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A: The criterion for calibration acceptance is that “the instrument under calibration meet the manufacturer’s advertised specifications.” Calibration has nothing to do with the absolute accuracy of a device.
Standard procedure is that if a unit under calibration fails to meet the advertised specification in a single area, then it may be marked “limited use only,” and the limit must be specified. For instance, if a multimeter were found to be out of calibration on the 1 V scale, it could be marked “Limited use only. 1 V scale out of tolerance.” Consequently, it could then be used on the 10 V scale. Ideally, any unit out of calibration on any scale would be removed from service until it was fully functional.
The accepted calibration procedure requires that the standard have an accuracy that is 10 times better than the device under test. For instance, a pressure transducer with an accuracy of 0.01% would, therefore, require a standard of 0.001%.
This may be easy to achieve with a 100-psi pressure transducer and a dead-weight tester. At the extreme end of sensitivity of some modern devices, this is no longer possible. For instance, calibration of 1 in. of a water column pressure transducer is difficult. In these cases, a ratio of 1 to 4 is accepted. However, there’s no case in which a ratio of worse than 1 to 4 acceptable.
Johns Hopkins University
Applied Physics Laboratory
Q: How do I calibrate a DP transmitter for use on a boiler drum? We’re using the wet leg measurement directly from the high and low measurement points on the drum to the DP transmitter.
A: The DP cell detects the weight of the column inside the drum between the two taps. This weight drops as temperature rises because of the swelling effect, and usually drops as the pressure rises, because steam bubbles are compressed. I said “usually” because the weight of steam per unit volume increases. Therefore, if you’re interested in the mass of water and steam in the drum, the DP cell will give you that information, but if you want to know if the top of the boiling liquid level has reached some maximum point, DP detection will not do. For that you can use a conductivty probe.
A: The pressure seen by the LP leg will be that due to a head of cold water matching the height of the LP leg, plus the drum pressure.
The pressure seen by the HP leg will be due to a leg of cold water (height of lower tapping to DP transmitter) plus a leg of hot water corresponding to the height in the drum (density very much lower, a function of drum pressure) plus a leg of steam (density a function of pressure, very significant in comparison with water density), plus the drum pressure.
So the “effective” density of the hot leg is the density of the water minus the density of the steam. For a drum level measurement to work from start-up to online requires a continuous knowledge of the density, and this will be changing with pressure/temperature.
The other twist is, of course, “what do you call level?” The liquid in the drum swells and shrinks depending on the vapor fraction in the particular cross section that you are measuring. A DP measurement is a measurement of the mass of a column of liquid + vapor adjacent to the tapping point. A boiler drum is not an equilibrium device. So it is quite easy for the top of the denser phase (liquid) to be elevated by a number of bubbles of the lower density (vapor) phase interspersed. A DP transmitter, by its very nature, can’t tell where the top of the liquid phase is, just where it would be if there were no bubbles in it.
Ian H. Gibson
Process, Control and Safety Systems
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