Blog Purpose

The CTG Technical Blog is intended as a source of information on subjects related to industrial and precision cleaning technology. The writer of the blog, John Fuchs, has 40+ years of experience covering the entire gamut of cleaning. Mr. Fuchs has extensive knowledge of ultrasonic cleaning technology having been employed by Blackstone-Ney ultrasonics and its predecessors since 1968. The blog is also intended to serve as a forum for discussion of subjects related to cleaning technology. Questions directed to the blog will be responded to either in the blog (if the topic has general interest) or directly by email. Emails with questions about the current blog should be entered in the comments section below. Off-topic questions related to cleaning may be sent to

Weirs – Four-Sided Overflow

November 7th, 2014

The final type of weir I’ll discuss in this series is the four-sided overflow weir in which liquid overflows all four sides of a process tank.

Four Sided Overflow

A four-sided overflow is often used for “precision cleaning” tanks where rapid and thorough clearing of contaminants from the process tank is a major concern.

This type of weir is normally found in equipment designed for “precision cleaning” applications and most commonly in rinses where efficient particle removal is a major consideration.  The theory behind the benefit of a four-sided overflow is shown in the illustration below.

Single vs. Four-Sided Weir

Laminar flow from the bottom to the top of the tank in a four-sided overflow tank reduces possible areas of stagnation

In a tank with a single-sided overflow, liquid usually enters the tank through a fitting located near the bottom of the tank on the side opposite the overflow.  In flowing from the inlet to the overflow, the path of the liquid allows for possible stagnation of liquid not in the direct path of flow.  Even if the inlet is placed higher on the tank wall or if a sparger is used, the opportunity for stagnation remains and may even become more pronounced, especially near the bottom of the tank.  With a four-sided overflow having a diffused inlet at the bottom of the tank, the opportunity for stagnation is significantly reduced.  The charts below show the different speeds of recovery of a one-sided over vs. a four-sided overflow.

4-Sided Overflow Rinse Charts

Logic and evidence supports the benefits of a four-sided overflow.  However, successfully executing a four-sided overflow is difficult, especially under low flow conditions (low flow rates are beneficial to ultrasonics and may reduce the use of DI water and/or chemistry).  This is because the entire rim of the tank becomes the width of the overflow which is 4 or more times as wide as a single-sided overflow in a similar-sized tank.  Four-way leveling also becomes a critical factor.  Tanks with four-sided overflow that are part of a multi-tank system should be equipped with a means to level each tank separately once the machine is in place.  In addition, the saw-tooth weir design is often used in a four-sided overflow tank to effectively reduce the overflow weir width (especially in larger tank systems) making it easier to maintain flow over all sides of the tank.

-  FJF  -

Maximizing Overflow Weirs for Skimming Applications

October 31st, 2014

The concept and purposes of an overflow weir are pretty simple but making them perform for maximum benefit can be a challenge.  For the best performance of a weir used in a skimming application, liquid should overflow the entire width of the weir.  An area of stagnation is formed behind an area where there is no overflow.  There are a several factors that can affect the nature of flow over a weir – -

  • Weir Width
  • Liquid Flow Rate
  • Weir Geometry
  • Level of the Weir
  • Liquid Properties

The first two, weir width and flow rate, go hand-in-hand.  In the case of a relatively narrow weir, a few inches for example, it doesn’t take much flow to achieve flow over the entire width of the weir.  As the width of the weir is increased, however, achieving full width flow requires a higher and higher flow rate to overcome the forces of surface tension of the liquid which favor the flow being confined to as narrow a stream as possible.

Note -  This is especially true if the liquid is de-ionized water (which has very high surface tension).

Although there are instances where a “perfect” weir won’t overflow its entire width due to surface tension, it’s worth checking to assure that non-uniform flow is not due to the weir geometry and/or the fact that it is not level.

Weir geometry and level are critical to achieve full width flow over the weir.  In the case at the left above, the reason for non-uniform flow may be that the weir is not level.  Tilting the tank or machine to raise the back side (as shown in the illustration) may improve flow.  In the case shown at the right, the cause for non-uniform flow could be a low spot in the weir at the overflow point.

Weir geometry and level are critical to achieve full width flow over the weir. In the case at the left above, the reason for non-uniform flow may be that the weir is not level. Tilting the tank or machine to raise the back side (as shown in the illustration) may improve flow. In the case shown at the right, the cause for non-uniform flow could be a low spot in the weir at the overflow point.

One way to help assure that liquid flows over the entire width of a weir is to increase flow.  When increasing flow is not an option, other measures may be required to achieve the desired effect.  One of the more common options is the saw-tooth weir as shown below.

A "saw-tooth" weir is one way to assure uniform flow over a weir at low liquid flow rates.

A “saw-tooth” weir is one way to assure uniform flow over a weir at low liquid flow rates.

In effect, the saw-tooth design reduces the width of the overflow weir thereby reducing the flow required to achieve full width flow.  In addition, because of the increased flow velocity through each “V” of the sawtooth, leveling of the tank (or the more difficult case of a machine with several tanks) becomes less critical.  This design is particularly beneficial in the case of a four-sided overflow weir in which liquid overflows all four edges of a tank.  The four-sided overflow increases the width of the weir to the length of all four sides of the tank unless the saw-tooth design is utilized.  Since many four-sided overflow weirs are in rinse tanks with single pass de-ionized water, a lower flow is a desirable.

The one possible “down-side” to the sawtooth weir is that there can’t help but be a small area of liquid stagnation behind each one of the projecting “teeth.”  Care should be taken to avoid withdrawal of parts through these stagnant zones which might result in re-deposition of contaminants trapped in the stagnant zones.

-  FJF  -

More About Overflow Weirs

October 28th, 2014

Overflow weirs have specific purposes as they are utilized in industrial cleaning equipment.  Also, as one might expect, there are several manifestations of the overflow weir which perform various functions.   A simple “standpipe” is a form of weir as is a fitting on the side of a tank located just below the desired liquid level.  An overflow fitting is generally placed on the side of the tank so that the bottom of the fitting is just below the desired liquid level.

Standpipes and overflow fittings are types of "weirs" that are commonly used in the design of cleaning tanks to establish liquid level or protect against excessive liquid level.

Standpipes and overflow fittings are types of “weirs” that are commonly used in the design of cleaning tanks to establish liquid level or protect against excessive liquid level.

In designing a weir, it is important to first recognize that the amount of liquid flowing over a weir, provided that the overflow depth remains constant, is directly proportional to the width of the weir.  Wider weirs are required when it is necessary to accommodate a large volume of flow without a significant change in overflow depth.  A 1″ diameter standpipe provides a weir approximately 3.14″ (C=πD) in width (the edge of the pipe circumference).  The flow in the case of a standpipe is also limited by the capacity of liquid to flow down the pipe due to the force of gravity.  In the case of a fitting mounted on the side of a tank, the calculation becomes a little more complex as the width of the “weir” changes as the level of liquid changes.  Ultimately, however, the flow rate is still limited by the ability of liquid to flow through the outlet pipe. Standpipes or sidewall fittings are only applicable in cases where a relatively small flow of liquid is anticipated.  In the case of an “emergency” overflow, weirs of this type should be designed to accommodate the maximum anticipated flow without the overall level in the tank reaching the top edge of the tank.  Larger flow rates require wider weirs, eg. the entire width of one side of a process tank.

Weirs used for the main purpose of establishing and controlling liquid level generally don’t benefit from any particular lip consideration.  In the case of surface skimming, however, lip design may be critical to achieve the desired effect.

A weir sloped as shown at the left above is generally used to skim oil from a surface.  A weir as shown on the right above is generally felt to assist in removing floating particles from a surface.

A weir sloped as shown at the left above is generally used to skim oil from a surface. A weir as shown on the right above is generally felt to assist in removing floating particles from a surface.

It is generally felt that a knife edge weir or a weir sloped away from the overflow will give the best result in applications that involve skimming oil from a surface.  When it comes to removing particles that float on a surface, however, a weir sloped toward the overflow is felt to provide superior results.  A weir used to remove floating contaminants is generally used in conjunction with a sparger.  The flow of the sparger moves particles toward the weir and also increases the flow over the weir.  The sloped exit facilitates increasing flow velocity and the slight “lifting” required to help pushing particles over the edge of the weir.

Next, I’ll talk about some of the more unusual and sometimes weird features of weirs, their purpose and how they came to be.  So, stayed tuned!

-  FJF  -

Overflow Weirs

October 24th, 2014

Overflow weirs are common features of industrial cleaning systems.  Simply, a weir is much like a dam that holds back a river to form a lake.  In the case of a cleaning system, the “lake” is usually a tank of liquid.  The “dam” is formed as a portion of the lip of the tank is set at a lower level than the rest to allow liquid to overflow.

Illustration of an overflow weir

A basic overflow weir as used in industrial cleaning applications.

When the liquid level in the tank exceeds that of the weir, liquid overflows the weir.  The flow is directly proportional to W (the weir width) provided that the overflow depth is maintained the same.  The volume of flow as the overflow depth changes is described by the following formula – -

F = CW√(gH³)     or dimensionally speaking,     x³/s = x√((x/s²)(x³))

where F is the flow rate, and C is some dimensionless coefficient that will undoubtedly be there. So, if H (overflow depth) doubles,the increase in flow would increase by 2^(1.5), or about 2.828 times greater.  (Thanks Yahoo Answers!)

In industrial cleaning systems, weirs perform two basic functions.  The first is to establish and control liquid level in a tank or other vessel.  The other is to selectively remove floating contaminants from the surface of a liquid.

In the first function, controlling level, weirs are extremely efficient and are very, very simple compared to alternative means of level control.  Maintaining a relatively uniform tank depth is often important, especially in tanks with ultrasonic transducers, as the liquid depth has an effect on the efficiency and intensity of the ultrasonic field.  An overflow weir performs this function without any moving parts and has the added benefit of inherently modulating or adjusting flow as required (see above) to relatively closely (although not exactly) maintain a pre-set liquid level in spite of varying incoming flow rates and other factors like the displacement of liquid due to parts being introduced into a cleaning or rinsing tank in a cleaning system.

In most cases, overflow weirs incorporate a reservoir to collect the liquid that overflows the weir for disposal, re-use or whatever.

An overflow weir maintains liquid level as liquid is displaced as parts are introduced into the process tank.

An overflow weir maintains liquid level as liquid is displaced as parts are introduced into the process tank.

When parts are introduced into the process tank, they displace an amount of liquid equivalent to their volume.  Without an overflow weir, the liquid depth in the process tank would increase.  With an overflow weir and a recirculation pump as shown in the above illustration, the liquid level is maintained when parts are introduced and returns to the optimum level once parts are removed.  This, of course, means that the overflow weir reservoir should be sized based on the volume of parts being cleaned with some additional volume to allow compensation for liquid evaporation and makeup as necessary.  It is also important to establish the level of liquid in the overflow weir reservoir to accommodate the expected variations.  Filling the reservoir to the same depth as the process tank will defeat its purpose.

In the next few blogs, I will discuss other functions of overflow weirs and explain some of the variations in weir designs and the logic behind them.

-  FJF  -

Rinse to Resistivity

August 8th, 2014

Previous blogs have discussed the importance of rinsing as part of the overall cleaning process.  Because of its importance, many schemes have been devised to assure that parts are adequately rinsed after cleaning.  Most of these schemes embrace the “more is better”  concept which often leads to overkill as there is often no practical way to verify the adequacy of rinsing.  Although rinsing is important, it is also expensive! Multiple rinses and high flow rates can add considerably to the cost of cleaning as equipment and utility costs escalate. Although it is not perfect solution in every case, “rinse to resistivity” may offer a means to assure (and verify) adequate rinsing while controlling cost, especially in highly critical applications.

Illustration showing rinse to resistivity concept

At the start, rinsing is removing ions from the part being rinsed. As rinsing progresses, fewer and fewer ions are present. Once the resistivity meter measures resistivity adequately low enough, the rinse process is considered complete.

The concept of rinse to resistivity is quite simple as shown above.  As I stated at the outset, however, it is not always viable in every cleaning application.  In order for rinse to resistivity to work, certain criteria must be met or considered.

  • The contaminant to be rinsed off must be conductive.  If the contaminant is not conductive (and there are some that are not) the resistivity meter will not be able to see any resistivity related to the contaminant and give a misleading result.  This is especially likely in the case of particles.  The resistivity may be below the target level even though a significant number of particles are still present in the rinse.
  • The rinse vessel volume should be as small as possible yet still accommodate the parts being rinsed.  If too large a vessel is used, one of two things can happen.  First, the contaminant on the part may not provide enough ions to register on the resistivity meter thereby indicating an adequate rinse when, indeed, little or no rinsing has taken place.  Second, the turnover time of a large vessel may significantly delay the detection of an adequate rinse condition.
  • Adequate flow rate must be supplied to provide a short turnover in the rinse vessel.  If the flow only provides one tank turnover every 20 minutes (for example) it may take 20 minutes to see the resistivity as measured at the outlet to increase enough to show a complete rinse.
  • The incoming rinse water must be significantly lower in resistivity than the target resistivity at the outlet.  If it is not, it may take longer to achieve the target resistivity.  Of course, if the resistivity of the incoming water is above the target resistivity level, complete rinsing will never be achieved.

Rinse to resistivity is a useful tool but must be applied correctly to be of benefit.  In many applications, rinse to resistivity is simply not applicable.  When considering rinse to resistivity, make sure that the quality people, the process people and the equipment manufacturer are on the same page.  Otherwise, the results may be less than expected.

-  FJF  -

Heat Alternatives for Cleaning

July 28th, 2014

I have stressed the importance of temperature to cleaning processes many times in previous blogs.  This blog will discuss the heat source options for achieving and maintaining the temperatures required for effective cleaning.

Electric -

Strip Heaters

Examples of electric strip heaters.

Electric heat is, without a doubt, the most prevalent heat alternative used for cleaning applications.  This is because electric heaters are relatively inexpensive, relatively small, easy to control, and because electricity is a utility readily available in most facilities.  Electric heaters are appropriate for applying heat to both liquids and gasses making them useful in heating liquids for cleaning and rinsing as well as air for drying.  The two basic types of heaters used to heat liquids are surface mount heaters (often called “strip” heaters) and immersion heaters.  Surface mount heaters have a flat profile which allows them to be affixed to the exterior of tanks using clamps, weld studs with bolts and other means.

Contoured Heaters

Examples of contoured electric heaters.

In the case of irregularly shaped tanks, the heaters can be formed to match the profile of the tank.  It is important that heaters of this type be firmly and permanently attached to assure efficient heat transfer from the heater through the tank wall and into the liquid.  In some cases, a heat conductive paste is used between the face of the heater and the tank wall to maximize heat transfer.  The surface of the heater not facing the tank wall can be covered with insulation to minimize heat radiation to the ambient and is especially important in enclosed spaces.

Immersion Heater

Example of an immersion heater.

Immersion heaters are placed directly in the liquid to be heated with electrical leads either looped over the top edge of the tank or exiting the tank through an appropriate fitting mounted on the tank wall.  In the case of immersion heaters, the efficiency is nearly 100% since the only path for heat is into the liquid.

In cases where air (or another gas) is to be heated, electric heaters may be provided as an open coil or with an enclosed element (similar to a strip heater) with fins to improve conduction and radiation.

Open Coil Heater

Example of an open coil heater for heating air.

Finned Heater

Example of a finned heater for heating air.

Steam -

The use of steam circulated through a heat exchanger immersed in a liquid is another means of heating liquids for cleaning and rinsing.  In most cases, steam is selected when the facility already has an existing source of steam making it economical.  The control of steam is usually accomplished using valves electrically actuated by a temperature control or by mechanical means using a capillary bulb or bi-metallic actuator.  Steam is an excellent choice in systems using flammable materials as, with steam, there is a greatly reduced chance of electrical discharge which might otherwise initiate combustion of flammable liquids or vapors.

Gas -

Gas heat utilizes a burner head attached to a combustion tube immersed in the liquid to be heated.  The application of gas heat is used primarily in very large tank systems because of the inherent size and configuration restrictions that apply to the combustion tubes which are several inches to a foot or more in diameter and must be of sufficient length to assure complete combustion.  The products of combustion of gas, of course, also need to be vented to the outside of the facility which is not always easy.  In general, tanks with a volume less than 800 to 1,000 gallons are not practical candidates for being heated by gas.

- FJF  -

Heat Capacity and Temperature Control

July 21st, 2014

How Much Heat? -

It is common for the design engineer to calculate the heat requirement for the tanks of a cleaning system based on the tank volume, the target operating temperature and the desired heat-up time from ambient temperature to operating temperature taking into account, of course, heat losses through the tank walls and from the surface of the liquid.  In most cases, this calculation is sufficient, but there are cases in which other factors must be taken into consideration.  The problem with heaters (especially electric heaters) is that they still continue to heat after being turned off.  In general, the larger the amount of heat, the more heat the heaters store in their mass as thermal inertia.  Even though the heat is turned off when the desired temperature is reached, the residual heat in the heaters may cause a “spike” in temperature that may be detrimental to the process.  Excess heater capacity can lead to temperature over-shooting and wide temperature variations due to higher thermal inertia.

Illustration showing the effect of thermal inertia.

In the above, the graph on the left shows the result of a heater with lower capacity while that on the right shows a heater of relatively higher capacity. Notice that although the heater with higher capacity achieves the temperature set point more quickly, there are greater temperature variations due to the greater thermal inertia of the higher capacity heater once the temperature target is achieved.

In another example, consider a case where cold parts introduced into the cleaning tank have significant mass or are being introduced in large numbers.  In such a case, since the parts are consuming heat as well, the heat required to maintain the desired temperature in the tank may be more than just that required to heat the tank up to the desired temperature in a given time.  Another consideration is in ultrasonic systems where ultrasonic energy provides heat to the tank.  As the majority of ultrasonic energy delivered to a cleaning tank ultimately turns into heat, a 2,000 watt ultrasonic transducer is very similar to a 2,000 watt heater in its effect on tank heating!  Depending on the heat load introduced into the tank by the parts, heat provided by the ultrasonic system may be adequate to maintain tank temperature without any additional heat at all.  In fact, there are rare cases where cooling must be employed to keep the tank temperature within the desired limits during ultrasonic operation.

Temperature Control -

Control of temperature in a cleaning tank may, at first, seem very simple – - a temperature sensor is used to turn the heat on when the temperature is below a set limit and off when the desired temperature is reached.  In many cases, a control of this type is adequate.  There are others, however, where due to thermal inertia, more sophisticated control schemes are required.  The two most common are the use of dual heaters and temperature controllers that anticipate the temperature target.  In the case of dual heaters, a higher capacity heater is used in combination with a lower capacity heater.  The two are used together until a temperature near the target temperature is reached.  At that point, the higher capacity heater turns off allowing its thermal inertia along with the lower capacity heater to achieve the temperature target.  In the case of anticipating temperature controls, a “smart” controller throttles the heat by cycling power on and off (time proportioning) or reducing voltage as the temperature starts to approach the target temperature.

Graphs showing the effect of time and voltage throttling

In the example on the left, the heater is turned off and on with decreasing on time as the set temperature is approached. In the example on the right, voltage is reduced as the temperature nears the set point. In both cases, the heater capacity is reduced by about 50% to maintain a stable temperature at the set point temperature.

Controllers of this type normally have adjustments to set the temperature differential at which the throttling process begins.  Some even track the rate of temperature increase to determine when to start the throttling process to achieve the desired temperature without over-shoot.

-  FJF  -



Temperature Sensor Selection and Placement

July 18th, 2014

Meaningful temperature measurements depend on the selection of the proper sensors and controls and the proper placement of sensors to accurately measure the targeted temperature.  For example, it was previously stated that bimetallic sensors, in most cases, ultimately sense the temperature of air around them.  This makes bimetallic sensors a good choice for measuring air temperature in a dryer.  To employ them in sensing the temperature of a liquid in a cleaning tank, however, requires that they be placed at a location where the air temperature is directly proportional to that of the temperature of the liquid to be measured.  This is not a simple task as heat must be transferred from the liquid through the wall of the vessel containing the liquid and then through the air to the sensor itself.  As we have noted before, air (or any gas) is not a good heat conductor.  As a result, a temperature measured in this way may either not reflect the actual temperature of the liquid in the tank or may result in a delayed response due to the time required for the heat to be transferred to the sensor through the tank wall and air.  Accurate measurements of liquid temperature require that the heat path from the liquid to the sensor be as short as possible giving thermocouples and resistance sensing devices an advantage when it comes to measuring liquid temperatures.  Capillary bulb type sensors can be used to measure the temperature of both liquid and air but, in the case of liquid, the capillary bulb itself should be either immersed in the liquid or bonded directly to the wall of the vessel using a suitable mechanical clamp and surrounded by a good conductor such as a highly heat-conductive epoxy.

Thermocouple Placement Illustration

In the above case of a tank with a side mounted heater and no circulation, a thermocouple at location “A” will read a higher temperature than those at “B” and “C.”

Although selection of the appropriate sensor is critical, even the best sensor will not provide the needed temperature indication if the sensor is not properly positioned.  In the case of a tank with heaters mounted on a single side, for example, liquid temperature may vary by tens of degrees depending on the position of the sensor in the tank relative to the placement of the heaters and the depth of the tank.  More heaters positioned around the tank will better distribute the heat but even distributed heat will not prevent temperature stratification in a large or deep tank.  A better solution is to distribute the heat using a pump loop or propeller type immersion agitator.

It is always a good practice to verify the accuracy of temperature measurements using a reference thermometer.  Although most temperature sensors are highly reliable, it is not uncommon for them to become detached or dislocated over time in which case, of course, they are not measuring the targeted temperature.  I prefer a laboratory grade mercury bulb thermometer encased in a metal sheath as a reference.  Hand-held thermocouple or RTD devices are also useful and especially so in cases where there is no direct access to the area where temperature is to be measured.

Note – Ultrasonic vibrations have been known to have an effect on the accuracy of some temperature sensing devices.  To prevent this possibility, reference temperatures should be measured with the ultrasonic energy off in ultrasonic tanks to assure accuracy.

-  FJF  -

More Temperature Sensors – Electrical

July 16th, 2014

Preceding blogs have described a number of mechanical means for measuring temperature based on the expansion and contraction of liquids and solids.  Another major classification of temperature sensors are based on electrical phenomenon.

Thermocouples -

A thermocouple sensor utilizes a junction of two dissimilar metals to measure temperature.  The principle is based on the fact that an electrical potential (voltage) is created in any conductor exposed to a thermal gradient along its length.  The two materials in a thermocouple are chosen such that an instrument (really a specialized voltmeter) can measure the difference in voltage produced by two conductors both subjected to the same thermal gradient.

Note – Contrary to popular belief, the voltage produced by a thermocouple is not actually created in the junction but rather along the length of the two conductors subjected to a thermal gradient.

In order to accurately measure this difference in voltage, the leads from the “hot junction” of the thermocouple (subjected to the temperature to be measured) must be joined to other conductors at what is called the “cold junction.”  Although this secondary connection historically was made in an ice bath to assure that any voltage measured by the sensor was only the result of the thermal gradient produced by the temperature to be measured, instruments today usually incorporate a reference “cold junction” and do not require such measures.  Thermocouple leads are connected directly to the instrument which is then able to convert the measured voltage to a temperature readout in the appropriate units.  Thermocouple junctions are made using several different combinations of dissimilar metals based on the application.  It is important that thermocouples and instruments indicating temperature be compatible to assure the accuracy of any measurement.

Resistance Temperature Detectors -

The resistivity of many materials changes with temperature.  This change in resistivity can be used to measure temperature.  Common forms of resistivity temperature detectors (RTD’s) use pure metals either deposited on a substrate or as wire wrapped around a non-conductive core to measure temperature.  The measuring instrument senses the resistance of the device and converts it into temperature readings.  In general, RTD’s are preferred over thermocouples due to their accuracy, long life and interchangeability.

Thermistors -

Thermistors are made of semi-conductor materials which change resistivity with temperature.  Their most common use is in applications where the change in temperature of the thermistor due to a flow of current through it results in a temperature increase. The resulting change in resistance due to an increase in temperature is used to control the flow of current through the device directly.    Thermistors can be designed to either increase or decrease resistivity on heating.  This makes them valuable in limiting in-rush currents and providing controlled heat in many applications.

Calibration -

All of the above can accurately measure temperature change but must be calibrated to measure absolute temperature.  In the case of thermocouples, the reference or “cold junction” is produced within the measuring instrument but, generally, requires initial calibration to assure accuracy.  Further calibration of thermocouple devices is required as changes in insulation characteristics and other factors may compromise accuracy over time.  In the case of RTD’s, the calibration is performed in the manufacture of the sensing device and is permanent.  Calibration of RTD’s is generally not required except for verification.  The way a thermistor responds to heat is determined by the mixture of semiconductor materials used in the manufacturing process and may vary somewhat from one device to another.  Small variances are of no consequence in many thermistor applications but, if they are used to measure temperature directly, proper calibration is required to produce an accurate reading.

Although there are other ways to measure temperature including non-contact infrared sensing devices, the types described in the last few blogs are those most commonly used in industrial cleaning equipment.

-  FJF  -

Temperature Sensors Continued

July 11th, 2014

In the preceding blog we looked at the expansion and contraction of liquids as a means of measuring temperature.  The expansion and contraction of solids (usually metals) is another way to measure temperature.

Most solids expand and contract in a relatively linear way in response to temperature changes.  In most cases, solids expand as they are heated and contract as they are cooled.  The amount of expansion and contraction of solids (the coefficient of thermal expansion) is typically measured in inches per inch per degree Fahrenheit (or the metric equivalent) and, for common materials, is usually less than 100 times 10 to the minus 6th inches per inch per degree of temperature change.  Because of this very small change in dimension, using a direct observation of the expansion and contraction of a solid to measure even relatively large changes in temperature is quite difficult and impractical.  Solids, obviously, will not travel up a capillary tube as described for liquids in the preceding blog.  There are, however, ways to magnify these small changes in the dimension of solids that make them practical as a means of measuring temperature changes.

The elegant solution is to use two pieces of solid material (usually metals) with differing coefficients of thermal expansion that are fused together as shown in the illustration below.

Bimetallic sensor illustration

Metals with different coefficients of thermal expansion are fused together to make temperature sensors. As one metal expands (or contracts) at a different rate than the other, the result is deformation of the sensor.

As a bimetallic sensor is heated, the two solid materials expand at different rates which causes the composite strip or disc to deform in order to equalize internal forces.  Depending on the materials used, this deformation can be thousands of times greater than the actual growth of either of the materials the bimetallic is composed of by itself.  This larger motion makes measuring temperatures by using the expansion and contraction of solids practical.

In the case of a bimetallic strip, the motion of the bimetallic can be further amplified by forming the strip into a spiral.  With one end of the spiral fixed, the tightening and relaxation of the other end can be used to measure temperature changes directly.  If the outer end of the spiral is fixed, the center of the spiral will rotate with temperature change.  This rotation can be used directly to measure changes by adding a pointer.  An everyday example of this is a simple dial thermometer which can be purchased in any hardware store.  If the inner end of the spiral is fixed, the outer end will rotate.  Thermostats used to control home heating systems often use this motion to tilt a mercury switch which controls the heat source.

Bimetallic discs deform with changes in temperature causing the center of the disk to be displaced perpendicular to the plane of the disc.  This is the principle behind the operation of common “click disc” thermostats.  Although commonly used to indicate that a target temperature has be achieved in an on/off control system, when properly designed, the disc will remain deformed even when the heat source is removed until it is mechanically returned to the initial position or “reset” making them useful in “safety” applications.  In most cases these click devices are pre-calibrated to actuate or “click” at a single temperature.  Devices using a bimetallic strip, however, are usually adjustable and can be calibrated to indicate a range of temperatures.

In general, bimetallic sensors are best used in applications requiring the measurement of the temperature of air.  Occasionally, the disc type sensors are applied to surfaces but, in fact, are measuring the temperature of the air near the surface and not the temperature of the surface itself.

Upcoming blogs will discuss the use of changes of electrical properties of materials to measure temperature.

-  FJF  -