a second state hydraulically couples the fluid supply to the service orifice of the flow metering reservoir;

a fluid sensing circuit, disposed proximate the flow metering reservoir, that produces a first signal indicative of when the flow metering reservoir contains an amount of fluid less than or equal to a first selected amount and a second signal indicative of when the flow metering reservoir contains an amount of fluid greater than or equal to a second selected amount that is greater than the first selected amount;

a valve control circuit, coupled to the valve and responsive to the first and second signals, for causing the valve to be in the second state upon the presence of the first signal, to then remain in the second state until the presence of the second signal, to return to the first state upon the presence of the second signal, and to subsequently remain in the first state in the absence of both the first and second signals; and

wherein the flow metering reservoir includes a vent open to the atmosphere, wherein the fluid sensing circuit includes first, second and third fluid sensors and further wherein (a) the second and third fluid sensors are disposed proximate the vent and a first distance apart along a generally cylindrical portion of the flow metering reservoir, and (b) the second signal is derived from the logical conjunction of the second and third fluid sensors.