How To: Choosing the Right Tank Level Technology

You have a tank.  That tank is filled with a liquid (we’re sticking to liquids here).  The level or volume of liquid in that tank is important to you.  How can you tell how much you have?  There are so many options for getting this info, but not every method is appropriate for each tank or liquid type.  Below is a list of level indication/measurement technologies and some info on where to use them.    

Types of technologies

Eyeball – Your eyes work well when you can see the product, but there is no way to transmit or record that data automatically to a control or inventory system.  You can look down into a tank, see or see through a clear tank.  You can augment your eyeball with a sight glass on the side of the tank or a mag gage with a float to see indirectly.  Use this for low cost/no cost low priority inaccurate applications.

Mechanical Automatic Tape Measure – This technology connects a measuring tape to the roof of your floating cover or to a float in your tank and has a readout at eye level.  This can be read using you eyes or can be transmitted back to the control system.  Use these when you are cost conscious and need a general idea of your level and know that mechanical system will need maintenance over time.

Displacer/Float – These can be simple floats that give an idea of the top of the liquid level and send a 4-20mA or relay back to the control system.  These can also be servo-controlled displacers that are the most accurate level sensors in existence.  They are mechanical systems and will fail and require maintenance over time, so use sparingly.

Hydrostatic/Differential Pressure – This works by measuring the pressure created by the weight of the fluid above the sensor pushing down on it.  This is usually a reliable measurement and is the go-to when it works.  Your liquid needs to have a consistent density to be consistent.  If you change materials temperature changes, the densities will change, and the level will be off.  If it is an enclosed tank, you will need to us a differential sensor to compensate for the varying or non-atmospheric pressure in the top of the tank.  If that pressure varied and wasn’t measured, it would appear that the level in the tank changed.  Don’t use this when there will be build up, pressure shocks, or damaging grit in your vessel.

Bubbler – This technology uses compressed air that is regulated at a specific flow rate and to bubble up from the bottom of a tank.  The pressure it takes to overcome the weight of the fluid is measured by a pressure transmitter and gives you the level of the fluid.  Use this in applications with buildup, hazardous environments, places where you don’t want your instruments to touch, etc.

Ultrasonic – This was the first open air, top down, contactless level device.  It emits ultrasonic waves which contact the liquid and reflect to the transmitter.  The time it took for the sound to travel out and back is measured and since the speed of sound is known, the distance can be calculated.  That gives you level.  Ultrasonic is fast.  It is also fairly accurate and inexpensive.  The only downside is that it doesn’t work in on soft/loose foam, in a vacuum, or when the temperature at the sensor differs from the air above the liquid.  This can be compensated for by a separate RTD but adds to the complication and cost.  Use this technology when the level changes rapidly and the tank is vented.

Radar – This is a top down, non-contact level measuring method.  It is like ultrasonic but uses electromagnetic waves of various frequencies to reflect off the liquid surface rather than sound waves.  This allows for measurement in pressurized and vacuum tanks, but there are other considerations.  Radar reflects off liquids more or less strongly depending on the dielectric (dk) constant of the liquid.  If the dk is too low (~<2.0) the radar will pass through the liquid and just measure the bottom of the tank.  Radar is slow compared to ultrasonic or hydrostatic pressure, so don’t use in rapid changing processes.  Steps also need to be taken to deal with condensation in the radar horn.  Radars are a good all-around choice for level.  My personal favorite after hydrostatic pressure. 

Guided-wave Radar – These are a modification of an open-air radar.  Instead of sending radar through the air like a flashlight, you send it down a rod or cable.  This has the benefit of concentrating the radar in a small area.  This allows for measuring of lower dk fluids (~>1.2).  The drawback to guided wave is that is in contact with the fluid and therefore is susceptible to buildup and interferes with cleaning systems and agitators. 

Capacitance – This was one of the first electronic level technologies.  It uses a change in capacitance to infer the level of liquid in a tank.  This is a contact measurement where the transmitter is one plate of a capacitor, the tank is the second plate, and the liquid or air between them is the dielectric.  By varying the dielectric, the capacitance changes.  The ratio of liquid and air contacting the sensor rod is proportional to the measured capacitance.  These are fast responding, work on many fluid types, and can even detect interface level.  They do have the downside of being in contact with the process and possibly getting buildup.  They also must extend the entire distance to be measured, so are a poor choice for tall tanks. 

Load Cells – These are strain gages mounted to a frame that “weigh” the tank.  They need to be arranged so that tank is the only weight they see and that there is no side loading.  This means the tank must be isolated from the process with expansion joints and loops and that vibration needs to be minimized.  Since they don’t touch the process, they can be used on just about anything.  These are just more expensive and not always doable because of the need for isolation or the tank is just too big.

Nuclear Absorbance – This technology works on everything, but you don’t use it unless you must.  It consists of a gamma radiation emitting source mounted to one side of the tank and a long sensor (scintillating tube) mounted to the other.  The amount of radiation that passes through the tank and liquid varies with the amount of liquid in the way.  This gives you the ability to “see” the level.  The downside to this is that you will now have radiation on your site and must report to the NRC and have a nuclear safety officer.  You will also have to pay for wipe tests and other safety related paperwork which raises the cost and complication.  Only use these where you must.


There are many, many choices when it comes to determining your tank level and using that information to control your process.  Take the initiative and call me or submit a contact form if you have level applications and questions.  We can analyze your process and determine the correct technology for your specific case.  We can help you out and we love the adventure of solving the problem. 

“Why” Is The Most Interesting Question

Every day we ask and answer a multitude of questions.  They all have one of the W’s at their core (Who, What, Why, Where, and When).  My kids know which one of these is the most interesting and valuable.  WHY.  Why this?  Why that? Why? Why? Why?  As annoying as it is to answer a 10x string of “Why Daddy?” I understand the need to know. 

You can’t plan and organize your life if you don’t understand why things happen.  Why people react the way they do.  Why systems work and why they might fail.  In order to thrive and understand their world, kids need to know why things are the way they are.  We can translate this to ourselves personally and to the process industry.  Below are some of the important “Why” questions that I ask myself and the associated answers (as of today.  Always pending change.)  Maybe you will have different answers or better questions.  If you do, please add them to the comments. 

Why seek outside council?

I fancy myself a Jack of All Trades – Master of Some.  We are all good at some things and not others.  There is no way to become proficient at absolutely every skill and well versed in every discipline.  If you can learn to accept that your time is better spent on the things you are truly good at or truly enjoy, you will come to the conclusion that delegating that expertise to others is a good thing.  That way everyone can be as productive as possible and things run smoothly.  When designing a plant or process, seek the help of those who work on the component systems regularly and ask them for advice.  For that matter, ask your operators and maintenance people how they would like to interact and maintain the systems before implementing changes.  An operator that has been running a portion of the plant for 20 years is an expert on running the plant.  Even if changes are necessary, their input is valuable and will help avoid pitfalls later. 

Why work to optimize a process?

Sometimes good enough is good enough.  Sometimes it is not.  It takes time to examine a system and determine how efficient it is.  Once you know where you are currently, you can evaluate whether or not you could improve.  If you can improve, how much time and money will it take to get where you want?  If your time and money investment will pay for itself and more, then go for it!  Improve that process!

Why is this not working?

This is one of my favorites.  Whether debugging code, fixing the car, or figuring out why my process can no longer maintain it’s temperature setpoint identifying a problem and taking the steps to troubleshoot is so much fun.  Being able to logically step through a process and get the answer is very gratifying.  Getting the answer to this “Why” is the best.

Why did this fail?

This one is similar to the above, but I see this as a root cause analysis rather than a system fix.  You may fix the problem over and over, but it keeps breaking.  That is a waste of time and resources.  Why not determine the root cause of the failure and fix that and prevent the failures from reoccurring?  The answers to these “Whys” can have massive paybacks.

Why ask why?

I can’t help myself.

Why call me?

Call us because we can help.  We love process control.  We are experts in instrumentation, valves, analytical, and heat trace.  You’re good at running your plant and we’re good at specifying and designing instrument and control systems.  Leave a comment below if you made it this far.

How To: Controlling Steam

Almost every process facility requires steam. It is used for heating, drying, power, etc. Steam is an excellent way to convey energy from one process to another. The heat and energy of the steam needs to be measured and controlled into the next stage of the process.

Steam flow and pressure is controlled with an orifice.  You can use a fixed orifice, a line sensed pressure regulator, or a full control valve. There is no adjustment with an orifice. A regulator is given a fixed output setpoint, but is not adjustable by the control system.  A control valve can vary the orifice size according to the output of the control system which has measured the flow, pressure, and/or temperature from process instrumentation and sensors.

When controlling system, it is always done with an orifice which creates a pressure drop. The higher the flow rate and pressure drop, the noisier the valve will be. There are two problems with noise. First is the loud environment and potential damage to your hearing. The second is that noise is vibration and that vibration can damage the physical components of the valve. 85dB or lower is the ideal noise level. You can avoid high noise by taking the pressure down in small increments rather than one big drop. This can be accomplished within a control valve or through a fixed drop low decibel cassette inline with the control valve. 

There is a way to make almost every steam control application low noise and highly reliable. You must select the proper method of controlling and regulating the steam with the proper number of pressure drops. For help with your next steam application, please contact us.

Methods of Measuring Solids Flow

If you are like me, you started your adventure into process measurement with measuring liquid and gas flows, temperatures, and pressures.  I got very good at designing systems using the proper instrumentation to measure and control these.  However, I didn’t pay much attention to solids measurement.  This has since changed.  I work with solids control in a large percentage of my projects now.  I have come to realize that the expertise gained in liquid control does not always directly translate to solids measurement. 

There are 5 main ways to measure solids flow.  They are as follows:

  • Belt speed measurement
  • Conveyor motor amp draw
  • Belt Scale Conveyor
  • Impact Mass Flow
  • Radiometric Absorption Conveyor

Belt speed measurement is the simplest method, but it is also the most inaccurate.  This method just measures the linear speed of your conveyor or screw and estimates the mass of the dry product it conveys per minute.  Sometimes this is ok, but it assumes a steady feed rate and steady product density.  This method shouldn’t be used for control or inventory tracking.

Conveyor motor amp draw measurement assumes that increased amps is an increase in mass being conveyed.  This is a better idea than purely monitoring the conveyor’s speed, but it too has massive drawbacks.  It assumes that the conveyor itself is steady state mechanically.  Does your conveyor have a consistent friction at all speeds and temperatures?  Probably not.  Do mechanical systems wear over time causing “calibration” drift?  Probably.  Does the amp increase vary linearly with mass increase?  Probably not.  I personally don’t like these systems as they don’t work very well and give a false sense of having measured something well, when in fact you have not.

Belt scale conveyors are the gold standard in mass flow accuracy.  They consist of load cells on independently supported rollers under a conveyor belt. There can be from one to 4 of these roller/load cells in series.  As solid product rolls over these load cells, it is weighed.  This weight and an output from a speed sensor combine to give you an extremely accurate mass flow.  These systems, depending on the number of load cells, can give a mass flow reading of up to +/-0.25%.  These systems aren’t very expensive in the scheme of things, but they do require a lot of straight space in your belt to install and work properly.  They also rely on moving parts and are exposed to the dust from the solids which will require maintenance and periodic recalibration with provided reference weights.  Overall, these are the most accurate systems you can get.

Impact mass flowmeters are the next most accurate solids flow measurement device.  These require that product be lifted above this “box” and dropped through it down to a bin or secondary conveyor.  This “box” consists of a top entry chute and a bottom outlet chute.  In the middle of this box is a angled plate attached to a load cell. As solids fall into the box, they strike the load cell plate and then fall off and out of the box.  The load cell sensor assembly is sensitive enough to track the rate and mass of the solids going through it and gives you a reliable +/-0.75 to 1.00 percent accuracy spec.  These require less maintenance than the belt scales, but still need cleaned periodically as the sensor is exposed to dust and the plate is open to wear from the impact of potentially abrasive solids.  These are a great option if you have the vertical space to fit it into your process.

Lastly is relatively unknown method of radiometric absorption.  This method uses a radio isotope to emit gamma rays at a known and regular rate.  The conveyor and the material being conveyed absorb these gamma rays.  The conveyor never changes density, so it absorbs the same amount of gamma all the time.  The change in absorption over time is therefore directly related to the change in product mass flow.  The levels of gamma rays emitted are so low as to make these safe for install in the field. In fact, (only with Ronan) no special certificate or government regulation applies to most of their installs.  These systems do not make contact with the product and are installed outside of the conveyor.  This means they require no maintenance as dust and abrasion are not a factor.  They have a pretty good accuracy rating of +/-1-2% in most applications. 

Now that you know the main ways to measure solids mass flow, it’s time to include this in your plant’s mass balance calculations and inventory controls if you haven’t already.  If you have questions about how to implement this in your process or how to correct an install this isn’t living up to your expectations please contact us.

How to mount your differential pressure transmitter in a DP flow application

I have had several questions come up in the last couple of days about where to mount differential pressure transmitters in relation to the pipe mounted flow element. There are three different types of fluids that you will want to measure and they require different mounting methods.

First, is dry gases. The DP transmitter must be mounted vertically above the pipe taps so that all potential liquid or condensation in the line will drain back into the pipe.

Second is liquids. The DP will be mounted vertically below the pipe so that the line will always stay full with liquid.

The third is condensing liquids such as steam. These require that the DP transmitter is mounted below the pipe taps, but they must have a vertical water column that is always filled with the same amount of condensate so that the water pressure on the impulse lines remains consistent. The water column will block the high-temperature steam from cooking your transmitter. If you mount it above the pipe and there is no water present, the 300F+ heat will break your transmitter.

This was a very short and basic introduction to mounting and configuring your DP flow system. For more info and a thorough explanation, please contact me via this site.

The Unplanned Outage

Oh Crap! All hands on deck! You’re in another unplanned outage and you are working 16 hour days trying to get the plant up and running again. This time it was a broken agitator shaft, but you’ve dealt with other equipment failures and operator error in the past. Regardless of what caused the outage, you are stuck working to fix it and get back up. Outages are a pain and they are expensive. You have to pay your people overtime to get the problem fixed. You have to spend big dollars to replace/repair the broken equipment. You are also not making any product and therefore not making any money. It’s a trifecta of terrible. What if we could take steps to minimize unplanned downtime? Would you do these steps or at least some of them? I think it is worth it to find the time when things are running well to prevent things from going badly.

First, you must figure out the potential failure points in the plant. On a grand scale it could be three things. Operator error, loss of utilities, or equipment failure (there could be more, but for the sake of this article, let’s go with those three).

In regards to operator error, can you predict what an operator could do to damage or shut down the process? If you can, then implement control system or SOP fixes to prevent them from being able to that damaging activity. Noone wants to shut down a plant, but accidents can happen. Take steps in the software to prevent those accidents.

Regarding the loss of utilities, I don’t know that there is much you can do. You could keep a spare transformer or other large utility items as spares, but these events are relatively rare and it would be really hard to predict which component (yours or the utilities) would fail.

Lastly you have equipment failure. This is preventable or at least predictable. We all do preventative maintenance. Do you do predictive maintenance? Changing oil in gear boxes and replacing consumable parts on a regular basis prevents unplanned down time, but what if there is something that doesn’t get these PMs that fails? What if you could predict this failure? instrumentation has a lot of diagnostic information ready to be used already. You just have to look and have I/O capable to reading and logging that data. You could also use a smart cloud service (like Siemens) to analyze that data for you. There are also SIL (Safety Integrity Level) devices and equipment. These have proven and tested and predictable mean time to failures and can be used to ensure the safe and reliable function of your process. Keeping spare parts and complete assemblies for all your critical components is worth the inventory cost, espessially if you will lose enough paying idle people, not making product, and paying expedite fees to ship an emergency part in from somewhere else.

So, while you are stuck in this unplanned shutdown, use this as an opportunity to make some changes to your inventory, SOPs, and data analysis to prevent the next one. If I could do a little work and spend a little money now to prevent spending a lot of money and doing a lot of work later, I would.

If you like this article (or not) leave a comment and let me know what you think. I will try to cator future articles to the interest of my readers.

Hello Process World!

Welcome to my new site dedicated to my writings about the process industry and how we measure and control our processes. Stay tuned for regular posts and articles mined from my experiences and those of my customers and colleagues. Please let me know how well these are received and I promise I’ll try to cater my posts to the interests of my viewers (usually).