Hi all. I wanted to design something to help us in our commercial kitchen, but don’t have the electronics background (chemical engineering background). I’ve done a little homework on what’s available out there, but I’m really in need of some expertise. I haven’t found any products out there off the shelf. Here’s what I would like to do:
The application has to do with boiling a liquid in order to reduce its volume. For instance, when making a gravy, you boil a dilute chicken broth to make it more concentrated. So anything we are boiling down will be mostly water, but will get a little thicker and maybe a little foamy as it gets boiled down. The problem is that this takes time, and it is easy to forget about it after an hour or two. I would like to make a liquid level alarm. I’m thinking of something like a meat thermometer where you stick the probe in the meat and a 3’ long wire connects to an electronic temperature indicator. In this case, I need something to measure level that connects to a simple alarm that can be set based on the desired level. I might start at 12" of level and want to alarm at 4", for instance.
Since it is water, the temperature will be very near 212F, so the level indicator will have to be able to withstand some heat, including the sauce pan and burners.
Of course I’d like the components to be as cheap as possible, but there are so many different things out there it is hard to know where to start. Any comments/direction are greatly appreciated!
I keep saying I should write a blog post on how to do Requirements. So far I got as far as the first line
What is your budget? “As cheap as possible” doesn’t cut it
2)What are you really trying to measure? Volume? Weight? Height? Viscosity? Time? Think carefully about that. If it foams up and you’re measuring height, how do you want to account for that?
How hot will it get? Boiling water is probably a good enough spec for that.
How accurate must it be?
Is anything likely to be objectively better than just setting a timer?
I’d like to keep everything under about $100. Not knowing what’s available, I’m not sure if that is realistic or not, but I’m hoping it is.
I want to measure height. If it foams up then I might need more of a weighted float than a sensor, or depending on the sensor, the foam might not be enough to interfere with the actual level reading.
Boiling water, 212F, plus a little due to other things getting concentrated as is boils. I’d have to measure gravy temp, and see how hot it can really get. That would be worst case scenario. I think specing for 212 thereabouts would be fine.
Accuracy? If I had to boil 12 of water down to 4" I’d be happy with 4.5 or 3.5".
Timers are not helpful enough. Most timers only ring for a short time and if you don’t hear it you have a problem. Also, It is a guessing game with a timer. Boiling 5 gal down to 1 gal can take a long time. I thought a level alarm would be a great help.
Maybe this is more difficult to do than I figured? How about if I have something like an open contact. For instance, I could have two rods near each other such that the boiling liquid completes the circuit. When the liquid falls below the tip of the rods the circuit will open and I would need something to ring an audible alarm when the circuit is open.
I wouldn’t think something like that would be too difficult, I just don’t know enough about electronics to figure out what to buy or build. Also, if the device is located above the sauce pan and is battery powered I could see heat problems there. I might need a wire that can run to a counter top device.
As a concept, it’s really quite simple. The devil is in the details. The sensor has to survive at temperatures above 100C, be food-safe, be removable for cleaning to avoid contaminating food, and a host of other details.
It could be done with a simple float, but a float that meets all those requirements would have to be custom made. There goes your budget!
It could be done with capacitive sensing, but the sensor would be affected by condensation and food spatter and not be very accurate.
You could use the method you proposed with stainless steel probes, but the probes need to be separated by something non-conductive. Teflon is the first thing that comes to mind that meets all the above requirements and is non-conductive. [US Plastics shows a 3/4" Teflon square bar for $38/foot. It could be machined to take two stainless steel probes. Online Metals has 1/8" stainless steel bar for under $1/foot, so maybe you’re onto something.
Now those two steel rods need to be excited by an alternating current, but nothing a simple circuit can’t handle. So there’s your level sensor. Now the problem becomes how to make it adjustable for varying liquid end heights.](PTFE Square Bar | U.S. Plastic Corp.)
I imagine something about the size of a pack of cigarettes or maybe smaller that could be clamped to the rim of the sauce pan. That would be the electronics. I could have two stainless rods extend down about 8 or 12 inches into the water to the desired height, and then the two rods could penetrate right through the cigarette box and upward maybe another 6 inches. They would be adjustable by pushing them up or down through the holes in the cigarette box. Maybe a thumb screw would be needed to clamp them in place, or just a spring type contact to maintain pressure on each rod. Not sure if that makes sense without a diagram.
I may be showing my ignorance, but aren’t battery powered devices like this typically DC, or is there something in the device that converts it to AC for the electronics to work?
I was also thinking about the heat. Instead of mounting it on the rim over the heat, why couldn’t I connect the stainless rods to basically a lamp wire, one “electrode” connected to each wire, and then connect the lamp wire to an electronic alarm sitting on the counter. This would be like an oven thermometer that has a 3’ stainless wire going right out the oven door to the digital thermometer.
So I need something that generates current for the rods and buzzes when the circuit is open (boiled down liquid). Whaddya got for that?
Thanks for looking up prices on some of that stuff. I’m willing to spend a few dollars playing around with ideas until I get something that works.
You would have to convert the DC to A/C, but that is quite trivial to do with a microcontroller or Arduino. Using DC in a conductivity sensor will cause the sensor to eventually fail due to the constant polarization.
Also, you’re correct about heat. Commercial electronics is rated to about 70C, and water vapor in the pot could go hotter than that.
Thanks, Lyndon. If I have something like a temperature sensor that runs on a 9v battery are you saying that a device like this converts to A/C internally in order for the circuit board to work?
Also, I am realizing that I don’t need any real electronics to make this work, since I am not converting anything to a temperature or level reading. I just need an audible buzzer, and on/off switch, and a way to tell the buzzer to sound if the circuit is broken (other than being turned off of course). This is like a “fail on” device. Am I getting this right?
Now that would give you what you’re looking for and more than what you need but it would be a good platform to start with.
The bigger problem is if you are making a gravy and not stirring it, it could stick and burn. Someone else mentioned foaming over and I have heard that placing a wooden cooking spoon across the pot will help break the foam and prevent overflows. Of coursse, a little oil acts as a great foambreaker (remember your Chem Eng days) but you might not want the oil in your recipe.
BTW, I too am a Chemical Engineer. But a little more versed in electronics. I started with the Radio Shack kits as a teen, broadcasting through my mom’s AM radio…
Hi Bill! Thanks for the comments and for posting that link. Honestly, I didn’t get a lot of interest in this forum other than from lyndon so I tried another forum. I did happen to get lots of feedback there, and think I am on to some good options for a simple, battery powered alarm.
All we are doing is boiling large volumes of what is really mostly water in to smaller volumes. Boiling over doesn’t happen until you have concentrated up to the point where you can call it gravy. Even then it won’t boil over much unless you add cornstarch to thicken it first. At that point if you are adding enough heat to boil over, it will boil over wooden spoon/oil or not!
FYI, you are not a little better versed in electronics than I… you are a LOT better versed! I never really got into the minutia of electronics, though I did find process control loops to be very interesting.
We used that kit on a hydroponics application. We suspect it would work for your application.
Most of the liquid controllers work using conductivity of the solution and this is no different. But it should give you a good basis and Velleman does a good job on their kits as well as explaining how it works.
Here is another approach might want to consider. The higher the concentration of a solution the lower the boiling point. Instead of measuring the liquid level measure the temperature. So, instead of measuring the change in volume you would be measuring the concentration which may actually give you a more consistent product from one batch to another.
SFE sells both a thermocouple encased in stainless steel (presumably food safe) and a thermocouple interface breakout board. The interface has 14 bit converter which should give good relative accuracy but the built in cold junction reference is only good to +/-2 degrees. For an application like this you probably need to find a way to get better absolute accuracy.
How about a magnet switch? Put a ceramic magnet, which can withstand higher temperature than a rare earth magnet, and very inert, inside pyrex glass tube. Connect the magnet to the top of a rod with a floating material at the bottom so the magnet stays out of the boiling solution. Place the magnet switch at a point where you want the alert to sound.
brianence:
Here is another approach might want to consider. The higher the concentration of a solution the lower the boiling point. Instead of measuring the liquid level measure the temperature. So, instead of measuring the change in volume you would be measuring the concentration which may actually give you a more consistent product from one batch to another.
SFE sells both a thermocouple encased in stainless steel (presumably food safe) and a thermocouple interface breakout board. The interface has 14 bit converter which should give good relative accuracy but the built in cold junction reference is only good to +/-2 degrees. For an application like this you probably need to find a way to get better absolute accuracy.
Thanks brianence. I think you’ve got the bold part backwards. The higher the concentration of a solute like salt, the lower the freezing point, and the higher the boiling point. However, my instinct tells me this would be a difficult way of measuring liquid level, especially among different kinds of solutions.
Also, @theropod, thanks for the suggestion. There are numerous level swiches similar to yours floating around the internet.
I think I came up with a pretty good solution that has no moving parts, which was one thing I was hoping to achieve.
