Ashcroft 1495 - Low Pressure Receiver Gauge - Manual (Page 6)

6
2.0 TEMPERATURE
2.1 Ambient Temperature – To ensure long
life and accuracy, pressure gauges should
preferably be used at an ambient tempera-
ture between –20 and +150F (–30 to +65C). At
very low temperatures, standard gauges may
exhibit slow pointer response. Above 150F, the
accuracy will be affected by approximately
1.5% per 100F. Other than discoloration of the
dial and hardening of the gasketing, non-liq-
uid filled type 1279 (phenolic case) and 1379
(aluminum case) Duragauge
® , with standard
glass windows, and Duralife ® gauges types
1008 and 1009, can withstand continuous
operating temperatures up to 250F. Liquid
filled gauges can withstand 200F but glycerin
fill and the acrylic window of Duragauge
®
gauges will tend to yellow. Silicone fill will
have much less tendency to yellow. Low pres-
sure, liquid filled types 1008 and 1009 gauges
may have some downscale errors caused by
liquid fill expansion. This can be alleviated by
“burping” the gauge by gently pushing the top
fill plug to one side to admit air to the case.
Although the gauge may be destroyed and cal-
ibration lost, gauges can withstand short
times at the following temperatures: gauges
with all welded pressure boundary joints,
750F (400C); gauges with silver brazed joints,
450F (232C) and gauges with soft soldered
joints, 250F (121C). For expected long term
service below –20F (–30C) Duragauge
® and
4 1 ⁄
2
˝ 1009 gauges should be hermetically
sealed and specially lubricated; add “H” to the
product code for hermetic sealing. Add varia-
tion XVY for special lubricant. Standard
Duralife gauges may be used to –50F (–45C)
without modification.
2.2 Accuracy – Heat and cold affect accuracy
of indication. A general rule of thumb for dry
gauges is 0.5% of full scale change for every
40°F change from 75°F. Double that allowance
for gauges with hermetically sealed or liquid
filled cases, except for Duragauge
® gauges
where no extra allowance is required due to
the elastomeric, compensating back. Above
250°F there may exist very significant errors
in indication.
2.3 Steam service – In order to prevent live
steam from entering the Bourdon tube, a
siphon filled with water should be installed
between the gauge and the process line.
Siphons can be supplied with ratings up to
4,000 psi. If freezing of the condensate in the
loop of the siphon is a possibility, a diaphragm
seal should be used to isolate the gauge from
the process steam. Siphons should also be
used whenever condensing, hot vapors (not
just steam) are present. Super heated steam
should have enough piping or capillary line
ahead of the siphon to maintain liquid water
in the siphon loop.
2.4 Hot or very cold media – A five foot capil-
lary line assembly will bring most hot or cold
process media within the recommended gauge
ambient temperature range. For media above
750F (400C) the customer should use his own
small diameter piping to avoid possible corro-
sion of the stainless steel. The five foot capillary
will protect the gauges used on the common
cryogenic (less than –300F (200C) gases, liquid
argon, nitrogen, and oxygen. The capillary and
gauge must be cleaned for oxygen service. The
media must not be corrosive to stainless steel,
and must not plug the small bore of the capillary.
2.5 Diaphragm seals – As mentioned above, a
diaphragm seal should be used to protect
gauges from corrosive media, or media that
will plug the instrument. Diaphragm seals
are offered in a wide variety of designs and
corrosion resistant materials to accommodate
almost any application and most connections.
Request bulletin OH-1 for details.
2.6 Autoclaving – Sanitary gauges with
clamp type connections are frequently steam
sterilized in an autoclave. Gauges equipped
with polysulfone windows will withstand
more autoclave cycles than those equipped
with
polycarbonate windows. Gauges
equipped with plain glass or laminated safety
glass should not be autoclaved. Gauge
cases should be vented to atmosphere (remov-
ing the rubber fill/safety plug if necessary)
before autoclaving to prevent the plastic
window from cracking or excessively distort-
ing. If the gauge is liquid filled, the fill should