OK I replaced the battery and it looks a little better, but I think I have some resistors floating around in the basement somewhere so I'll need another day to get good readings, hopefully.

 

I think I have some resistors floating around in the basement somewhere

Sounds like you need another water-level sensor down there, then! .

 

@alec: LOL. Good one. Actually it's not as funny as I first thought. About a month ago somebody messed with the spigot in the back yard. They turned the water on 100% and it ran to ground next to the foundation for about 5 hours. We started getting flooding in the basement because the level float on the older sump failed and the new sump was clogged and the back up battery system kicked in...until the battery went dead. Overall we didn't lose anything, but we'll never know who opened that spigot.

 

That's really poor. Get yourself a free meter from Harbor Freight if/when you can.

Free meter? I was surprised to find them online for about $5. Maybe my wires are corrupted. I paid a lot more for this meter but it seems useless for ohm readings. I'll try to get a new one and take some new readings. I did have some old resistors at home, but the readings were inconsistent. They would all drift upwards slowly, even when I crossed the leads. I'll get back with new readings.

Thanks.

 

Yeah, it could just be your batteries as noted.

The HF meter is free with a coupon, usually free with any other purchase. I get about 2 such coupons every week, so I am never out of one. But the meter is worth $5, and I wouldn't hesitate to pay that if I needed it.

 

OK, so I receive the multimeter from Harbor Freight yesterday and today I tested some old resistors I had sitting around. With wires crossed I was getting 1.2 ohms. The resistors I tested checked out perfectly with the new meter. However, I have a new mystery. I can't get ANY reading other than a 1 on any liquid I try! I put the leads 1/2" apart in tap water and get a 1. I add a ton of salt and still get a 1. I change the range from 200 ohm to 2000 kohms and still get a 1. Is it possible that it just can't read resistance properly in liquid? The multimeter came with a battery in it, and the resistors I tested, between 4 and 18 ohm, all checked out perfectly. Am I missing something?

 

A "1" indicates the value is off scale. You need to use a higher range. Resistance in liquid depends on many factors including surface area. You might try using a screw or bolt to immerse as this would give a big increase in surface.

 

A "1" indicates the value is off scale. You need to use a higher range. Resistance in liquid depends on many factors including surface area. You might try using a screw or bolt to immerse as this would give a big increase in surface.

I used every range from 200 to 2000 Kohm and still got 1. I'll try your idea about increasing the size of the leads with screws or similar.

 

Hi folks This looks like a great forum! I've been reading some background material and trying to see if the answer to my question is in the forum, but I haven't found anything. Let me say briefly that I "used to be" a chemical engineer, so I have some technical background, but it was CHEMICAL. I'm pretty lost when it comes to electronics. I understand basic generation a/c and d/c, circuits in school 30 years ago, etc. What I know about circuit boards would fill 1/2 a paragraph, lol.

I am trying to design what I think is a simple device to sound an audible alarm when boiling water in a pot reaches a low level. I wanted to achieve this without a mechanical float, and I found a circuit that seems to fit the bill, sort of. Below is a copy of the circuit, but the designer at circuitsgallery.com doesn't seem interested in helping people figure out why it doesn't often work. The principle, as I understand it, is that a probe is inserted into the water do the desired depth. This provides a ground and keeps a transistor closed (learned about how transistors work last night, lol). When the water drops below that level, the voltage in the line opens the transistor and causes the alarm.

I have a number questions, but I'd like to ask just a couple to get started (don't want to scare anybody off with 1000 questions about something so simple).

1. Would you consider this generally a good design, and one that could be modified to work for my specific situation. For instance, if I want to use an 18" long stainless rod as the probe, I imagine I might have to change some of the components in the circuit in order to make it work.

2. Can this be made to work using a battery rather than a/c power supply? I would like to make it portable and to use as little battery power as possible.

Hopefully I've described things adequately. Any help, or direction if this is the wrong forum would be greatly appreciated!

Dan

I don't know the 'pot' however use the weight of the pot. water out weight goes down.
measure, at the same time, the 'pot' temperature above 100 deg celcius pot is near to dry or dry. ( steam can go up to 200++ deg Celsius)
picbuster

 

I don't know the 'pot' however use the weight of the pot. water out weight goes down.
measure, at the same time, the 'pot' temperature above 100 deg celcius pot is near to dry or dry. ( steam can go up to 200++ deg Celsius)
picbuster

Steam temp will be the same as water temp unless it is superheated, which it isn't just coming out of the pot. I'm looking for a simple way to alarm on low level, and using a probe in water with no moving parts seemed like something to try first.

Thanks for the comments.

 

wayneh - are these probes supposed to be able to measure resistance of liquids? I find that if I use the largest range, 2000 kohm, I get a reading only if I hold two washers against the probes. The problem is that when I remove the washers from the water, I still get a reading! I think at such a high resistance range, it is measuring resistance across my body as I am holding the washer onto the probe with my fingers.

I'm at a bit of a loss measuring the water's resistance. Even salted water seems to be more resistance than the probe can measure. How necessary is this measurement to designing a circuit? It seems like there would be a significant difference between a milliamp current going into water vs no water. Of course thinking about it I guess that's what the ohmmeter is doing, right?

 

Hi folks This looks like a great forum! I've been reading some background material and trying to see if the answer to my question is in the forum, but I haven't found anything. Let me say briefly that I "used to be" a chemical engineer, so I have some technical background, but it was CHEMICAL. I'm pretty lost when it comes to electronics. I understand basic generation a/c and d/c, circuits in school 30 years ago, etc. What I know about circuit boards would fill 1/2 a paragraph, lol.

I am trying to design what I think is a simple device to sound an audible alarm when boiling water in a pot reaches a low level. I wanted to achieve this without a mechanical float, and I found a circuit that seems to fit the bill, sort of. Below is a copy of the circuit, but the designer at circuitsgallery.com doesn't seem interested in helping people figure out why it doesn't often work. The principle, as I understand it, is that a probe is inserted into the water do the desired depth. This provides a ground and keeps a transistor closed (learned about how transistors work last night, lol). When the water drops below that level, the voltage in the line opens the transistor and causes the alarm.

I have a number questions, but I'd like to ask just a couple to get started (don't want to scare anybody off with 1000 questions about something so simple).

1. Would you consider this generally a good design, and one that could be modified to work for my specific situation. For instance, if I want to use an 18" long stainless rod as the probe, I imagine I might have to change some of the components in the circuit in order to make it work.

2. Can this be made to work using a battery rather than a/c power supply? I would like to make it portable and to use as little battery power as possible.

Hopefully I've described things adequately. Any help, or direction if this is the wrong forum would be greatly appreciated!

Dan

Boiling water makes a sensor easier, cool water leaves a film that continues a leakage path that your sensor must differentiate from actual immersion. With boiling water, you only have water vapour when the level drops.

In normal applications, you should energise the probes with AC to prevent electrolysis. No idea whether that also applies to boiling water, but if you're boiling a chemical solution, electrolysis will probably be much worse.

If you use an isolated battery supply for the sensor; it can be as simple as a CMOS square wave generator that's floating from any ground reference. A capacitor coupling in the sense current path forms part of a charge pump circuit which generates the voltage you sense.

There are various designs online, but its basically as simple as tying a 1/2 VCC(VDD) reference to the container so the squarewave excursion is negative as well as positive.

 

Maybe helpful?
http://www.octiva.net/projects/ppm/

 

Hey, I just did about 1 cup of water in a glass and used a cheap meter an got very inconsistent results.
The sharp probes were about 1 cm apart. The surface area of the probes were asymmetrical.

I then changed to an expensive Keithley 195 DVM and got consistent results.

About 70K in water and 15K with a few shakes of a salt shaker.

The specs of the 195: http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&ved=0ahUKEwj8ufrInefNAhXCZj4KHQViCNkQFgheMAg&url=http://www.testequipmenthq.com/datasheets/KEITHLEY-195A-Datasheet.pdf&usg=AFQjCNEFHZB2UHw4lMdZ5U8G8YG61chDZQ&sig2=cBQ199yhXrfpX3BtVnwDvQ&cad=rja

Pure water isn;t very conductive, I think the DI water system ran somewhere between 15 and 18 Mohm-cm. Again, I think the same R=pL/A formula applies. p=resistivity in ohm-cm.

http://www.labmanager.com/news/2010...rement-of-purified-water?fw1pk=2#.V4FiCJH0Fok

These guys use weird units of S/m for Conductivity instead of resistivity of some foods.

I could, but I don't know if I have time to do it though, use an autoranging Picoammeter and a voltage source and I should be able to get similar results.

An OP-amp I-V converter isn't too hard to build.

 

Just thinking out load a bit. Concentrating would mean boiling without a lid. That to me means clamping something to the side of the pot. The clamp might be died to the diameter. e.g. Two moon crescents on fixed, one variable. So, you do have a "pot diameter variable".

You do have a high temperature humidity environment, so you have a IP (Ingress Protection) to consider. Enclosures have IP ratings.

The resistance read basically depends on the area of the exposed electrodes. While one can include the POT, I'm not sure that;s practical.

My xtal ball sees a protective cage around the electrodes. This would prevent things like pieces of meat from bridging the electrodes, but I;m not sure how important that is. If anything, it could prevent accidental bending. Screen conductive or non-conductive or holes in a stainless tube.

Proof of concept can ignore the AC excitation.

So for the most part R=pL/A. L is the distance between the electrodes. R is the resistance, and A is the cross-sectional area. or a wire, this is easy to visualize. p (Rho) is a material property. It can be in units of conductance/area or ohms-area since 1/R=S(Siemens).

Although I've built I-V converters, I didn't pay much attention to be able to have zero output for zero input. In one case I used a 400 M-ohm resistor.

It's easy to bias an I-V converter. Just apply a voltage to the non-inverting input. The transfer function becomes Vout=Vb-Irf

An electrometer OP amp may work, or it may be overkill. It may need a matched FET differential pair which is hard to come by.

You don't care if it overloads. Nothing bad happens.

So, it may be easy to apply say 2V and the the output would decrease to ~0 when the resistance lowers.

Possibly add a low battery indicator/test function.

==

One idea might be to use and RF transmitter or an IR LED and transmit an IR signal to a nearby receiver.
In the short term it could just detect IR. In the longterm, the code could be modified for each sensor and it could have alarm silence controls and which sensor etc.

So, in this case, the alarm becomes powered by the wall and only the sensor is battery powered.

Linear's DXS transmitters and a commercial receiver might also be used. I planned to use one of their pendents as a doorbell. Their pendents are totally sealed and when the battery dies you need a new pendant. They also transmit low battery alarms.

I have the 8 channel receiver: https://www.surveillance-video.com/transmission-ssr00070.html/ (this is a 4 channel one) and a medical alarm PERS-2400) that uses the transmitters. e.g. https://www.surveillance-video.com/transmission-sst00124.html/

 

Thanks for the various replies. I don't understand half of it, though. (or 3/4 of it).

@keepitsimple: The original idea was to send a small current down the wire into the liquid that acts as a ground. There is just one wire or probe, not two. When the liquid level falls below the probe, resistance goes up and the alarm sounds. I have been thinking about various versions such as wireless (probably too expensive). My other thought was to use about a 3 foot long coated wire that exposes only the tip to the liquid and runs out of the pot and over to a battery powered alarm where the electronics would be housed. This would be like an oven thermometer. I suppose the ideal would be to have everything small and clipped above the pot. It could be insulated to protect from heat and humidity.

However, I think all the above is beyond the initial problem for me, which is to figure out how to make the single probe idea work. Some here suggested measuring the resistance of the liquid in order to calibrate the circuit (I think) but I have found out that you can't measure ohms in a liquid with a cheap ammeter. Oh well, I guess you can never have too many ammeters.

So getting back to the initial problem, what can I build to start testing out the feasibility of a single probe alarm? Do I really need to know the resistance of the liquid to accomplish this? If so, I might have to rely on other people who have done such measurements and assume I'm not too far off...

 

Look here: http://electronicdesign.com/analog/whats-all-transimpedance-amplifier-stuff-anyhow-part-1

You, sort of, want to build what;s in figure #1for a "proof of concept", but things always get complicated. If you put a buffered voltage source (Vb) at the (+) input, the transfer function changes to Vout=Vb-I*Rf. (Note the - sign).

If you don't zero the amplifier or have a place for the bias current to go, the amp will saturate. The Keithley 480 (which I have) http://www.tunl.duke.edu/documents/public/electronics/Keithley/keithley_485_picoammeter_manual.pdf is a manually ranged picoammeter.
The theory of operation might be worth reading.

There would have to be some techniques to use a single supply. I built one using two 9v batteries to measure a light source. Actually plugged the output into a TRMS (True RMS) AC meter. for a work project.

I also worked on one with bias and I built a very sophisticated one that was 4-terminal.

The OP amp parameters that are important are:
1) Ib - this can be in the order of Pa (1e-12) or Femtoamperes (1e-15 A) even.
2) Vos - The offset voltage which is a strong function of temperature.

There's a special class of amplifiers known as an-auto-zero amplifier that you might be able to use, but not sure.

I picked an OP amp in one case that had uV of offset and I think I used a 400 Meg-ohm resistor somewhere (I think in the measuring circuit) and I only had only had 40 pA of offset current. That very large resistor meant there was a place for Ib to go, otherwise the amplifier will saturate with the input disconnected.

So, let;s say Vb was 1V and you had a comparitor that looked for <0.5V. Your feedback resistor can be just about anything that could measure Those offset voltages and currents are really low.<100K. So anything around R>100K/1V or 100K. In the case of Ib and Vos, you need some
R < Vos/Ib; e.g. 50 uV/40 pA or 1e14 ohms, but those offset voltages are really low.

You also have the low-power aspect to deal with and possibly a low battery indicator.

There are expensive Electrometer grade amplifiers that have both parameters low, but they are EXPENSIVE.

The "problem" with this design is that you can't use a DC voltage in the long term. It will cause plating. What plates out is probably dependent on polarity and who knows, it may plate to the pot and not the electrode.

So, later you add square wave excitation and a low pass filter and your probably there.

Electronic design means sprinkling bypass caps where they belong etc.

You should keep the distance from the edge of the pot and the electrode consistant.

The feedback resistor has to be "high enough" to not cause saturation when not immersed. "Saturation" being defined as close to the negative rail which could be ground.

Another way to implement battery test is to place a fixed resistor in circuit.

 

@keepitsimple - Thanks for the ideas, but your post is not simple and stupid enough for me. I'm struggling with limited knowledge to keep up. What do you think of analog kid's suggestions in posts 12 and 15? If I can run a circuit on very low DC current, what is wrong with his scenario?

I thought I was on my way to getting a good design from analog kid to try when the suggestion of checking the liquid resistance was made. I waited a week for a new ammeter and then found that cheap meters can't measure liquid resistance. So now I'm wondering how important is that reading, anyway? We know the conductivity of anything I am going to boil is very high, with low resistance. Can we make some simple assumptions about the difference between immersion and the "dry" probe and design something that can be tweaked based on actual use?

 

I think if I was doing this I'd look at an IR emitter-detector across a gap. You would want a wavelength that is absorbed by water so that you would see a big change when the liquid level drops below the gap.

A vibrating object is another possibility, like an ultrasonic membrane or a piezoelectric disk, or even a tuning fork. I think it would be easy to detect air versus water but I've never actually seen such a thing.

 

The idea of a low-pass filter on it's own won't work, You would have to precision rectify first, then a low pass filter. Frequency only matters somewhat. You would want to think that 5 time constants (RC) the signal is at 90% it's final value. Lowering the duty cycle could lower power consumption.