information flows from both the desired inventory and the inventory level. The
average time during which this inventory discrepancy is adjusted is termed here the
‘time to meet inventory discrepancy’. This time is considered here as a constant. For
a precise definition, the material requisition rate requires information on both
inventory discrepancy and the time to meet inventory discrepancy.
It may be observed here that if the inventory level exceeds the desired inventory
value, the inventory discrepancy becomes negative. This entails a negative value of
the material requisition rate, thus resulting in an outward flow of material from the
inventory level. In case the inventory is at its desired level, the inventory discrepancy
is zero, the material requisition rate is zero, and the inventory level continues to be at
its desired value. (In the simulation example of Chap. 13, water either remains in the
reservoir or gets spilled. It is still a flow, with the desired inventory level at V MAX .)
Two physical flows are shown in Fig. 6.5, namely (1) the flow of men and (2) the
flow of material. The accumulations (increases) in the flow of men arise from (a) the
men in training and (b) the trained manpower. The flow rates are (i) the hiring rate,
(ii) the rate at which men complete their training, and (iii) the rate at which men leave
the organisation. Two time constants are associated with this flow, namely (a) the
average training time, and (b) the average length of service. These constants, along
with the levels, help in defining the rate variables.
The only accumulation in the flow of material is the inventory. It increases with
production rate and decreases with the shipment rate. The production rate (units/day)
is defined as the product of trained manpower (man) and the labour productivity
(units/man/day). Labour productivity is a constant. Figure 6.5, thus, shows how two
physical flows are linked by an information flow.
6.5.5 Delays
Physical flows are often subject to delays, such as training of workers, order
processing (clerical, mailing, fulfilling), shipment, etc., and possess the following
properties:
(a) In a physical flow, the rate variable is subject to a delay.
(b) An accumulation will occur during the delay.
(c) Two factors affect the outflow rate: (1) the amount accumulated in the delay and
(2) the average time a unit stays in the delay.
(d) When conditions are steady (inflow rate = outflow rate = constant), the accumulation (level) in the delay is also constant, being equal to the inflow rate times
the average delay time constant.
(e) The transient response features of delays vary from situation to situation. In
some cases, there is a sharp and quick response between inflow and outflow
rates. In other cases, the outflow rate response may suffer a time lag, with
different rates of increase.
6.5 Elements of System Dynamics Modelling
179
average time during which this inventory discrepancy is adjusted is termed here the
‘time to meet inventory discrepancy’. This time is considered here as a constant. For
a precise definition, the material requisition rate requires information on both
inventory discrepancy and the time to meet inventory discrepancy.
It may be observed here that if the inventory level exceeds the desired inventory
value, the inventory discrepancy becomes negative. This entails a negative value of
the material requisition rate, thus resulting in an outward flow of material from the
inventory level. In case the inventory is at its desired level, the inventory discrepancy
is zero, the material requisition rate is zero, and the inventory level continues to be at
its desired value. (In the simulation example of Chap. 13, water either remains in the
reservoir or gets spilled. It is still a flow, with the desired inventory level at V MAX .)
Two physical flows are shown in Fig. 6.5, namely (1) the flow of men and (2) the
flow of material. The accumulations (increases) in the flow of men arise from (a) the
men in training and (b) the trained manpower. The flow rates are (i) the hiring rate,
(ii) the rate at which men complete their training, and (iii) the rate at which men leave
the organisation. Two time constants are associated with this flow, namely (a) the
average training time, and (b) the average length of service. These constants, along
with the levels, help in defining the rate variables.
The only accumulation in the flow of material is the inventory. It increases with
production rate and decreases with the shipment rate. The production rate (units/day)
is defined as the product of trained manpower (man) and the labour productivity
(units/man/day). Labour productivity is a constant. Figure 6.5, thus, shows how two
physical flows are linked by an information flow.
6.5.5 Delays
Physical flows are often subject to delays, such as training of workers, order
processing (clerical, mailing, fulfilling), shipment, etc., and possess the following
properties:
(a) In a physical flow, the rate variable is subject to a delay.
(b) An accumulation will occur during the delay.
(c) Two factors affect the outflow rate: (1) the amount accumulated in the delay and
(2) the average time a unit stays in the delay.
(d) When conditions are steady (inflow rate = outflow rate = constant), the accumulation (level) in the delay is also constant, being equal to the inflow rate times
the average delay time constant.
(e) The transient response features of delays vary from situation to situation. In
some cases, there is a sharp and quick response between inflow and outflow
rates. In other cases, the outflow rate response may suffer a time lag, with
different rates of increase.
6.5 Elements of System Dynamics Modelling
179