Circulating Bath Dual Level Sensing for Heater Overheat Prevention
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Solution Overview
Problem
Conventional circulating baths face challenges in preventing overheating due to thermal lag in high temperature limit bulbs, especially in compact designs, which can lead to temperature overshoot and failure to meet safety standards like UL 61010-1, particularly when using high wattage density heating elements and reduced surface dimensions.
Innovation Solution
The implementation of a dual independent fluid level sensing system coupled with a high temperature limit switch, where both fluid level sensors and the high temperature limit switch are connected to a control circuit to ensure timely disconnection of the heater from power, preventing overheating, and incorporating a redundant monitoring system to prevent false shutdowns and software faults.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a high temperature limit switch with a sensing bulb is used to prevent overheating, then safety is improved, but thermal lag causes temperature overshoot and reduced operating temperatures
Solution Approach 1:
The patent applies preliminary action by using a second fluid level sensor to detect low fluid conditions before the high temperature limit switch can respond to overheating. This advance detection allows the control circuit to shut down the heater proactively, preventing thermal lag from causing temperature overshoot. The sensing bulb's thermal mass is compensated for by having the second sensor positioned to detect level changes that would precede dangerous temperature rises.
Solution Approach 2:
The patent implements feedback through a control circuit that continuously monitors fluid level via two sensors and adjusts heater operation accordingly. The control circuit receives signals from both fluid level sensors and the high temperature limit switch, creating a closed-loop system that responds to temperature and level conditions. This feedback mechanism ensures the heater shuts down not only when the limit switch triggers but also when fluid level drops, preventing the thermal lag problem from causing safety issues.
2Reliability
If the high temperature limit switch is set to activate at a lower set-point temperature to account for thermal lag, then safety is improved, but the maximum operating temperature is reduced
Solution Approach 1:
The second fluid level sensor performs preliminary detection of low fluid conditions before temperature becomes critical. By positioning this sensor to detect levels that would precede dangerous heating, the system shuts down the heater proactively, eliminating the need to lower the temperature set-point. The control circuit integrates signals from both sensors, allowing operation at higher temperatures when fluid levels are adequate while providing safety shutdown when levels drop.
3Device complexity
If a single fluid level sensor is used to monitor low fluid conditions, then device complexity is reduced, but reliability is compromised due to potential false readings or sensor failures
Solution Approach 1:
The patent applies local quality by positioning two fluid level sensors at different locations within the reservoir to monitor different aspects of fluid level conditions. The first sensor monitors general fluid presence while the second sensor provides redundant verification. This spatial distribution of sensing functions allows the system to distinguish between true low fluid conditions and false sensor readings, improving reliability without significantly increasing overall system complexity.
Solution Approach 2:
The control circuit implements beforehand cushioning by requiring agreement from both fluid level sensors before triggering a heater shutdown. This redundancy protects against false shutdowns caused by single sensor failures or erratic readings. The control circuit is designed to tolerate temporary sensor discrepancies while still providing safety shutdown when both sensors confirm low fluid conditions, cushioning the system against false positives.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution effectively prevents overheating by accurately monitoring fluid and surface temperatures, ensuring the circulating bath operates within safe limits, meeting stringent safety standards while maintaining high heater performance and compact design, even under conditions of thermal lag and potential sensor failures.
Implementation Method 1
a first fluid level sensor configured to monitor a low fluid condition and a second fluid level sensor configured to monitor the same low fluid condition
Implementation Method 2
The high temperature limit switch typically includes a temperature sensing bulb that is attached to the top of a heater coil to monitor both heater and fluid temperatures. As the temperature of the bulb rises, fluid located within the bulb expands. This expanding fluid is typically coupled through a capillary tube to a diaphragm located within the switch housing. When the temperature of the expanding fluid exceeds a set-point temperature, the pressure exerted by the expanding fluid causes an electrical contact within the limit switch to open.
Implementation Method 3
a heater configured to be operatively connected to a source of power and a control circuit
Data Source
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AI summary
A circulating bath (10) including a heater (18) configured to heat a fluid (34) in a reservoir (22). The heater (18) is configured to be operatively connected to a source of power (50) by a control circuit (39) operatively coupled to the heater (18). First and second fluid level sensors (14) (16) are operatively coupled to the control circuit (39), and provide signals indicative of a low fluid condition in the reservoir (22). The control circuit (39) is configured to receive the signals from the fluid level sensors (14) (16), and disconnect the heater (18) from the source of power (50) in response to receiving a signal from either of the first and second fluid level sensors (14), (16) indicative of a low fluid condition.