Endress+Hauser NRF81 - Page 210

Operating menu Tankside Monitor NRF81
210 Endress+Hauser
HTMS modes
Two HTMS modes can be selected in the HTMS mode parameter (→  211). The mode
determines whether one or two pressure values are used. Depending on the selected mode
a number of additional parameters are required for the calculation of the product density.
The HTMS P1+P3 option must be used in pressurized tanks in order to compensate
for the pressure of the vapor phase.
HTMS mode
(→  211)
Measured variables Required additional parameters Calculated variables
HTMS P1 • P
1
• L
P
• g
• H
P1
• L
W
(optional)
ρ
P
HTMS P1+P3 • P
1
• P
3
• L
P
• ρ
V
• ρ
A
• g
• H
P1
• H
P3
• L
W
(optional)
ρ
P
(more precise calculation
for pressurized tanks)
Minimum level
The density of the product can only be calculated if the product has a minimum thickness :
Δ Δ
P P, min
≥
A0028864
This is equivalent to the following condition for the product level:
L V H L
P P P
- ³ + =D
,min min1
A0028863
L
min
is defined in the Minimum level parameter (→  212). As can be seen from the
formula it always must be bigger than H
P1
.
If L
P
- V falls below this limit, the density is calculated as follows:
• If a previous calculated value is available, this value will be kept as long as no new
calculation is possible.
• If no value was previously calculated, the manual value (defined in the Manual upper
density parameter) will be used.
Hysteresis
The level of the product in a tank is not constant but slightly varies, due for example to
filling disturbances. If the level oscillates around the changeover level (Minimum level
(→  212)), the algorithm will constantly switch between calculating the value and
holding the previous result. To avoid this effect a positional hysteresis is defined around
the changeover point.