Looking for a water-sensing power shutdown system

Discussion in 'The Projects Forum' started by amainiero, Nov 1, 2017.

  1. amainiero

    Thread Starter New Member

    Nov 1, 2017
    3
    0
    Hello people of the forum,

    I’m currently working on a project which has need for a unit which can sense the water level within a container using a probe (this will be high temperature upwards of 250 degrees f). When the water level drops below the sensor, I’d like the system to either cut power until the fluid is back above the sensor, or shutdown permanently until reset. I’ve looked around for a while for a unit like this, but all the water sensors seem to be made of cheap plastic rather than metal probes.

    Do any of you know of a device which can do this?
     
  2. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    7,648
    1,867
    Given that water boils at 212°F, for the temperature to be 250°F the pressure must be about twice atmospheric pressure. That's got to be taken into account to. Could you use a float switch?
     
  3. Reloadron

    Distinguished Member

    Jan 15, 2015
    4,196
    2,030
    I can tell you how I did it and a popular method. We used a differential pressure gauge to measure water levels of grade A water in a tank. This was water at about 600 degrees F with a 3,000 PSI nitrogen blanket applied. In your case 250 degrees F is not that bad to work with as water boils at about 212 F at sea level where atmospheric pressure is about 14.7 PSIA. Per Albert your saturated steam pressure at 250 F will be about 30 PSIG. You choose a differential pressure gauge with a housing max pressure exceeding the maximum pressure you expect to see. In your case 50 PSIG would be fine. You drill and tap the bottom and top of the tank. The difference between the top pressure and bottom pressure represents how many inches of water are in the tank. 1 PSIG = 27 Inches of water column. You can buy differential pressure gauges made to measure inches of WC (Water Column). This is how I have done it in the past with pressurized tanks.

    A Google of "differential pressure water level sensor" will give you dozens of ideas and pictures.

    Ron
     
    AlbertHall likes this.
  4. Externet

    AAC Fanatic!

    Nov 29, 2005
    1,340
    169
  5. amainiero

    Thread Starter New Member

    Nov 1, 2017
    3
    0
    The float switch sounds like a viable solution as far as automated shutoff goes. My issue is I know quite little about circuitry (young student in college, haven’t gotten to those classes yet).
    I’m not sure how I would use a float switch in order to disable power going to my heating element. It’s 1000w, 120v ac; most floats seem to be 10w dc at 120v. What exactly would that entail?
     
  6. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    7,648
    1,867
    I like that!
     
  7. Externet

    AAC Fanatic!

    Nov 29, 2005
    1,340
    169
    I do not find a pressure differential measurement to be a good sensing in a pressurized vessel, the differential may be just so many inches of water. If the vessel pressure is many hundreds of inches of water.
    If there is 20 psi in the vessel, that is like 555 inches of water. If the water level is -say 12 inches-, it would not be a significantly different reading 555 and 543 for reliable detection.
     
  8. Reloadron

    Distinguished Member

    Jan 15, 2015
    4,196
    2,030
    If a float switch is practical the low power switch would in turn drive a relay. A 1 KW heater at 120 VAC is only about 8.3 amps you are switching and a heater element is a resistive load. You could have the float switch drive either a mechanical relay or a SSR (Solid State Relay) using either a low control voltage like 12 or 24 VDC on a mechanical relay or 3 to 32 volts on a SSR. I am guessing ON/Off control and not a more elaborate temperature control. This would go better if you can describe your project in some detail. Sometimes we want to shut off heaters if a tank water level gets too low. Sometimes when a tank level is low we want to add water to the tank. This is where you draw out in even block diagram what you want to happen, logically. Anyway, if a simple float switch will fit and can be designed in then that switch can drive a larger switch to handle your heater current and allow a margin. Also consider your budget. The trick is to well define your objective and in detail what you want to happen. There are also capacitive level sensors which can be drilled and tapped into a tank at any level. Just remember when you get above 212 F at sea level you will need to add pressure to the tank or your water boils off. This is where saturated steam tables come into play. Also certain sensors depending on how they are made can react with water depending on the water purity. So it comes down to exactly what you want to do, how you would like to go about it and budget as I see this is school project.

    Ron
     
  9. Reloadron

    Distinguished Member

    Jan 15, 2015
    4,196
    2,030
    You measure the top and bottom pressure the tank pressure matters not as long as the differential pressure gauge housing is rated. If there is 20 PSI in the vessel and the vessel is 60 inches tall and the vessel is full the differential pressure a little over 2 PSI The top of the tank is 200 PSI, the bottom of the tank is 201 PSI, the delta is 1 psi so the tank has 27 inches of water in the tank.

    Ron
     
  10. Externet

    AAC Fanatic!

    Nov 29, 2005
    1,340
    169
    Correct, Ron, agree. But a 1 psi differential in 200psi is what I believe a poor resolution for reliable detection, as 0.5%
     
  11. amainiero

    Thread Starter New Member

    Nov 1, 2017
    3
    0
    Currently, the system is a 50 gallon water distilling tank with a 1kw heating element in the bottom. I already have a temperature regulating device which keeps it within whatever range I set it to; but as the water level decreases over time and I am not always free to check the system, I need a means for it to shut off when the water gets below a certain level so that it doesn’t butn out the element. Your explanation of a relay seems simple enough, are 12v 0.5-1a relays to drive 120vac 9a switches common?
     
  12. Reloadron

    Distinguished Member

    Jan 15, 2015
    4,196
    2,030
    OK, now I understand much better what you have. You are not maintaining a pressurized vessel as you are distilling. That makes a big difference. Your heating element at 1.0 KW 120 VAC is really not that big of a load. At 120 VAC less than 10 Amps and it is a resistive load. Your sensor should be chosen based on what will fit in the tank arrangement you have. Be it a capacitive sensor or a float switch as long as the sensor can handle the temperature just about any sensor will do. Your only objective here is when the water level gets low to turn off the heaters so they don't self destruct.

    The sensor choice will also be based on your control voltage. Relays, including the solid state relays I mentioned can be controlled using 12 VDC, 24 VDC or even 120 VAC. Pretty much whatever you wish. The idea is the sensor is a low current device which in turn will control a much higher current device. The high current switching device needs to be able to handle the heater current with some overhead. The simple approach would be a float level switch if the tank can accommodate it. Personally I like using low voltage DC control and would use my sensor to drive an SSR but even a mechanical relay would work fine. Pretty much a matter of your budget.

    Ron
     
  13. Reloadron

    Distinguished Member

    Jan 15, 2015
    4,196
    2,030
    OK, here is what I am getting at. The tank pressure never figures into it. The tanks I worked with for example were running at about 3,000 PSIG. They ranged from 60 inches to as high as 80 inches in height. The tank pressure never enters into things. The only matter of importance was that the transducer housing and lines be able to handle the tank pressure. The actual measured differential pressure was only a matter of the weight of the water column ion the tank. Even if the top of the tank was atmospheric pressure the weight at the bottom would only be the weight of the column of water.

    We used Rosemount Differential Pressure Transducers which when rated for 3,000 PSI were expensive but the actual measured differential pressures were onlt several PSID. We used the same transducers to measure flow using pressure differential across an orifice in a line, also lines at 3,000 PSIG. Today Rosemount is a part of Emerson and the last transducer I bought before retiring was about $4,000. Here is what they are. Again, it is important to understand in systems like this that the system pressure has nothing to do with the differential pressure.

    Ron
     
  14. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    7,648
    1,867
    Remember this is one differential pressure measurement not the difference between two pressure measurements.
     
  15. Reloadron

    Distinguished Member

    Jan 15, 2015
    4,196
    2,030
    I got to thinking about this. I way overshot overthought this. You have a heater system already. You apparently have a temperature controller of some sorts. Exactly how are you controlling your heating element as of now? Your existing controller has some form of output unless it is a simple bi-metallic or capillary tube type (like on an older home oven). If you have a controller this may be as simple as placing a normally closed sensor inline with the controller output. When the sensor detects low water level it will open and the heaters shut down. What exactly do you have right now.

    Ron
     
  16. Picbuster

    Well-Known Member

    Dec 2, 2013
    900
    117
    Common used method is weight the water ( pressure sensors under the tank zero at empty) best solution for hot and pressured tanks.
    or use ultrasonic.
    or the old way use a gauge glass with a magnet in a floater in the gauge glass and sensor outside.

    Picbuster
     
Loading...