You may well have done this calculation for yourselves and if so then all is well.
As you know of course, the remaining Oxygen in a cylinder, is readily assessed at least in terms of % by checking the pressure gauge. The pressure being directly proportional to the volume of gas.
A quick look will tell you whether the cylinder is say 1/2 full , 1/4 full etc. How long will that last? (See table II below)
.Calculating Nitrous Oxide Usage
The only way to gauge a nitrous Oxide cylinder's contents is to weigh it.
Many GDPs use a mobile stand with size E cylinders and these can be weighed readily, by standing with the cylinder on a set of domestic bathroom scales. Note the total weight in Kg. and then weigh yourself. No cheating!! Now deduct your own weight from the total to give the Gross Weight (cylinder + liquid Nitrous Oxide).
You have to know the Tare (or empty) weight of the cylinder (written on the collar label) and deduct that from the Gross weight.
A full Size E nitrous Oxide cylinder has a Gross weight 8.8kg (A)
The Tare Weight is 5.9kg (B)
Therefore the Liquid Nitrous Oxide weighs (A-B) = 2.9kg (see the table below)
Now a full size E Nitrous Oxide cylinder contains 1360Litres of gaseous Nitrous Oxide (written on the collar label).
Calculating how long that amount of gas will last is almost like asking how long is a piece of string however the table below is based on:
1) Adult use
2) An average flow rate of 7L/m
3) A gas ratio of 60% Oxygen: 40% Nitrous Oxide (again as a probable mean ratio for adults)
Using these parameters produces the results in the table I below showing how much operating time (on average) remains for reducing weights of Nitrous Oxide cylinders, together with the % of gas remaining and the time (yes in minutes and hours) that such a cylinder will last.
If however the cylinder is used only for children, then it will last proportionately longer, as flow rates may be, say 3.5-4L/m and the ratio may well be 80% Oxygen to 20% Nitrous. This will be close to double the usage time available from the same cylinder weight. A mix of adults and children will be somewhere between.
If the cylinder is used for an adult case which requires a ratio of perhaps 40% Oxygen and 60% Nitrous Oxide then this is an increase of 50% of the required Nitrous Oxide, such that the full cylinder would only last 4 hours. If by chance the flow rate to match the patient's minute volume was required to be set to 10L/m ( unusual but not impossible) then the flow has increased by another 42% from the base calculation assumption. In this case the same cylinder would then only last about 2 hrs and 20 mins. Thus depending on all of these factors the same sized full cylinder might last between 2.3hrs and perhaps around 16 hours !!
The next size up, Size F cylinders, are also commonly used. In this case the full cylinder holds aprox 100% more nitrous Oxide (3600L) so just double the quantities and time available for those of the E cylinder table I below. However you would still need to do the same weighing exercise to gauge part-full cylinders.
A few GDPs also use use Size G cylinders. These are considerably larger and heavier and so weighing them by the above method is unlikely to be easy and there is a real risk of toppling such a cylinder with danger of injury, so cannot be advised. Size G Nitrous Oxide cylinders contain 9000L when full = 6.6 x the usage time or about 53 hours for that average use of the average adult only. Increasing pro rata when mainly children are treated or possibly reduced if depending on the number of adults requiring higher ratios of nitrous and larger Tidal Volumes.
The moral is always to arrange for sufficient numbers of cylinders to be held and/or ensure timely orders to be placed to avoid running out during a procedure but if using E or F cylinders and you have a part full nitrous oxide, you could now have a chance to calculate or at least guestimate how long that would last!
Table I
Table II
For a more detailed review of all aspects of Inhalation Sedation and to gain 7 hrs vCPD
as an update, towards complying with the required 12hrs every 5 years (IACSD April 2015)
with an opportunity for case discussions with colleagues,
we run a series of update days around the UK.
To be clear these updates are suitable for Dentists, DCPs and RDNs with experience of utilising Inhalation Sedation, prior to 22nd April 2015, under the new
11th Sept 2015 Perth
12th Sept 2015 Glasgow
17th October 2015 Newbury
Dates to be announced for
Bolton & London in the autumn of 2015
and another in Newbury 2016
Please e-mail: richard@the-ra-coach.co.uk
for full details and an application form
Richard Charon BDS
SAAD and DSTG listed mentor
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