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Selection Process

When you want to implement a monitoring solution how do you go about the purchase? Do you :-

  1. Get the vibration sensors, cables and controller from one vendor?

  2. Do you get each item separately from different vendors?

  3. Which item do you buy first? Do you verify compatibility before buying?

A recent encounter makes for an interesting case study in the buying process. A customer requested for a quote for 4-20 mA vibration transducers. In the same message the customer, Mr X, appended the specifications which is for a mV/g accelerometer.

I informed Mr X that if he wanted to use a 4-20 mA transducer he should be aware that it will not be compliant to the specifications put out by the plant. After explaining to Mr X, he agreed that he should use a mV/g accelerometer. I quoted him a transmitter that can convert the mV/g signal to 4-20 mA as per the specifications.

Then later Mr X said that he has the controller already. Before buying he wanted to test if the mV/g accelerometer can work with his controller. This was a bit unusual because normally the specifications of his controller should be able to answer this question. Or he could also check with the controller vendor.

Anyway, since his worksite is nearby I went to visit him and brought along an mV/g accelerometer. When I saw his controller I had a feeling that it should work with a 4-20 mA transducer but not with a 100 mV/g accelerometer. Anyway, Mr X connected the accelerometer and he could not get a reading.

On the spot I checked the controller's model on the internet and it was a temperature controller and no mention of it being able to connect to a voltage sensor, much less a vibration sensor. Mr X called the controller vendor and the vendor said the controller will work with a 4-20 mA transducer but he was unsure about the mV/g accelerometer as he was not familiar with it.

I asked Mr X if the controller he has is a sample unit that he bought to test or he has bought the entire quantity required. He replied that he has bought the quantity required. Mr X said he will ask the vendor to check if they have a controller that can work with a mV/g accelerometer.

And so the story ends here for the time being while Mr X is supposed to check with the controller vendor for an alternative model. Except a few days later Mr X has still not checked with the vendor. So I wrote to the vendor, sent him the specifications for a 4-20 mA transducer and a 100 mV/g accelerometer. And what do you know, the vendor quoted the same controller that Mr X has.

So it seems that the controller vendor was not paying attention as few days ago when Mr X spoke to him. Anyway, I replied back to the vendor and asked him to confirm that his controller can work with a mV/g accelerometer since the datasheet and manual that he sent did not specify this clearly.

This case has raised some points of interest :-


Why did Mr X bought a controller that was not in compliant with the specifications? In the aftermath of the testing, Mr X said that the output required is in 4-20 mA so perhaps that was the reason why he bought that controller.

I can agree with his thinking but in the event an audit is carried out and the controller and sensor is found to be non-compliant with the original tender specifications then Mr X runs the risk of rejection and having to replace all the controllers. I suggested to him that perhaps he should discuss with the enduser and get their agreement on using 4-20 mA transducer instead of mV/g accelerometers. Then Mr X's problem would be solved.


Why did Mr X not get 1 unit controller to test first instead of buying outright the entire quantity? As things stands now he stands the risk of having to buy another batch of controllers if the user insists on compliance to the specifications.


Did Mr X send the specifications of the tender to the controller vendor to check, verify and confirm that it could be used with mV/g accelerometers before buying?


Mr X asked to test the vibration sensor after buying the controller. Did he ask the controller vendor to do a test to verify compliance to specifications? If no, then why not?

This case study illustrates the importance of checking with all vendors if one were to buy the controller and sensor from different parties.

As it is, I have this feeling that the controllers that Mr X has bought is going to be proved to be non-workable as far as complying to the tender specifications.

The best solution for Mr X is to have a discussion with the enduser and persuade them to accept the use of a 4-20 mA transducer while acknowledging the non-compliance with the published tender specifications.

I know the price of the controller is low but I would think that the vendor should take some responsibility to ensure that Mr X is getting the correct item. It would not be very nice to sell him something that he cannot use. I hope I can help Mr X settle his problem.

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