Temperature Sensors

In preceding blogs I have identified temperature as probably the single most important variable in a cleaning process.  Devices that sense and control temperature, therefore, are a critical part of a cleaning system.  Let’s take a quick look at a few of the types of temperature sensors that are available and suggest where each is applicable in cleaning.

Temperature sensors can be divided into two basic categories.  Those which are mechanical and those which are electrical in function.

First, let’s look at mechanical type sensors.

In a thermometer of this type, expansion and contraction of the liquid in the bulb is amplified as the liquid is pushed up into the attached small diameter tube.

Most gasses, liquids and solids expand in volume as their temperature is increased and shrink in volume as their temperature is decreased (with the notable exception of water which grows a little after it freezes).  Mechanical temperature sensors rely on the expansion and contraction of a sensing element (usually a metal, or a confined liquid or gas) to measure  changes in temperature.  A mercury thermometer is probably the best known example of a temperature sensor that uses the expansion and contraction of a liquid (mercury) to indicate temperature.  Since the expansion and contraction of the mercury is relatively small, an arrangement is used which consists of a large volume of contained mercury (the bulb) which is directly connected to a very small diameter tube.  In this arrangement, the small diameter tube amplifies the result of expansion and contraction of the mercury in the bulb as the mercury is pushed up into the tube.  The result is that a very small amount of expansion of the mercury in the bulb creates a relatively larger change in how far the mercury is pushed up into the small diameter tube.  This makes the device more accurate.  With all the sensitivity about the health concerns of mercury in today’s environment, mercury is often replaced with another liquid, colored alcohol for example, for safety’s sake.

Thermometers of this type are commonly used to measure the temperature of a liquid or gas in laboratory or domestic settings.  They are simple to use and allow precise, permanent calibration making them ideal for reference use.  In this simple form, however, thermometers of this type do not offer the ability to provide feedback to control temperature.

Industrial temperature sensors utilizing the expansion of a liquid to control temperature are often called capillary thermostats.  A large bulb, usually containing a liquid, connects via a small diameter (capillary) tube to a remotely located instrument with a diaphragm which flexes in response to the amount of liquid displaced from the bulb caused by temperature changes.  Since the volume of liquid in the capillary tube is insignificant compared to the volume of liquid in the bulb, the displacement of the diaphragm is primarily the result of the expansion of liquid in the bulb and not that in the capillary.  Movement of the diaphragm or bellows can be used to close or open an electrical circuit through a switch or directly control the flow of a liquid or gas (refrigerant or steam, for example) through the use of a valve.

Illustration of a capillary bulb thermometer.
In a capillary type thermostat, expansion of liquid in the sensor bulb results in physical displacement of a diaphragm or bellows connected to it by a small diameter (capillary) tube.  This motion can be used to actuate switches, valves, etc.

In order to accurately sense temperature, the bulb of a capillary thermostat needs to be positioned in such a way that the entire bulb is heated or cooled as the temperature being sensed changes.  Immersion of the bulb in a liquid or gas, for example, will give the most accurate temperature reading.  Clamping a bulb to a tank containing a liquid, on the other hand, does not always result in an accurate temperature measurement as only one side of the bulb is seeing the heat source while the other side remains at ambient temperature.  In some cases, an insulating material is placed over the sensing bulb on the side opposite to the surface being measured to minimize this potential inaccuracy.

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–  FJF  –

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