Gauge Guards are used to protect vacuum or pressure gauges, transducers and switches from the effects of process liquid contact with instrument mechanisms. The distinction between “chemical” type guards and “slurry” guards is that slurry guards have oversized process connections and provisions for purging solids trapped in inlet piping and guard housings. Appcor is the only manufacturer of slurry style guards, having installations since 1976 in Mining, Sand & Gravel, Chemical and Pulp & Paper industries.

The Gauge - To prime or not to prime? Contrary to the usual recommendation of guard manufacturers, most gauges used with Appcor guards do not have to be primed with transmitting fluid. Guards themselves must be filled with fluid.

Industrial duty vacuum and pressure gauges (to 600 PSI range) use a single C-shaped, 2.0-2.5 cm radius, 1.5-2.0 cc internal volume (typical for 3½”face gauge, and includes male ¼” NPT connection), bronze Bourdon tube and linkage mechanism to activate the indicator needle as the tube responds to changes in pressure. Whether filled with air or fluid, Bourdon tubes react identically to pressure changes.

Flat formed elastomeric diaphragms, installed in most others’ gauge protectors, frequently do not have the flexibility required to adequately compress air in the Bourdon tube to transmit accurate gauge readings. In this case air must be displaced from the Bourdon tube by hypodermic injection of transmitting fluid into the 1.5 cc cavity.

The geometry of all Appcor guards, and use of Rolling Diaphragms, results in up to 10.6 cc displacement of transmitter fluid, more than enough to accurately flex a 2.0 cm radius air filled bourdon tube. Naturally, if gauges are not threaded directly into the guard but are remotely connected to the guard with piping or hoses, these connections must be completely filled with transmitting fluid.

4½” face gauges, which usually have a ½” NPT connection, use a 4.0 cm radius Bourdon tube with total volume of 11.0 cc. These gauges should be at least partially primed with fluid.

Guard materials of construction vary with maximum rated operating temperatures of the guard assemblies. When specifying guard components, select materials suitable for maximum process temperatures.

Maximum 150ºF - PVC with Nitrile Diaphragm
(Most commonly used and least expensive)

Maximum 210ºF - CPVC with Viton Diaphragm

Maximum 280ºF - PVDF with Viton Diaphragm

Appcor guards are machined from solid bar stock with prototypes pressure tested to 1000 PSI at 70ºF. At their maximum rated temperatures, all Appcor guards are rated for operation to 200 PSI. Do not operate Appcor guards above 200 PSI. All guard hardware is of 316 stainless construction. Appcor guards are not suitable for high purity semiconductor or sanitary applications.

Vacuum and pressure gauge assemblies should not exceed 150º F. Should process temperatures exceed 150º F the instrument should be located away from the guard by use of extended connections. All connecting lines should be filled with transmitting fluid. The instrument side of guards can be filled with glycerin, silicone, mineral oil, or any light weight industrial oil.

¼” NPT instrument connections are fitted with Viton O-rings to prevent leakage of transmitting fluid. Teflon tape is not required on gauge threads. Over tightening of NPT gauge threads into guard housings is prevented.