KROHNE BM 70 A_P EN - Manual (Page 16)

Installation and operating instructions BM 70 A/P Page: 16 11/00
5.5 Transient recovery time
The transient recovery time is determined by the setting parameter "time constant" (1 ... 100 s).
The transient recovery time relative to 1% deviation from the steady-state value is approx. 4.6 times
the time constant.
However, in the case of unusually rapid changes in level, the transient recovery time may deviate
from this figure.
5.6 Turn-on drift / turn-on characteristics
A rough measured value is initially displayed about 1 minute after the BM 70 A has been switched on.
The first self-calibration is completed after about 2 minutes. Full measuring accuracy is attained after
30 minutes operating time.
The measured value from the BM 70 P is accurate directly after the startup phase (approx. 1.5
minutes).
Typical turn-on drift (example):
Measured value
Time/min 0 1 2 30
BM 70 A
BM 70 P
5.7 Long-term drift
The long-term drift is within the specified error of measurement.
5.8 Effect of ambient temperature
Temperature coefficient, signal output:
Ex-e current output HART
®
: < 100 ppm/°C (typically: 30 ppm/°C)
Ex-i current output HART
®
: < 100 ppm/°C (typically: 30 ppm/°C)
Current output: < 200 ppm/°C (typically: 70 ppm/°C)
Digital interfaces: no temperature effect
The temperature has no effect on the measured value as the device carries out a regular self-calibra-
tion.
The effect of temperature of the atmosphere above the liguid product results theoretically in -
1ppm/°C for air.
In respect of proof of temperature effect with the aid of reference measurements in liquid tanks, it
needs to be borne in mind that liquids generally have a high coefficient of cubic expansion (organic
liquids: typically 0.15 %/°C)!