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Infrared Thermometers

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Sentry Optronics offers an outstanding, time tested line of IR Thermometers.  These instruments offer the highest performance, and are manufactured to the highest standards.  Performance specifications equal or surpass instruments of two or three times the cost.  Sentry's quality standards, and performance requirements make these thermometers the best value on the internet.  Shop and compare our prices and industry best 18 month warranty and you will find that there is no better value available.

New Item - See our low introductory pricing!

ST 642 $75.00

In Stock  Newest addition to our fine line of Sentry Products

This family of thermometers have not only expanded range, but several unique features.  Both have the ability to use a K-type thermocouple.  Using the thermocouple and performing a simple test enables this unit to automatically adjust emissivity.  Also, there is a visual alarm for high and low temp. in addition to the audible.   Click here for specifications
ST 643 $89.00

In Stock
Optional K type Tcouple $14.95

In Stock


Each family of Infrared Thermometers begins with the basic "Point and Shoot" version available for under $60.00.   Within each style, higher model numbers indicate advanced features such as, Data logging, Min./ Max./Avg. and alarm setting.  The most advanced units offer adjustable emissivity.  Every Thermometer is covered by our industry leading Full 18 Month Warranty.  Also, Martrow's Satisfaction Guarantee Policy is second to none when it comes to your confidence in getting what you paid for!


Most items are in stock and ship the same day if ordered during normal business hours.  Other items usually deliver within 7 - 10 business days.  Standard 9V battery included with every unit.


ST 620


In Stock

  New sleek design.

  Rugged and reliable.  -25F to 999F temp. range.  12:1 D/S Ratio

Click here for more information and complete specifications

Important Questions for Thermometer Shoppers

ST 622


In Stock
ST 623 $83.25

In Stock
ST 630 49.95

7 - 10 days

  Most Economic Model      

  Compact design IR Thermometer offers a good  temperature range, for the lowest price!

 -4F to 930F temp. range.  8:1 D/S Ratio

Click here for more information and complete specifications

ST 632 $54.25

7 - 10 days
st631.jpg (16781 bytes) ST 631 $47.25

In Stock   New Dual Function Thermometer

  The new ST 631 has the unique feature of multiple modes. Clinical mode offers industry leading accuracy for body temp readings.  Household mode offers a nice temperature range for around the home use.

Click here for more information and complete specifications.

ST 650


7- 10 days

  Most Popular Model      

  Classic IR Thermometer  -25F to 999F temp. range.  12:1 D/S Ratio

Click here for more information and complete specifications

ST 652


7 -10 Days

ST 653


In Stock

ST 656


In Stock

  Low temp. model for Food Service industry.

  Expanded lower range  -58F to 600F temp. range.  12:1 D/S Ratio

Click here for more information and complete specifications

ST 658  $92.25

In Stock

ST 660


In Stock

  Wide Temperature range model

  -58F to 1830F.  12:1 D/S Ratio.  Most versatile unit in this price range

Click here for more information and complete specifications

ST 662 $119.75

In Stock
ST663 $128.25

In Stock
ST 670 $156.00

7 - 10 Days

  HDS Series High D/S ratio Very High Temp     Range up to 3000F+  See specifications

  -25.6 to over 3000F with the ST 677!

Click here for more information and complete specifications

ST 672 $255.00

7 - 10 Days
ST 675 355.75

In Stock
ST 677 $446.25

7 - 10 Days

ST 685


In Stock

  Long Distance measurement.  30:1 D/S Ratio.

  Wide Temperature range-58F to 1830F

Click here for more information and complete specifications

ST 688


In Stock

  Longest Distance measurement.  50:1 D/S Ratio.


  Wide Temperature range -58F to 1830F

Click here for more information and complete specifications

ST 688 Plus $326.75

7-10 Days

  Longest Distance measurement.  50:1 D/S Ratio.


  Widest Temperature range -58F to 2732F

Click here for more information and complete specifications


ST 689 $355.00

7-10 Days

  Longest Distance measurement.  50:1 D/S Ratio.  K Type Thermocouple for direct measurements, USB output for process control/data output.


  Wide Temperature range -58F to 1830F

Click here for more information and complete specifications





How do infrared thermometers work?

IR thermometer operation is based on the physical principle that all bodies having a temperature greater than absolute zero (0 K, –273.16C, –459.69F) radiate energy. The heat in such bodies causes molecular vibrations that induce electron vibrations. These electron vibrations in turn provide the electromagnetic coupling to produce emission. The wavelengths of the emitted radiation are temperature dependent, which means that the wavelength is associated with temperature; the amplitude is also influenced by a surface characteristic--emissivity--of the radiating body. Emissivity is the ratio of the energy radiated by an object at a given temperature to that emitted by a perfect radiator (blackbody) at the same temperature.”


All matter--animate or inanimate, liquid, solid, or gas--constantly exchanges thermal energy in the form of electromagnetic radiation with its surroundings. If there is a temperature difference between the object in question and its surroundings, there will be a net energy transfer in the form of heat; a colder object will be warmed at the expense of its surroundings, a warmer object cooled. And if the object in question is at the same temperature as its surrounding, the net radiation energy exchange will be zero.” “In either case, the characteristic spectrum of the radiation depends on the object and its surroundings' absolute temperatures. Non-contact temperature measurement, involves taking advantage of this radiation dependence on temperature to measure the temperature of objects and masses without the need for direct contact.”


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How do I use my infrared thermometer?

While there are many different shapes and styles of handheld infrared thermometers, and many features available, the basic function is very similar in very nearly every variation. The first step in taking a measurement, is determining the desired target.  All portable infrared thermometers regardless of design will have a means of "aiming" the instrument at the target.  Some versions simply are shaped in such a way that pointing at the target is the targeting method.  Others have a guide such as a laser spot, or group of laser spots that indicate the area being targeted.  The temperature measurement is taken when the "trigger" is activated.  This is usually a button located either underneath the pyrometer (gun style), or on top (television remote control style). The accuracy of the measurement depends on several factors.  See What is the distance to spot ratio?  to understand the elements governing accuracy.

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What is the distance to spot ratio?

D/S Ratio- Distance to Spot ratio refers to a very important feature of your infrared thermometer.  This ratio is the size of the area being evaluated by the infrared thermometer as it relates to distance.  In other words, the area being measured becomes larger as the distance increases.  This has a profound impact on the accuracy or precision of the reading.  If the target you are measuring is 6 inches in size, and your infrared thermometer has a D/S ratio of 8:1, than the maximum distance you can reliably measure the temperature of the target is 48 inches.  Beyond this distance, not only is the target being measured, but whatever else falls within the "spot" is being measured as well.  This means that if a very hot object is the target, and it is in cooler surroundings, than measurements taken beyond the maximum distance will include cooler elements, and lowering the "average" of what is in the "spot".


D/S Ratio X Target Size,   or   8:1 X 6 = maximum measure distance of 48 inches.


As the target size decreases, or the distance to the target increases, a larger D/S Ratio becomes necessary.  Using the same example above, and changing first the target size, and then the D/S ratio, you can see that this formula helps you decide the correct D/S ratio and therefore the Infrared Thermometer for your needs.


D/S Ratio X Target Size,   or   8:1 X 2 = maximum measure distance of 16 inches.


D/S Ratio X Target Size,   or 12:1 X 2 = maximum measure distance of 24 inches.


D/S Ratios vary greatly, so carefully compare this feature of IR Thermometers when comparison shopping.  This ratio, and temperature range are the two biggest factors to consider when shopping for an Infrared Thermometer!

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What are some common household applications for infrared thermometers?

While infrared thermometers are primarily used in industry, there as several useful applications for around the home!

Heating and cooling efficiency- This is a popular use for those who live in extreme climate areas.  Very cold winters, or very hot summers mean that escaping heating or cooling drives up the electric bill!.  During the extreme weather time of the year, the user can locate escaping heat, or cold around windows and doors and dramatically improve the heating, or cooling efficiency of their home!  Even a reasonably small area around a window or two that you can seal, may well return your investment in your thermometer in a single season by reducing heating or cooling costs!

Electrical Troubleshooting- A troublesome circuit breaker "popping" frequently, can be an indication of loose connections within the electrical box.  The qualified do-it-yourselfer can easily locate a loose connection by "scanning" through the electrical box.  Overall, the contents of the box should be within a temperature "range", but any loose connections will be drawing additional amperage and causing extra heating as a result.  Of course, as always when dealing with electricity, only qualified persons should make corrections, and only after taking the necessary precautions.

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What are some common industrial applications for infrared thermometers?

There are literally hundreds of applications in industry for infrared thermometers.  Some uses would not occur to the average person.  Here is a list of some common applications, and a few unusual ones as well!

HVAC- Heating Ventilation and Air Conditioning Technicians are one of the most frequent users of portable infrared thermometers.  It is vital in their job, to take accurate measurements, usually from a distance to gauge the efficiency of the unit by measuring the degree of cooling (or heating) of the air.

Home Inspections- Home inspectors have many uses as listed in the categories above.  They also might use their infrared thermometer to measure hot water temperature, look for insulation leaks etc.  This is becoming on of the most popular applications for pyrometers.

Health Inspectors- Perhaps not an example that would occur to everyone, health inspectors closely monitor many temperature related things in and around food handling and preparation.  Heating bins and surfaces hot enough?  Refrigerators and freezers cold enough to protect food from spoiling?  Restaurant owners, does your health inspector use an infrared thermometer?  If so, getting the same tool he uses is wise to avoid surprises!

Process Controls- This is a very broad category, but the following list details some industries that utilize handheld infrared thermometers for monitoring and maintaining temperature sensitive aspects of their business.  Some examples are:

Measuring the surface temperature of curing concrete to determine if cracking is a danger.

Measuring asphalt prior to pour to ensure smooth application.

Measuring ink temperatures prior to mix for accurate color matching in printing presses.

Monitoring the temperature of lamination materials for proper adhesion

Verifying part temperature following an oven baking operation to ensure proper drying, ductility, curing of paints and coatings etc.

Any application where taking temperature is either impossible through direct contact because of barriers or distance.

Taking temperatures in a hazardous environment where the user can maintain a safe distance by using non contact infrared thermometer technology.

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What should I consider when shopping for an infrared thermometer?

There are many factors to consider when shopping for an infrared thermometer.  There are many features and variables available, and of course, a very, very wide range of prices.  The following list of factors is structured so that the factors at the top of the list will likely have the greatest impact on cost.  You will notice that some of these factors are related in the way that they affect the value of the instrument.  So ask your self the following questions before shopping, and use the list below to help guide you in selecting the best infrared thermometer for your needs.  You can than start comparison shopping with an arsenal of information that will enable you to get the best value!  The one factor not listed below is Warranty from the provider.  While shopping, use this to determine what distributor has the greatest confidence in the product they are selling!

Question: What is the minimum and maximum temperature I might expect to be measuring?

Question: What is the maximum distance I may be measuring from?

Question: What is the smallest target object I may need to measure?

Question: How accurate do my results need to be?

Question: Do I need features like, Overtemp. alarm, Logging multiple readings etc.

Question: Do I need a targeting guide like a laser spot for accuracy?

Temperature Range- One of the most important feature that affects the cost of an infrared thermometer is the temperature range of the instrument.  A couple of important things to note, is that most thermometers fall into several groups that have a natural separation of costs.  Medical Use, Smaller Range, Medium Range, Lower Minimum Medium Range, Wide Range, Very Wide Range.

Obviously an infrared thermometer intended for medical purposes only needs a limited range.  There is a much greater need for accuracy however.  These are often very inexpensive and intended only for this purpose.  A more expensive unit could take measurements in the appropriate range, but the accuracy would likely not qualify, eliminating it as useful for this purpose.  Of course, plus or minus 2 degrees F is the difference between normal, and a fever!

Smaller Range infrared thermometers vary somewhat, but generally have a total range of  450 F or so.  A typical range for an instrument in this category might be -4F to 425 F.  Somewhat lower and higher variations will be noted, but this is an average for this category.  Usually inexpensive, and useful for many applications, this group is your choice if this range meets your needs.

Medium Range infrared thermometers typically are more consistent in their grouping regarding min. and max. temperature.  The average temperature range for this group is -25F to 999F.  This is the group that has the most versatility without adding a great deal of cost in most cases.

Lower Minimum Medium Range infrared thermometers are also sometimes called "Food Service IR Thermometers".  This is the case because the lower end of the range reaches down below -40F. (Usually -50F or more)  This is important to the food service industry to ensure the safety and freshness of frozen foodstuffs.  This is the group most likely used by health inspectors.

Wide Range IR thermometers represent the first big jump in cost.  For this reason, the health inspector for example, will likely select the prior category.  Number one because of cost, and number two, because he would likely not need the extreme upper range that this unit offers.  Although the lower end is the same as the previous group, and the accuracy is the same, the extra cost may not be attractive to that consumer, unless the thermometer will be used in other environments requiring the expanded upper range.  Typical ranges for this group go below -50F to 1000+ F.

Very Wide Range infrared thermometers are obviously the most costly group of IR thermometers, if we ignore other features than range alone.  Typical temperature ranges for this group can be nearly -60F to 1800+ F!  Found mostly in high end industrial applications, these units are rarely found in everyday uses as mentioned above.  They are reserved for special applications and as such are priced accordingly!

Conclusion: The better value will be the instrument with the greatest range for the same cost, all other factors being equal.

Distance from the target / target size-  These go hand in hand as we look at the Distance to Spot Ratio.  This feature is as important as temperature range when accurate readings are needed.  See what is the distance to spot ratio.  This determines many things among which are, the smallest target you can measure and from what distance.  Distance to Spot Ratios (D/S) will vary dramatically, and have an equally dramatic impact on cost.  D/S can be as low as 4:1 and as high as 50:1 or more!

Conclusion: The better value will be the instrument with the highest D/S ratio for the same cost, all other factors being equal.

Instrument Accuracy- Accuracy seems to be, on the surface a simple factor.  However, if cost is a concern as it generally is, than selecting an infrared thermometer that meets your needs as opposed to exceeding your needs, may mean a reduced cost for a suitable instrument.  I always suggest erring on the side of caution, and making sure the accuracy is adequate and then some.

Conclusion: The better value will be the instrument with the greatest accuracy for the same cost, all other factors being equal.

Features- The importance of various features is completely dictated by the intended end use.  If simple measurements are all that is required, than data logging, averaging, and high temperature alarms etc. are not necessary.  In many cases however, these features may be available for a minor added cost when selecting an upgraded unit, or even the same cost from another brand name!  This is where the diligent shopper can ensure that the best infrared thermometer for his or her needs is acquired by visiting several outlets and comparison shopping.

Conclusion: If extra features are needed, the instrument with the best features for the same cost, all other factors being equal will be the greatest value, and most useful.

Laser spots / Targeting Guides- The need for targeting aids such as laser spots or sighting scopes is primarily dictated by the need for accuracy.  It is important to note, that recent versions of infrared thermometers available now, almost always have the laser option.  The one negative to this feature is that it is a drain on the battery.  Many versions have a laser on/off feature, which can be disabled to either achieve extended overall battery life, or preserve remaining life when the battery has been in service for a while and is becoming drained.

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Gregory S. Rowe
Copyright 1999  Martrow Technical Products. All rights reserved.
Revised: November 30, 2011 .