Bar (Customer) asked a question.
Hello, I am interested in wiring a single wire fluid level probe into a metal container (Boiler with a heating element). The metal container has a two male fast-on terminals (Spade) connection. The type of level probe can be found here:
https://www.espressoparts.com/products/auto-fill-probe-level-water-filling-probe
Do these act like switches . Can they be connected as a sinking input? ie 24 VDC to the top of the sensor and then connect one fast-on to X3 for example?
The metal container will also be grounded in case of an AC fault. Should their be any protection before X3 in case there is an electrical fault?
I have seen another forum where it is recommended to use another controller with them
https://www.plctalk.net/forums/threads/wiring-a-one-wire-level-sensor.146332/
Thanks for your help.
@OkiePC (Customer)
I tagged the person who responded to that other post as they post here as well.
They should be used with a controller that limits how much current can pass through the liquid and detect the change in resistance when the probe is immersed. Here's one NOTE it is 24VAC powered:
https://www.automationdirect.com/adc/shopping/catalog/process_control_-a-_measurement/level_sensors_-a-_controllers/level_controllers/clc1-f-24
I am pretty sure that 24VDC powered liquid level monitoring relays are out there.
EDIT: Where was this little guy when I needed him?
https://www.ato.com/liquid-level-control-relay?srsltid=AfmBOop9pWdDOlMljFAHBbwlaCndG77KEVqlkq4u8ai6kGieuBKzkrg6
I inherited a system once where the OEM used regular 24VDC relays with similar probes, but it was unreliable if there was debris on the probe and it became less conductive, not to mention the potential for a short allowing the full power supply current to pass through the tank. I ended up shoehorning the liquid level relays into that panel to make it reliable. Luckily, there were some unused relays I could remove to make it fit without getting too ugly.
I have come across several systems that used controllers that are basically transformers and 230VAC for these probes, but nah, I shudder at the thought of applying 230V to water even if it is current limited.
https://us.rs-online.com/product/b-w-controls/1500gl1s7ocx/70714756/
Thanks for your detail reply. I'm trying to learn here, so bear with me. It seems those modules have a lot of extra functionality so they can control fluid level via a pump/valve without a PLC. The discrete input on the BRX PLC has a 3kΩ impedence which should limit the current to 8mA on a 24VDC circuit with no other resistor inline. Since the boiler shell is also connected to ground, could a resistor (3kΩ) not be put in front of the probe to limit the current to ground? (This would allow the PLC input to see 12V)
Thanks
I agree with above posts that this is a conductivity level probe (uses the conductivity of the liquid). It is used with a conductivity-type control relay. I like using the AutomationDirect ProSens level control relays, such as the CLC1-F-120. These mount on a DIN rail. This one (CLC1-F-120) is 120VAC-powered. They also offer this relay in 24VAC-powered and 240VAC-powered. This one is a "Tank Fill' type relay. They also offer "Tank Drain" types. These include a "Sensitivity" adjustment to set the resistance between the probe and metal tank wall (or common probe) when the relay switches. They also include a status-indicating LED and a built-in delay. The metal tank wall must be connected to the "COM" terminal. This relay can be used with (2) probes, where you select both the high and low levels. We use (2) probes when controlling a pump for example, so the pump does not short-cycle. But this can also be used with (1) probe, as in your case. If using a plastic tank, then another "common" probe is used to connect to the "COM" terminal. The relay contacts can be used to provide the input to your PLC.
I'm glad you find this helpful, glad to help The ProSense level control relay does everything for you, no need to try to figure out resistors. And it does have functionality (1-probe, 2-probe, tank-fill, tank-drain). The level control relay measures the resistance between the probe and the tank wall. It uses a 5VDC-pulsed probe voltage (which is a UL listed method). If the resistance is above the setpoint, it assumes liquid is not touching the probe and switches the relay, (relay energizes or relay de-energizes depending on tank-fill or tank-drain model). If the resistance is below the setpoint, it assumes liquid is touching the probe and switches the relay back. The "Level Control Relay Installation Instructions" describes the operation for 1-probe, 2-probe, tank-fill, and tank-drain. Scroll down to get to the installation instructions.
https://www.automationdirect.com/adc/shopping/catalog/process_control_-a-_measurement/level_sensors_-a-_controllers/level_controllers/clc1-f-120
Yes, this relay contact can be used to control a valve or pump directly, without the need for a PLC. This relay contact can also provide an input to a PLC.
I would not recommend trying to make your own level controller by simply connecting a PLC input to the level probe and your 24VDC power supply. The PLC input wants to see 12-24VDC or 0VDC. Putting the liquid in series with the control voltage would cause the input voltage to vary with the conductivity of the liquid and liquid level. It would be an analog signal with an indeterminate range. Just too many things to go wrong.
Thanks again. Very helpful. I'm currently using a PLC to prototype this, but the final product will have an embedded board for control. Would you have time to share a link to a document concerning the 5VDC pulsed probe voltage as a UL listed method so I can share it with our EE? Or perhaps another search term for it? Thanks!
I'm guessing they do not have a 24DC powered version because its circuit needs the AC to produce the DC pulse? Thanks
This one has all the same features and can be powered from 24 to 240 volts AC or DC:
https://www.ato.com/liquid-level-control-relay
I wish it was around last time I needed one, because it is small too.
As far as the 5VDC-pulsed signal for measuring resistance, click on this link, scroll to one of the level control relays, CLC1-F-120, for example, and click on "Specifications". Scroll down to "CLC Liquid Level Controller Specifications" and the 5VDC is listed under "Probe Voltage".
https://www.automationdirect.com/adc/shopping/catalog/sensors_-z-_encoders/level_sensors_-a-_controllers/level_controllers#
As far as the UL listing, this is interesting. The CLC series controllers are "UL Listed". When being tested for UL Listing, they used the 70169-D 8-pin socket, so to maintain the "Listing" you must use the 70169-D 8-pin socket, as noted on the spec sheet. But I do not like the 70169-D socket. I prefer the 750-2C-SKT 8-pin socket. The 750-2C-SKT has recessed screws and is "finger-safe". The CLC series controllers are also "UL Recognized" as a component. And the 750-2C-SKT is also "UL Recognized" as a component. We have built various panels that had to be inspected by 3rd party inspectors to verify the panels were built to UL standards. And our panels passed using the "UL Recognized" components. Of course, you need to check with your inspectors and engineers.
That's a good question. I do not know why they do not offer this in a 24VDC version. It would be nice. Before ADC offered these level control relays, we used the Macromatic level control relays from Grainger (which looks identical to the ProSense, suspect it's private-labeled). Macromatic from Grainger did not offer a 24VDC version either. We were glad when ADC offered these level control relays, we could buy more panel components from one great supplier for less money!
You may be right, these units may use the AC to produce the DC pulse, but from the above post from OkiePC, they are available. It looks like Grainger and Digikey offers the Omron K8AK-LS1 level control relay, a VAC/DC level control relay, but I have never used these before.
In many (24VDC) cases, we used point level sensors (24VDC):
https://www.automationdirect.com/adc/shopping/catalog/sensors_-z-_encoders/level_sensors_-a-_controllers/capacitance_level_sensors#
We also used the ifm LMT series point level sensors (24VDC):
https://www.ifm.com/us/en/search?query=LMT&searchSessionId=1769618129414.p50sniv#/