Repairing a water level detector

[@xi@g@me]

Hello all!
My company's system administrator asked me to fix a water level detector by replacing the sensor which is completely rusted and does not work any more.
The detector has some kind of alarm that triggers when current flows through the sensor. It's basically made by a sensor controlling the base of a transistor, which will be used to amplify signal (with 3 chained transistors) to power the alarm. All of this is powered by a 9V battery.
The problem is that the sensor he gave me to replace the old one is current passing by default, so I had to build a small inverter circuit (with a pair of resistors and an NPN transistor)... and that circuit is consuming current. I used the same resistor the circuit uses, which is about 5.6KOhms, which makes a permanent consumption of nearly 1.6mA (against nearly 0 with the old circuit). According to the usual charge of a 9V battery, this battery would be discharged in about 13 days, which is really a bad thing!
Perhaps I should just replace the defecting sensor with a pair of wires ? Or is there a way for me to reduce the used current ? I don't really know how to compute the maximum resistance I could use for the inverter circuit (I'm not english, and don't know the terms I should use on my favorite search engine)

 

[@xi@g@me]

hmm... I computed that with the transistor I use to invert the signal (9015, gain of approx. 120), if I wanted to still have 1.6mA to the first amplifying transistor I could use a 630kOhms resistor :
- voltage through the resistor would be 9 - 0.6 (voltage drop of the resistor)
- current must be 1.6mA, so 0.0016A
- gain is 120
- 8.4 / 0.0016 * 120 = 630kOhms (R = U / I)
By experiment it seems like that both resistors of the inverting system must be of 630kOhms, I can't really explain why. Anyway, I managed to go up to 820kOhms resistors, and reduce permanent current usage to 0.2mA, increasing the lifespan of a regular battery (500mAh) to more than 3 months. I guess that's correct, but I still wonder if I did the thing correctly ?

 

[dave9]

My 9V powered water level detector's sensor is just a pair of wires, terminating in a plastic frame that seals them in so they don't corrode, with external stainless steel contacts. If you lack stainless contacts you could try stainless bolts and if you have trouble soldering to them, put the wire on with a terminal and a nut to hold it on.

 

[@xi@g@me]

Yeah, that's an easier solution I suppose ^^ I was wondering if I just shouldn't replace the failing sensor with a pair of wires, but as It's not my detector I tried to stick to what I've been asked to do.

EDIT : was right, he disagrees the water level is used in a large company (150+ people) for a tank that contains remains of coffee and stuff from a machine, and no-one cares about it. He told me that the former sensor was also "stainless steel" but finally got rusted any way. Using an active sensor like this one has the advantage of triggering the alarm if the sensor gets out of order.

Seems like he does not care replacing a battery every 3 months. Let's see if he will stick to that schedule

 

[ChosunOne]

Seems like he does not care replacing a battery every 3 months. Let's see if he will stick to that schedule

Here's an idea: Replace the corroded sensor electrodes with stainless steel bolts as dave9 suggested, then replace the bolts every 3 months, or 6 months, or year, as needed. I think 2 stainless steel bolts will be cheaper and probably almost as easy to change as a 9-volt battery.
(I say "probably" because I don't know what model device you're working with. Some devices require removing screws to replaces batteries.)

Using an active sensor like this one has the advantage of triggering the alarm if the sensor gets out of order.

This raises an interesting question: The sensor electrodes might eventually fail, but the battery is guaranteed eventually to fail, and you hope it will be 3 months before it does. Does a failing/dying battery trigger any kind of trouble alert or alarm?
I've had experience with getting "new" batteries that were left on the shelf too long before being sold, and died much sooner than expected.

 

[@xi@g@me]

Here's an idea: Replace the corroded sensor electrodes with stainless steel bolts as dave9 suggested, then replace the bolts every 3 months, or 6 months, or year, as needed. I think 2 stainless steel bolts will be cheaper and probably almost as easy to change as a 9-volt battery.
(I say "probably" because I don't know what model device you're working with. Some devices require removing screws to replaces batteries.)

No screws for this device, changing the bolts would take some more time. But yeah I agree this would have been a better solution to use bolts. I just don't do it because he does not want that solution.

This raises an interesting question: The sensor electrodes might eventually fail, but the battery is guaranteed eventually to fail, and you hope it will be 3 months before it does. Does a failing/dying battery trigger any kind of trouble alert or alarm?
I've had experience with getting "new" batteries that were left on the shelf too long before being sold, and died much sooner than expected.

Good point. I thought about that, too, there is no alarm indeed and that's a problem. But if I add a low battery circuit that triggers the same alarm then I'll reduce the battery life time even more. I guess a wall plug adapter would give all the advantages, but that may be overkill, wouldn't it be ?



Anyway, agree : having bolts would have been the best solution, and I'll try to convince my sys admin once again to stick to that. But I'm still curious on how I first fixed the problem: was that a good solving process ? I need to know for my personal experience