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 jfuchs@ctgclean.com.

Sound Physics – Nodes and Antinodes – Part II

December 10th, 2014

When a sound wave encounters an object or discontinuity in the sound conducting medium, at least part of the sound is reflected.  The way the sound reflects depends in large part on the rigidity of the reflecting surface.  If the object is hard, like a substantial piece of metal, the wave reflects as shown below.

When a sound wave reflects from a hard surface, the phase of the reflected wave is reversed in phase.

When a sound wave reflects from a hard surface, the phase of the reflected wave is reversed in phase.

The hardness of the material prevents displacement.  Notice that the surface does not yield to the arrival of the compression of the sound wave.  As a result, there is a pressure generated at the interface in lieu of displacement.  This results in a displacement node (no displacement) at the interface.  When the sound wave encounters a surface that is softer, like a piece of plastic or rubber, the softer material allows displacement at the interface and the sound wave reflects as shown below.

When a sound wave reflects from a soft surface, the phase of the reflected wave is the same as the incoming wave.

When a sound wave reflects from a soft surface, the phase of the reflected wave is the same as the incoming wave.

Notice that in this case, the surface yields to the arrival of the compression of the sound wave.  As a result, there is displacement generated at the interface in lieu of pressure.  This results in a pressure node (no change in pressure) at the interface.  This is very much like the discussion of resonance of a tube with and open end vs. a tube with a closed end that appeared in an earlier blog.

So, what does this mean when it comes to cleaning?  In the case of a hard surface, because of the reverse phase reflection, the sound wave reflecting on itself looks like the following.

When a sound wave is reflected from a hard surface, a pressure anti-node (maximum pressure deviation) is formed at the reflecting surface.

When a sound wave is reflected from a hard surface, a pressure anti-node (maximum pressure deviation) is formed at the reflecting surface.  Click on animation to start in a new window.

The formation of a displacement node where the sound wave intersects the reflecting surface results in a pressure anti-node (maximum pressure variation). Since cavitation and implosion are a result of changing pressure, the pressure anti-node at the interface provides an ideal site for cavitation and implosion at the surface for ultrasonic cleaning.  When a sound wave reflects from a soft surface the result is as shown below.

When a sound wave reflects from a soft surface, a pressure anti-node (relatively constant pressure) is created at the interface.

When a sound wave reflects from a soft surface, a pressure anti-node (relatively constant pressure) is created at the reflecting surface.  Click on the animation to re-start in a new window.

In this case, a pressure node (area of minimum pressure variation) forms at the surface and, as a result, cavitation and implosion do not occur adjacent to the surface.  In essence, a large portion of the sound energy is absorbed as the surface yields to the incoming compression with the reflected portion forming cavitation bubbles at some distance from the surface where  they have minimal benefit to the cleaning process.

Although the models shown above are very simplistic, they illustrate the physics behind the variations in cleanability of surfaces we observe that can be associated with their physical properties.

-  FJF  -

 

 

 

 

Sound Physics – Nodes and Antinodes – Part I

November 17th, 2014

I’ve made reference in the blog before to the fact that some surfaces are good candidates for ultrasonic cleaning while others are either difficult or impossible to clean.  In general, surfaces that are hard (metal, glass) are easily cleaned using ultrasonics while softer surfaces (rubber, soft plastic) resist ultrasonic cleaning.  The reason has to do with the physics of sound (are you surprised?).  In this series of several blogs, I will first introduce some basic physics and then expand on this knowledge to suggest ways achieve better ultrasonic cleaning results based on the knowledge of the physics of sound.

To visualize sound waves, we often use a graphic representation similar to the one below.

Simple Sound Diagram

As a sound wave travels through an elastic media it consists of a traveling series of compressions and rarefactions.  It is common to interpret the Y axis in the above diagram to indicate areas of negative and positive pressure.  However, the thing responsible for the variations in pressure is the inertia of the media.  Where there is pressure, it is because the media is compressed due to the fact that, because it is elastic, the transmission of motion from one molecule or atom to the next is slightly delayed as the inertia of the particle receiving the motion is overcome.

Inertia is the resistance of any physical object to any change in its state of motion, including changes to its speed and direction.

The motion of atoms of molecules of the media, then, is in response to pressure but it is slightly delayed by inertia.  As forward motion takes place, pressure is relieved.  This may be better understood graphically by studying the following illustration which was published in an earlier blog.

The above illustration shows how sound waves are transmitted from particle to particle in matter through spring-like atomic or molecular forces.

The above illustration shows how sound waves are transmitted from particle to particle in matter through spring-like atomic or molecular forces.

The negative pressure portion of the sound wave is a result of the inertia of motion in the medium in the direction opposite to the direction that the wave is traveling.

When traveling through a boundless media, sound waves will continue to propagate until they finally decay into heat as a result of the internal friction of the sound conducting medium.   In liquids, cavitation bubbles will form and implode throughout the liquid as long as there is sufficient energy left in the sound wave.  However, this all changes when a sound wave encounters a discontinuity in its path in the form, for example, of a tank wall or a part being ultrasonically cleaned.

When a sound wave encounters a discontinuity there are several possible outcomes – reflection, absorption and/or transmission depending on the physical nature and geometry of the discontinuity.

Note – In the upcoming blogs, I will be using examples chosen to illustrate principles of the physics of sound relevant to ultrasonic cleaning.  My apologies to the physicists and physics “purists” out there as I try to bring this complex topic to a level that I can understand and share with the readers of this blog.

-  FJF  -

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  -