stabmaster
Chemical
- May 30, 2005
- 50
Hello. I'm having a controller developed for my process, a solar thermal collection tank (a tank full of hot water).
The liquid level is controlled using a high and a low sensor. I am using conductive probes (no moving parts and relatively inexpensive); however, all commercially available probes are usually either switches or really large gague probes with very expensive junctions.
We mount our temperature sensors at the end of a threadded PVC pipe cut to length and submerged into the tank. We would like to do the same with the conductive level probes. Here is a rough schematic:
the probe might be threadded on the dry side. This side may have terminals, a plug, or wires coming out. The submerged side may have probes, connectors, nobs, or anything that will conduct electricity and is connected to the wire side.
Design considerations:
I would like to find something off the shelf. I don't really care what it is intended for: metal on both sides and some pipe thread in the middle should suffice.
I have decided that AC voltage would prevent electrolysis, so the gague of the metal is not as important as it will not be losing mass.
I need a corrosion resistant metal because it will be suspended in moist air much of the time. Is stainless steel the only reasonable choice?
I am going to use a pull-up resistor for the circuit to detect that the probe is on-line when not submerged. I do not know yet how to figure in the resistance due to the unknown "probe constant" ("k value") of this yet to be discovered probe.
Domestic water may have a resistivity of, say, between 5-500 microsiemens per centimeter.
Probe constants vary from 0.01-50 "per centimeter".
Therefore, the expected conductance ranges from .05-25,000 microsiemens.
In general, the higher conductivity of the fluid, the higher probe constant required. Since domestic water is not considered "highly conductive", I'll consider probe constants from 0.05-1.0, for argument sake.
This translates to a resistance range of 2kOhm -4000kOhm.
On a 24VAC circuit, this translates to 0.006mA to 12mA. Does this make sense? I am rusty on my electrical circuits.
Power requirements would be 0.024 mW to 24 mW.
So the proper pull-up resistor must be on the order of, say 40MOhm? Say, an order of magnitude greater than the greatest resistance expected from the conductive probe.
Will there be a problem with EM interference? Should the pull-up resistor be mounted in the pipe (in the tank), or in the controller? I don't know enough about this to make the call myself. The tank is mostly rubber and foam with very little aluminum on the outside. A copper pipe may be used if EM interference is a problem?
Now this leads to design considerations of the probe. The probe constant may be on the larger end (say near 50) so that the resistance is 2-20kOhm thus decreasing the sensitivity to interference. Is this true? I don't generally have any clue as to the design parameters of a sensor. I'm just using the knowledge i've gained from a course in electrical circuits and no intuition. Please help me gain some intuition on the matter.
The liquid level is controlled using a high and a low sensor. I am using conductive probes (no moving parts and relatively inexpensive); however, all commercially available probes are usually either switches or really large gague probes with very expensive junctions.
We mount our temperature sensors at the end of a threadded PVC pipe cut to length and submerged into the tank. We would like to do the same with the conductive level probes. Here is a rough schematic:
the probe might be threadded on the dry side. This side may have terminals, a plug, or wires coming out. The submerged side may have probes, connectors, nobs, or anything that will conduct electricity and is connected to the wire side.
Design considerations:
I would like to find something off the shelf. I don't really care what it is intended for: metal on both sides and some pipe thread in the middle should suffice.
I have decided that AC voltage would prevent electrolysis, so the gague of the metal is not as important as it will not be losing mass.
I need a corrosion resistant metal because it will be suspended in moist air much of the time. Is stainless steel the only reasonable choice?
I am going to use a pull-up resistor for the circuit to detect that the probe is on-line when not submerged. I do not know yet how to figure in the resistance due to the unknown "probe constant" ("k value") of this yet to be discovered probe.
Domestic water may have a resistivity of, say, between 5-500 microsiemens per centimeter.
Probe constants vary from 0.01-50 "per centimeter".
Therefore, the expected conductance ranges from .05-25,000 microsiemens.
In general, the higher conductivity of the fluid, the higher probe constant required. Since domestic water is not considered "highly conductive", I'll consider probe constants from 0.05-1.0, for argument sake.
This translates to a resistance range of 2kOhm -4000kOhm.
On a 24VAC circuit, this translates to 0.006mA to 12mA. Does this make sense? I am rusty on my electrical circuits.
Power requirements would be 0.024 mW to 24 mW.
So the proper pull-up resistor must be on the order of, say 40MOhm? Say, an order of magnitude greater than the greatest resistance expected from the conductive probe.
Will there be a problem with EM interference? Should the pull-up resistor be mounted in the pipe (in the tank), or in the controller? I don't know enough about this to make the call myself. The tank is mostly rubber and foam with very little aluminum on the outside. A copper pipe may be used if EM interference is a problem?
Now this leads to design considerations of the probe. The probe constant may be on the larger end (say near 50) so that the resistance is 2-20kOhm thus decreasing the sensitivity to interference. Is this true? I don't generally have any clue as to the design parameters of a sensor. I'm just using the knowledge i've gained from a course in electrical circuits and no intuition. Please help me gain some intuition on the matter.
