Robust dynamical method and device for detecting the level of a liquid using resistance temperature detectors

Patent metadata

Field	Value
Publication number	US-9810566-B2
Application number	US-201514836963-A
Country	US
Kind code	B2
Filing date	Aug 27, 2015
Priority date	Sep 10, 2014
Publication date	Nov 7, 2017
Grant date	Nov 7, 2017

Abstract

A method for indicating the level of a liquefied gas in a cryogenic tank having a resistance temperature detector ( 1 ). The controller applies a heating pulse to the detector and performs a single resistance measurement after the heating pulse. The overheating of the sensor and the time interval for the measurement are found in a separate set of test experiment. As a result, for temperature sensors with negative temperature coefficient, the resistance of the sensor in gas is below some unique characteristic value, which can be used like a threshold criterion to distinguish between the liquid and the gas in a wide temperature range. For sensors with positive temperature coefficient, the resistance of the sensor in gas is larger than some unique characteristic value, which can be used like a threshold criterion to distinguish between the liquid and the gas in a wide temperature range.

First claim

We claim:

1. A method of operation of a device for determining a position of a gas/liquid interface of a liquefied gas, helium, superfluid helium, neon, hydrogen, nitrogen or oxygen in a cryogenic tank, the device having at least one self-heated resistance temperature detector mounted inside the cryogenic tank with a support made from a material with low thermal conductivity in comparison to a thermal conductivity of surrounding media, the detector being connected to a current or voltage pulse generator, the device further having means to read out a temperature of the detector in dependence on an electric resistance of that detector, the method comprising the steps of:
a) applying at least one current or voltage heating pulse from the pulse generator to the detector, a power and duration of that heating pulse being sufficient for overheating the detector at an end of the heating pulse to a temperature T heated above a temperature of a detector environment T env plus a temperature resolution Δ(T env ) of the detector at the temperature of the detector environment, wherein T heated >T env +Δ(T env );
b) performing a temperature measurement with the detector at an end of a time interval t off after the end of at least one heating pulse, wherein a power and a duration of the heating pulse is determined in a preceding test experiment such that, for at least one time interval t off , a condition is fulfilled, the condition specifying that a difference between a temperature of the detector measured at an end of the time interval t off in gas and a temperature of the detector measured at the end of the time interval t off in liquid is greater than 2Δ(T env ), where Δ(T env ) is the temperature resolution of the detector at the temperature of the detector environment, a power and/or a duration of the heating pulse thereby being increased by steps until at least one value of the time interval t off is found which satisfies the condition;
c) choosing a threshold temperature T threshold , such that, if the detector is immersed in liquid, then the threshold temperature T threshold is above a temperature of the detector measured at the end of the time interval t off plus the temperature resolution Δ(T env ) of the detector at the temperature of the detector environment T env and, if the detector is immersed in gas, then the threshold temperature T threshold is below the temperature of the detector measured at the end of the time interval t off minus the temperature resolution Δ(T env ) of the detector at the temperature of the detector environment T env ;
d) deducing a position of a gas/liquid interface of the liquefied gas in the cryogenic tank below a position of the detector if the temperature of the detector measured at the end of the time interval t off after the end of the heating pulse is above the threshold temperature T threshold ; and
e) deducing a position of the gas/liquid interface of the liquefied gas in the cryogenic tank above the position of the detector if the temperature of the detector measured at the end of the time interval t off after the end of the heating pulse is below the threshold temperature T threshold .

2. The method of claim 1 , wherein the power and/or the duration of the heating pulse and/or the time interval t off and the threshold temperature T threshold required to deduce the position of the gas/liquid interface are adapted to the detector environment temperature T env for a range between a minimum and maximum environment temperature T env min , T env max by varying the power and/or the duration of the heating pulse and/or the time interval t off in a special test experiment until a single threshold temperature T threshold is defined which is valid to distinguish between detector-in-liquid and detector-in-gas states through an entire environment temperature range.

3. The method of claim 2 , further comprising the steps of:
f) adjusting the power and/or the duration of the heating pulse at the maximum environment temperature T env max in liquid to values sufficient to overheat the detector at the end of the heating pulse to a value at which the temperature T heated liquid of the overheated detector in liquid must satisfy the following condition: T heated liquid >>T env max +2Δ(T env max ), where Δ(T env max ) is the temperature resolution of the detector in a vicinity of the maximum environment temperature T env max ;
g) immersing the detector in liquid at the maximum environment temperature T env max and determining an optimum time interval t off by varying t off starting from zero by small portions until the detector, which was previously overheated with a power and duration of the heating pulse according to step f), is cooled down to a temperature T detector in liquid (t off , T env max ) close to the maximum environment temperature T env max plus the temperature resolution of the detector Δ(T env max ) in the vicinity of the maximum environment temperature T env max , such that a g condition T detector in liquid (t off , T env max )≈T env max +Δ(T env max ) is satisfied;
h) immersing the detector into the gas and reducing the environment temperature T env stepwise to the minimum environment temperature T env min , wherein an h condition T detector in gas (t off , T env )−Δ(T env )>T detector in liquid (t off , T env max )+Δ(T env max ) is checked for each step of the environment temperature T env , where T detector in gas (t off , T env ) is a temperature of a previously overheated detector in gas at an environmental temperature step T env measured at an end of the time interval t off after the end of the heating pulse and Δ(T env ) is the temperature resolution of the detector at temperature T env ;
i) if the h condition fails for at least one environment temperature step T env , increasing the power and/or the duration of the heating Pulse and adjusting the time interval t off to fulfill the g condition and repeating step h) until the h condition is fulfilled for all environment temperatures T env between the minimum and maximum environment temperature T env min , T env max ; and
j) choosing the threshold temperature T threshold in a range T detector in liquid (t off , T env max )+Δ(T env max )<T threshold <T detector in gas (t off , T env *)−Δ(T env *), where T detector in gas (t off , T env *) is the detector temperature measured in gas at an environment temperature T env * at the end of the time interval t off after the end of the heating pulse, and Δ(T env *) is the temperature resolution of the detector at a temperature T env *, wherein T env * is an environment temperature for which the detector temperature measured in gas at the end of the time interval t off after the end of the heating pulse is lowest for all environment temperatures T env between the minimum and maximum environment temperature T env min , T env max .

4. The method of claim 1 , wherein a position of a gas/liquid interface of He4 or of superfluid He4 is determined in a cryogenic tank with a self-heated resistance temperature detector being mounted inside the cryogenic tank with a support made from a material having low thermal conductivity in comparison to a thermal conductivity of He4 or of superfluid He4, a cross section of the support being reduced to suppress additional thermal contact to an environment by superfluid film, wherein the power of the heating pulse and/or the duration of the heating pulse are/is additionally cross-checked to be high enough to cancel creeping effects caused by the superfluid film while the detector is just above the superfluid helium gas/liquid/interface.

5. The method of claim 1 , wherein the detector is connected to a pulse generator capable of producing a sequence of at least two heating pulse

Assignees

• Bruker Biospin Ag

Inventors

• Eshchenko Dmitry
• Schauwecker Robert

Classifications

• G01F23/248

  Constructional details; Mounting of probes · CPC title

• G01F23/247 Primary

  for discrete levels · CPC title