When it comes to accuracy – you can count on Van Walt Ltd

September 15, 2014

But on this occasion we need the help of all you professionals working out in the field. Please read on…

A question that has been keeping me awake at night: Is your manual dip correct?

This has come about In the light of higher accuracy water level sensors we have recently launched from Instrumentation North West (INW) – AquiStar® Smart Sensors. These vented sensors have a “typical” accuracy of 3mm making them one of the most accurate transducer now available for environmental research. They are particularly appropriate for measuring surface water fluctuations.

However, before deployment they require a manual dip to water to establish a reference point from which to analyse the data. Currently this is done with a water level dip meter but this is prone to a number of problems:

1. The measurement is only as good as the operator on the day
2. Often the tape measure is a little bowed and this affects the readings
3. Sensors are frequently deployed within a stilling well at an angle from the lake or river shore. In the absence of an accurate inclinometer (and the application of Pythagoras’ theorem) this manual water level will throw up some inaccuracies.
4. In measurements of elevation of land a differential GPS system is used. This is a skilled job and the equipment is very expensive so probably not an option?

Contrary to absolute loggers (like Diver Level Loggers), INW sensors require a manual dip at the start of the deployment but using a poor manual dip will substantially increase the measurement inaccuracy.

So how do we get this datum? In my opinion it may be best to enter a “0” manual dip and insert the datum at a later point but of course we’d still be left with the value of the datum, but this could be obtained anytime in the deployment time sequence.

The question remains: How do we get the (accurate) datum? Can we work without it? (This may be the best route). I’d really like to hear your thoughts on this conundrum – post a reply below or email me on vincent@vanwalt.com.

Vincent van Walt

2 Responses to “When it comes to accuracy – you can count on Van Walt Ltd”

  1. Clive

    The new sensors look interesting. I’m not sure there is a difference between either system and the troubles of a slanted stilling well, either the measured string on a diver is at an angle or the manual dip tape is. There is far more error in measuring a string to a certain length. Divers are far better programmed via a manual dip for this and other reasons. Dip tapes without mm intervals are as much of a problem, I’ve also measured differences of 10cm or more in tapes without a rated accuracy tape. Loggers are only as accurate as your dip tape.

  2. Van Walt

    Thanks Clive for your interesting reply and I agree with what you write. I think I may not have been entirely clear: Both absolute and vented loggers absolutely require a manual dip. With Diver it can be taken at the beginning or at the end of a time sequence. With the Aquistar, the programming cannot be started unless a manual dip measurement is input at the beginning of the time sequence and prior to deployment and this is why perhaps my suggestion for the insertion of value=zero might be more expedient. The datum can then be measured at a later time and taken into account when the data is manipulated e.g. in Excel. The other point I was trying to make is that any error in a dip measurement, be it because of the operator or poor quality uncalibrated tape will be more pronounced as a function of the typical accuracy.

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