American Magnetics 185 & 186 Liquid Level Instruments (CE-Marked) - Manual (Page 29)

Rev. 3 19
Calibration
Calibration methods
3.3 Calibration Methods for Model 185/186 Instruments
The most straightforward calibration method is the Open Dewar
Calibration
which requires the customer to have access to a filled dewar
where the full active length of the sensor can be dipped. The Closed Dewar
Calibration
method can be performed in situations where it is not feasible
for the customer to dip the sensor into an open dewar, such as situations
where the target liquid is under pressure. The closed dewar calibration is
more complex and may require initial preparations to insure success.
Occasionally customers ask AMI to calibrate an instrument and sensor for
a liquid which is not available at AMI for calibration purposes and/or for a
sensor which is too long to be calibrated at our facilities.
For the case of the target liquid being unavailable, AMI uses liquid
nitrogen as the reference liquid and an Approximate Calibration is
performed using mathematical manipulation of the ratio of the dielectric
constants between liquid nitrogen and the desired liquid. This procedure is
outlined in the Approximate Calibration section beginning on page 25. The
technique is intended to provide the instrument with an approximate
calibration so that it can be used immediately by the customer. However,
the customer is still expected to perform a more accurate calibration where
feasible, such as the open dewar or closed dewar calibration, with the
target liquid.
For the case where a sensor is too long to be calibrated in AMI facilities,
AMI will perform a partial length open dewar calibration in liquid
nitrogen, and then calculate the MAX calibration point. A dielectric ratio
may also be subsequently utilized to adjust for a target liquid other than
liquid nitrogen. The customer is expected to perform a more accurate open
dewar or closed dewar calibration if feasible.
As a quick guide for selection of the best calibration method available, a
calibration selection diagram is presented below. If the instrument and
sensor are purchased as a unit from AMI, then the factory calibration will
be adequate in most cases. However, for the exceptions noted in the
previous paragraphs (which are approximate calibrations), the customer
should perform a more accurate open dewar or closed dewar calibration. A
customer performed calibration is also required for sensors that are
purchased as a separate item from the instrument, since the instrument
and sensor were not both available for calibration at AMI facilities.